Editing Preterm birth (section)

== Causes ==
Preterm labor can be caused by medical conditions including infections, environmental or drug exposures, cervix insufficiency, uterine abnormalities, or amniotic fluid problems.<ref name=":2">{{Cite web |date=2023-05-09 |title=What are the risk factors for preterm labor and birth? {{!}} NICHD - Eunice Kennedy Shriver National Institute of Child Health and Human Development |url=https://www.nichd.nih.gov/health/topics/preterm/conditioninfo/who_risk |access-date=2025-04-08 |website=www.nichd.nih.gov |language=en}}</ref> Some women require preterm labor induction for medical conditions.<ref name=":2" /> Other preterm births are spontaneous. In some cases, preterm labor occurs spontaneously and there is no identifiable cause.<ref name=":2" /> There are maternal and pregnancy-related risk factors that increase the risk of a women experiencing preterm labor.

=== Risk factors of preterm labor ===
The exact cause of spontaneous preterm birth is difficult to determine and it may be caused by many different factors at the same time as labor is a complex process.<ref>{{cite web | title = Preterm birth | publisher = World Health Organization | date = 19 February 2018 | url = https://www.who.int/news-room/fact-sheets/detail/preterm-birth | access-date = 20 May 2020}}</ref><ref>{{cite journal | vauthors = Frey HA, Klebanoff MA | title = The epidemiology, etiology, and costs of preterm birth | journal = Seminars in Fetal & Neonatal Medicine | volume = 21 | issue = 2 | pages = 68–73 | date = April 2016 | pmid = 26794420 | doi = 10.1016/j.siny.2015.12.011 }}</ref> The research available is limited with regard to the cervix and therefore is limited in discerning what is or is not normal.<ref name="Vink_2018" /> Four different pathways have been identified that can result in preterm birth and have considerable evidence: precocious fetal endocrine activation, uterine overdistension ([[placental abruption]]), decidual bleeding, and [[Intrauterine infections|intrauterine inflammation or infection]].<ref>{{Cite book|url=https://www.ncbi.nlm.nih.gov/books/NBK11353/|title=Biological Pathways Leading to Preterm Birth| vauthors = Behrman RE, Butler AS | collaboration = Institute of Medicine (US) Committee on Understanding Premature Birth and Assuring Healthy Outcomes |date=2007|publisher=National Academies Press (US)}}</ref>

Identifying women at high risk of giving birth early would enable the health services to provide specialized care for these women and their babies, for example a hospital with a special care baby unit such as a [[neonatal intensive care unit]] (NICU). In some instances, it may be possible to delay the birth. [[Risk score|Risk scoring systems]] have been suggested as an approach to identify those at higher risk; however, there is no strong research in this area so it is unclear whether the use of risk scoring systems for identifying mothers would prolong pregnancy and reduce the numbers of preterm births or not.<ref name="ReferenceB">{{cite journal | vauthors = Davey MA, Watson L, Rayner JA, Rowlands S | title = Risk-scoring systems for predicting preterm birth with the aim of reducing associated adverse outcomes | journal = The Cochrane Database of Systematic Reviews | volume = 2015 | issue = 10 | pages = CD004902 | date = October 2015 | pmid = 26490698 | pmc = 7388653 | doi = 10.1002/14651858.CD004902.pub5 }}</ref>

====Maternal factors====
{|class="wikitable" style="float:right; margin-left:1em; font-size:88%"
! Risk factor !! [[Relative risk]]<ref name=Os2013>Unless otherwise given in boxes, reference is: {{cite journal | vauthors = Van Os M, Van Der Ven J, Kazemier B, Haak M, Pajkrt E, Mol BW, De Groot C | s2cid = 8036202 | title = Individualizing the risk for preterm birth: An overview of the literature | journal = Expert Review of Obstetrics & Gynecology | volume = 8 | issue = 5 | pages = 435–442 | year = 2013 | doi = 10.1586/17474108.2013.825481 }}</ref> !! 95% [[confidence interval|confidence<br /> interval]]<ref name=Os2013/>
|-
| Fetal [[fibronectin]] || 4.0 || 2.9–5.5
|-
| Short cervical length || 2.9 || 2.1–3.9
|-
| Prenatal Care Absent<ref name="sciencedirect1254">{{cite journal | vauthors = Vintzileos AM, Ananth CV, Smulian JC, Scorza WE, Knuppel RA | title = The impact of prenatal care in the United States on preterm births in the presence and absence of antenatal high-risk conditions | journal = American Journal of Obstetrics and Gynecology | volume = 187 | issue = 5 | pages = 1254–1257 | date = November 2002 | pmid = 12439515 | doi = 10.1067/mob.2002.127140 }}</ref> || 2.9 || 2.8–3.0
|-
| [[Chlamydia infection|Chlamydia]] || 2.2 || 1.0–4.8
|-
| Low socio-economic status || 1.9 || 1.7–2.2
|-
| Large or small pregnancy weight gain || 1.8 || 1.5–2.3
|-
| Short maternal height || 1.8 || 1.3–2.5
|-
| [[Periodontitis]] || 1.6 || 1.1–2.3
|-
| [[Celiac disease]] || 1.4<ref name="TersigniCastellani2014">{{cite journal | vauthors = Tersigni C, Castellani R, de Waure C, Fattorossi A, De Spirito M, Gasbarrini A, Scambia G, Di Simone N | display-authors = 6 | title = Celiac disease and reproductive disorders: meta-analysis of epidemiologic associations and potential pathogenic mechanisms | journal = Human Reproduction Update | volume = 20 | issue = 4 | pages = 582–593 | year = 2014 | pmid = 24619876 | doi = 10.1093/humupd/dmu007 | doi-access = free | hdl = 10807/56796 | hdl-access = free }}</ref> || 1.2–1.6<ref name="TersigniCastellani2014"/>
|-
| [[Asymptomatic bacteriuria]] || 1.1 || 0.8–1.5
|-
| High or low [[Body mass index|BMI]] || 0.96 || 0.66–1.4
|-
| || ''[[odds ratio]]'' || 
|-
| History of spontaneous preterm birth || ''3.6'' || 3.2–4.0
|-
| [[Bacterial vaginosis]] || ''2.2'' || 1.5–3.1
|-
| Black ethnicity/race || ''2.0'' || 1.8–2.2 
|-
| Filipino ancestry<ref>{{cite web |url=http://health.hawaii.gov/mchb/files/2013/05/prematurity20101.pdf |title=Premature Birth Fact Sheet |access-date=2014-08-08 |url-status=live |archive-url=https://web.archive.org/web/20140808062311/http://health.hawaii.gov/mchb/files/2013/05/prematurity20101.pdf |archive-date=8 August 2014}}</ref> || ''1.7'' || 1.5–2.1
|-
| Unwanted pregnancy{{ r | UIP_Shah | p=1 | q=An unintended pregnancy was further classified as mistimed (not intended at that time) or unwanted (not desired at any time). ... There were statistically significantly increased odds of PTB among unintended (OR 1.31, 95% CI 1.09, 1.58), and unwanted (OR 1.50, 95% CI 1.41, 1.61) but not for mistimed (OR 1.36, 95% CI 0.96, 1.93) pregnancies. Unintended, unwanted, and mistimed pregnancies ending in a live birth are associated with a significantly increased risk of LBW and PTB. }} || ''1.5'' || 1.41–1.61
|-
| Unintended pregnancy{{ r | UIP_Shah | p=1 | q=An unintended pregnancy was further classified as mistimed (not intended at that time) or unwanted (not desired at any time). ... There were statistically significantly increased odds of PTB among unintended (OR 1.31, 95% CI 1.09, 1.58), and unwanted (OR 1.50, 95% CI 1.41, 1.61) but not for mistimed (OR 1.36, 95% CI 0.96, 1.93) pregnancies. Unintended, unwanted, and mistimed pregnancies ending in a live birth are associated with a significantly increased risk of LBW and PTB. }} || ''1.31'' || 1.09–1.58 
|-
| Being single/unmarried<ref name=Raatikainen2005/> || ''1.2'' || 1.03–1.28 
|}
[[File:Premature Births in England and Wales 2011 (cropped).png|thumb|upright=1.4|Percentage premature births in England and Wales 2011, by age of mother and whether single or multiple birth]]
Risk factors in the mother have been identified that are linked to a higher risk of a preterm birth. These include age (either very young or [[Advanced maternal age|older]]),<ref name=Goldenberg2008>
{{cite journal | vauthors = Goldenberg RL, Culhane JF, Iams JD, Romero R | title = Epidemiology and causes of preterm birth | journal = Lancet | volume = 371 | issue = 9606 | pages = 75–84 | date = January 2008 | pmid = 18177778 | pmc = 7134569 | doi = 10.1016/S0140-6736(08)60074-4 }}</ref> high or low body mass index (BMI),<ref>{{cite web| vauthors = Moldenhauer JS |title=Risk factors present before pregnancy|url=http://www.merck.com/mmhe/sec22/ch258/ch258b.html|work=Merck Manual Home Edition|publisher=Merck Sharp & Dohme|url-status=live|archive-url=https://web.archive.org/web/20100817051002/http://www.merck.com/mmhe/sec22/ch258/ch258b.html|archive-date=17 August 2010}}</ref><ref>
{{cite journal | vauthors = Hendler I, Goldenberg RL, Mercer BM, Iams JD, Meis PJ, Moawad AH, MacPherson CA, Caritis SN, Miodovnik M, Menard KM, Thurnau GR, Sorokin Y | display-authors = 6 | title = The Preterm Prediction Study: association between maternal body mass index and spontaneous and indicated preterm birth | journal = American Journal of Obstetrics and Gynecology | volume = 192 | issue = 3 | pages = 882–886 | date = March 2005 | pmid = 15746686 | doi = 10.1016/j.ajog.2004.09.021 | url = https://zenodo.org/record/1258686 }}</ref> length of time between pregnancies,<ref>{{cite journal | vauthors = Smith GC, Pell JP, Dobbie R | title = Interpregnancy interval and risk of preterm birth and neonatal death: retrospective cohort study | journal = BMJ | volume = 327 | issue = 7410 | pages = 313–0 | date = August 2003 | pmid = 12907483 | pmc = 169644 | doi = 10.1136/bmj.327.7410.313 }}</ref> [[endometriosis]],<ref name="PMID28181672">{{cite journal |last1=Berlac |first1=Janne Foss |last2=Hartwell |first2=Dorthe |last3=Skovlund |first3=Charlotte Wessel |last4=Langhoff-Roos |first4=Jens |last5=Lidegaard |first5=Øjvind |title=Endometriosis increases the risk of obstetrical and neonatal complications |journal=Acta Obstetricia et Gynecologica Scandinavica |date=June 2017 |volume=96 |issue=6 |pages=751–760 |doi=10.1111/aogs.13111 |pmid=28181672}}</ref> previous spontaneous (i.e., [[miscarriage]]) or surgical [[abortion]]s,<ref name="rcog-care">{{cite web| work = Evidence-based Clinical Guideline No. 7| publisher = [[Royal College of Obstetricians and Gynaecologists]]| title = The Care of Women Requesting Induced Abortion| pages = 44, 45| date = November 2011| access-date = 31 May 2013| url = http://www.rcog.org.uk/files/rcog-corp/Abortion%20guideline_web_1.pdf| archive-url = https://web.archive.org/web/20120529004416/http://www.rcog.org.uk/files/rcog-corp/Abortion%20guideline_web_1.pdf| url-status = dead| archive-date = 2012-05-29}}</ref><ref>
{{cite journal | vauthors = Virk J, Zhang J, Olsen J | title = Medical abortion and the risk of subsequent adverse pregnancy outcomes | journal = The New England Journal of Medicine | volume = 357 | issue = 7 | pages = 648–653 | date = August 2007 | pmid = 17699814 | doi = 10.1056/NEJMoa070445 | s2cid = 14975701 | doi-access = free }}</ref> unintended pregnancies,<ref name="UIP_Shah">{{cite journal | vauthors = Shah PS, Balkhair T, Ohlsson A, Beyene J, Scott F, Frick C | title = Intention to become pregnant and low birth weight and preterm birth: a systematic review | journal = Maternal and Child Health Journal | volume = 15 | issue = 2 | pages = 205–216 | date = February 2011 | pmid = 20012348 | doi = 10.1007/s10995-009-0546-2 | s2cid = 20441901 }}</ref> untreated or undiagnosed celiac disease,<ref name="TersigniCastellani2014" /><ref name="SacconeBerghella2015">{{cite journal | vauthors = Saccone G, Berghella V, Sarno L, Maruotti GM, Cetin I, Greco L, Khashan AS, McCarthy F, Martinelli D, Fortunato F, Martinelli P | display-authors = 6 | title = Celiac disease and obstetric complications: a systematic review and metaanalysis | journal = American Journal of Obstetrics and Gynecology | volume = 214 | issue = 2 | pages = 225–234 | date = February 2016 | pmid = 26432464 | doi = 10.1016/j.ajog.2015.09.080 | hdl = 11369/330101 | hdl-access = free }}</ref> fertility difficulties, heat exposure,<ref>{{cite journal| vauthors = Barreca A, Schaller J |year=2020|title=The impact of high ambient temperatures on delivery timing and gestational lengths|journal=Nature Climate Change|volume=10|pages=77–82|doi=10.1038/s41558-019-0632-4|issn=1758-6798|s2cid=208538820}}</ref> and genetic variables.<ref name="Bhattacharya_2010" />

Studies on type of work and physical activity have given conflicting results, but it is opined that stressful conditions, hard labor, and long hours are probably linked to preterm birth.<ref name="Goldenberg2008" /> [[Obesity]] does not directly lead to preterm birth;<ref>{{cite journal | vauthors = Tsur A, Mayo JA, Wong RJ, Shaw GM, Stevenson DK, Gould JB | title = 'The obesity paradox': a reconsideration of obesity and the risk of preterm birth | journal = Journal of Perinatology | volume = 37 | issue = 10 | pages = 1088–1092 | date = October 2017 | pmid = 28749482 | doi = 10.1038/jp.2017.104 | s2cid = 25566593 | author5-link = David K. Stevenson }}</ref> however, it is associated with diabetes and hypertension which are risk factors by themselves.<ref name="Goldenberg2008" /> To some degree those individuals may have underlying conditions (i.e., uterine malformation, hypertension, diabetes) that persist. Couples who have tried more than one year versus those who have tried less than one year before achieving a spontaneous conception have an adjusted [[odds ratio]] of 1.35 (95% [[confidence interval]] 1.22–1.50) of preterm birth.<ref name="Pinborg2013">{{cite journal | vauthors = Pinborg A, Wennerholm UB, Romundstad LB, Loft A, Aittomaki K, Söderström-Anttila V, Nygren KG, Hazekamp J, Bergh C | display-authors = 6 | title = Why do singletons conceived after assisted reproduction technology have adverse perinatal outcome? Systematic review and meta-analysis | journal = Human Reproduction Update | volume = 19 | issue = 2 | pages = 87–104 | year = 2012 | pmid = 23154145 | doi = 10.1093/humupd/dms044 | doi-access = free }}</ref> Pregnancies after [[in vitro fertilisation|IVF]] confers a greater risk of preterm birth than spontaneous conceptions after more than one year of trying, with an adjusted odds ratio of 1.55 (95% CI 1.30–1.85).<ref name="Pinborg2013" />

Certain ethnicities may have a higher risk as well. For example, in the U.S. and the UK, [[Black people|Black]] women have preterm birth rates of 15–18%, more than double than that of the white population. Many Black women have higher preterm birth rates due to multiple factors but the most common is high amounts of chronic stress, which can eventually lead to premature birth.<ref name="Braveman_2017">{{cite journal | vauthors = Braveman P, Heck K, Egerter S, Dominguez TP, Rinki C, Marchi KS, Curtis M | title = Worry about racial discrimination: A missing piece of the puzzle of Black-White disparities in preterm birth? | journal = PLOS ONE | volume = 12 | issue = 10 | pages = e0186151 | date = 2017-10-11 | pmid = 29020025 | pmc = 5636124 | doi = 10.1371/journal.pone.0186151 | bibcode = 2017PLoSO..1286151B | doi-access = free | veditors = Ryckman KK }}</ref> Adult chronic disease is not always the case with premature birth in Black women, which makes the main factor of premature birth challenging to identify.<ref name="Braveman_2017" /> [[Filipinos]] are also at high risk of premature birth, and it is believed that nearly 11–15% of Filipinos born in the U.S. (compared to other Asians at 7.6% and whites at 7.8%) are premature.<ref name="hawaii.edu">{{cite web |url=http://www.hawaii.edu/news/2014/01/14/preterm-birth-by-filipino-women-linked-to-genetic-mutational-change/ |title=Preterm birth by Filipino women linked to genetic mutational change |date=14 January 2014 |access-date=2014-08-08 |url-status=live |archive-url=https://web.archive.org/web/20140811122154/http://www.hawaii.edu/news/2014/01/14/preterm-birth-by-filipino-women-linked-to-genetic-mutational-change/ |archive-date=11 August 2014}}</ref> Filipinos being a big risk factor is evidenced with the [[Philippines]] being the eighth-highest ranking in the world for preterm births, the only non-African country in the top 10.<ref>{{cite web |url=http://www.smartparenting.com.ph/community/news/unicef-philippines-has-one-of-the-highest-premature-birth-rates-in-the-world |title=Smart Parenting: The Filipino Parenting Authority |access-date=2014-08-09 |url-status=dead |archive-url=https://web.archive.org/web/20140814165910/http://www.smartparenting.com.ph/community/news/unicef-philippines-has-one-of-the-highest-premature-birth-rates-in-the-world |archive-date=14 August 2014}}</ref> This discrepancy is not seen in comparison to other Asian groups or Hispanic immigrants and remains unexplained.<ref name="Goldenberg2008" /> Genetic make-up is a factor in the causality of preterm birth. Genetics has been a big factor into why Filipinos have a high risk of premature birth as the Filipinos have a large prevalence of mutations that help them be predisposed to premature births.<ref name="hawaii.edu" /> An intra- and transgenerational increase in the risk of preterm delivery has been demonstrated.<ref name="Bhattacharya_2010">{{cite journal | vauthors = Bhattacharya S, Amalraj Raja E, Ruiz Mirazo E, Campbell DM, Lee AJ, Norman JE, Bhattacharya S | title = Inherited predisposition to spontaneous preterm delivery | journal = Obstetrics and Gynecology | volume = 115 | issue = 6 | pages = 1125–1133 | date = June 2010 | pmid = 20502281 | doi = 10.1097/AOG.0b013e3181dffcdb | hdl-access = free | s2cid = 10113798 | hdl = 2164/2233 }}
*{{lay source |template = cite news| url= http://news.bbc.co.uk/1/hi/health/8701628.stm|title =  Premature birth risk is genetic, researchers suspect |date = May 25, 2010 |website= BBC News }}</ref> No single gene has been identified.

Marital status has long been associated with risks for preterm birth. A 2005 study of 25,373 pregnancies in Finland revealed that unmarried mothers had more preterm deliveries than married mothers (P=0.001).<ref name="Raatikainen2005">{{cite journal | vauthors = Raatikainen K, Heiskanen N, Heinonen S | title = Marriage still protects pregnancy | journal = BJOG | volume = 112 | issue = 10 | pages = 1411–1416 | date = October 2005 | pmid = 16167946 | doi = 10.1111/j.1471-0528.2005.00667.x | s2cid = 13193685 | doi-access = free }}</ref> Pregnancy outside of marriage was associated overall with a 20% increase in total adverse outcomes, even at a time when Finland provided free maternity care. A study in Quebec of 720,586 births from 1990 to 1997 revealed less risk of preterm birth for infants with legally married mothers compared with those with common-law wed or unwed parents.<ref>{{cite journal | vauthors = Luo ZC, Wilkins R, Kramer MS | title = Disparities in pregnancy outcomes according to marital and cohabitation status | journal = Obstetrics and Gynecology | volume = 103 | issue = 6 | pages = 1300–1307 | date = June 2004 | pmid = 15172868 | doi = 10.1097/01.AOG.0000128070.44805.1f | s2cid = 43892340 }}</ref>{{update inline| {{PMID|20690038}}|date=May 2013}} A study conducted in Malaysia in 2015 showed a similar trend, with marital status being significantly associated with preterm birth.<ref>{{Cite journal |last1=Zain |first1=Norhasmah Mohd |last2=Low |first2=Wah-Yun |last3=Othman |first3=Sajaratulnisah |date=2015 |title=Impact of Maternal Marital Status on Birth Outcomes Among Young Malaysian Women: A Prospective Cohort Study |url=https://www.jstor.org/stable/26725703 |journal=Asia-Pacific Journal of Public Health |volume=27 |issue=3 |pages=335–347 |doi=10.1177/1010539514537678 |jstor=26725703 |pmid=25005933 |issn=1010-5395|url-access=subscription }}</ref> However, the result of a study conducted in the US showed that between 1989 and 2006, marriage became less protective of preterm births which was attributed to the changing social norms and behaviors surrounding marriage.<ref>{{Cite journal |last1=El-Sayed |first1=Abdulrahman M. |last2=Galea |first2=Sandro |date=September 2011 |title=Changes in the Relationship between Marriage and Preterm Birth, 1989–2006 |journal=Public Health Reports |volume=126 |issue=5 |pages=717–725 |doi=10.1177/003335491112600514 |pmid=21886332 |issn=0033-3549|pmc=3151189 }}</ref>

====Factors during pregnancy====
Medications during pregnancy, living conditions, air pollution, smoking, illicit drugs or alcohol, infection, or physical trauma may also cause a preterm birth.

Air pollution: Living in an area with a high concentration of air pollution is a major risk factor for preterm labor, including living near major roadways or highways where vehicle emissions are high from traffic congestion or are a route for diesel trucks that tend to emit more pollution.<ref>{{Cite journal| vauthors = Currie J |date=October 2009|title=Traffic Congestion and Infant Health: Evidence from E-ZPass|url=https://www.nber.org/papers/w15413.pdf|journal=National Bureau of Economic Research}}</ref><ref>{{Cite news|url=https://www.cbc.ca/news/technology/air-pollution-study-1.5339472|title=Harmful air pollution 'definitely too high for the public' near city roads, study suggests| vauthors = Chung E |date=30 October 2019|publisher=CBC News|access-date=2 November 2019}}</ref><ref name="Kor2017" />

The use of [[fertility medication]] that stimulates the ovary to release multiple eggs and of [[In vitro fertilisation|IVF]] with [[embryo transfer]] of multiple embryos has been implicated as a risk factor for preterm birth. Often labor has to be induced for medical reasons; such conditions include [[high blood pressure]],<ref name="Goldenberg1998">{{cite journal | vauthors = Goldenberg RL, Iams JD, Mercer BM, Meis PJ, Moawad AH, Copper RL, Das A, Thom E, Johnson F, McNellis D, Miodovnik M, Van Dorsten JP, Caritis SN, Thurnau GR, Bottoms SF | display-authors = 6 | title = The preterm prediction study: the value of new vs standard risk factors in predicting early and all spontaneous preterm births. NICHD MFMU Network | journal = American Journal of Public Health | volume = 88 | issue = 2 | pages = 233–238 | date = February 1998 | pmid = 9491013 | pmc = 1508185 | doi = 10.2105/AJPH.88.2.233 }}</ref> [[pre-eclampsia]],<ref name="Banhidy2007">
{{cite journal | vauthors = Bánhidy F, Acs N, Puhó EH, Czeizel AE | title = Pregnancy complications and birth outcomes of pregnant women with urinary tract infections and related drug treatments | journal = Scandinavian Journal of Infectious Diseases | volume = 39 | issue = 5 | pages = 390–397 | year = 2007 | pmid = 17464860 | doi = 10.1080/00365540601087566 | s2cid = 5159387 }}</ref> maternal diabetes,<ref name="Rosenberg2005">{{cite journal | vauthors = Rosenberg TJ, Garbers S, Lipkind H, Chiasson MA | title = Maternal obesity and diabetes as risk factors for adverse pregnancy outcomes: differences among 4 racial/ethnic groups | journal = American Journal of Public Health | volume = 95 | issue = 9 | pages = 1545–1551 | date = September 2005 | pmid = 16118366 | pmc = 1449396 | doi = 10.2105/AJPH.2005.065680 }}</ref> asthma, thyroid disease, and heart disease.

Certain medical conditions in the pregnant mother may also increase the risk of preterm birth. Some women have anatomical problems that prevent the baby from being carried to term. These include a weak or short [[cervix]] (the strongest predictor of premature birth).<ref>
{{cite journal | vauthors = To MS, Skentou CA, Royston P, Yu CK, Nicolaides KH | title = Prediction of patient-specific risk of early preterm delivery using maternal history and sonographic measurement of cervical length: a population-based prospective study | journal = Ultrasound in Obstetrics & Gynecology | volume = 27 | issue = 4 | pages = 362–367 | date = April 2006 | pmid = 16565989 | doi = 10.1002/uog.2773 | s2cid = 24970386 | doi-access = free }}</ref><ref name="Fonseca2007">
{{cite journal | vauthors = Fonseca EB, Celik E, Parra M, Singh M, Nicolaides KH | title = Progesterone and the risk of preterm birth among women with a short cervix | journal = The New England Journal of Medicine | volume = 357 | issue = 5 | pages = 462–469 | date = August 2007 | pmid = 17671254 | doi = 10.1056/NEJMoa067815 | s2cid = 14884358 | doi-access = free }}</ref><ref name="Romero2007">
{{cite journal | vauthors = Romero R | title = Prevention of spontaneous preterm birth: the role of sonographic cervical length in identifying patients who may benefit from progesterone treatment | journal = Ultrasound in Obstetrics & Gynecology | volume = 30 | issue = 5 | pages = 675–686 | date = October 2007 | pmid = 17899585 | doi = 10.1002/uog.5174 | s2cid = 46366053 | doi-access = free }}</ref><ref name=Goldenberg1998/> Women with vaginal bleeding during pregnancy are at higher risk for preterm birth. While bleeding in the third trimester may be a sign of [[placenta previa]] or [[placental abruption]]—conditions that occur frequently preterm—even earlier bleeding that is not caused by these conditions is linked to a higher preterm birth rate.<ref>
{{cite journal | vauthors = Krupa FG, Faltin D, Cecatti JG, Surita FG, Souza JP | title = Predictors of preterm birth | journal = International Journal of Gynaecology and Obstetrics | volume = 94 | issue = 1 | pages = 5–11 | date = July 2006 | pmid = 16730012 | doi = 10.1016/j.ijgo.2006.03.022 | s2cid = 41368575 }}</ref> Women with abnormal amounts of [[amniotic fluid]], whether too much ([[polyhydramnios]]) or too little ([[oligohydramnios]]), are also at risk.<ref name=Goldenberg2008/> [[Anxiety]] and [[Major depressive disorder|depression]] have been linked as risk factors for preterm birth.<ref name="Goldenberg2008" /><ref name="Dole2003">
{{cite journal | vauthors = Dole N, Savitz DA, Hertz-Picciotto I, Siega-Riz AM, McMahon MJ, Buekens P | title = Maternal stress and preterm birth | journal = American Journal of Epidemiology | volume = 157 | issue = 1 | pages = 14–24 | date = January 2003 | pmid = 12505886 | doi = 10.1093/aje/kwf176 | url = http://171.66.121.65/cgi/reprint/157/1/14 | url-status = dead | s2cid = 44325654 | doi-access = free | archive-url = https://web.archive.org/web/20071008202838/http://171.66.121.65/cgi/reprint/157/1/14 | archive-date = 8 October 2007 | url-access = subscription }}</ref>

The use of [[tobacco]], [[cocaine]], and excessive [[alcohol (drug)|alcohol]] during pregnancy increases the chance of preterm delivery. [[Tobacco smoking|Tobacco]] is the most commonly used drug during pregnancy and contributes significantly to low birth weight delivery.<ref>
{{cite journal | vauthors = Parazzini F, Chatenoud L, Surace M, Tozzi L, Salerio B, Bettoni G, Benzi G | title = Moderate alcohol drinking and risk of preterm birth | journal = European Journal of Clinical Nutrition | volume = 57 | issue = 10 | pages = 1345–1349 | date = October 2003 | pmid = 14506499 | doi = 10.1038/sj.ejcn.1601690 | s2cid = 27688375 | doi-access = free }}</ref> Babies with [[birth defect]]s are at higher risk of being born preterm.<ref>
{{cite journal | vauthors = Dolan SM, Gross SJ, Merkatz IR, Faber V, Sullivan LM, Malone FD, Porter TF, Nyberg DA, Comstock CH, Hankins GD, Eddleman K, Dugoff L, Craigo SD, Timor-Tritsch I, Carr SR, Wolfe HM, Bianchi DW, D'Alton ME | display-authors = 6 | title = The contribution of birth defects to preterm birth and low birth weight | journal = Obstetrics and Gynecology | volume = 110 | issue = 2 Pt 1 | pages = 318–324 | date = August 2007 | pmid = 17666606 | doi = 10.1097/01.AOG.0000275264.78506.63 | s2cid = 32544532 }}</ref>

[[Passive smoking]] and/or smoking before the pregnancy influences the probability of a preterm birth. The [[World Health Organization]] published an international study in March 2014.<ref>[[The Lancet]] 28. März 2014: [http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2814%2960082-9/abstract Effect of smoke-free legislation on perinatal and child health: a systematic review and meta-analysis]. This study is registered with PROSPERO, number CRD42013003522</ref>

Presence of [[anti-thyroid antibodies]] is associated with an increased risk preterm birth with an [[odds ratio]] of 1.9 and 95% [[confidence interval]] of 1.1–3.5.<ref>{{cite journal | vauthors = van den Boogaard E, Vissenberg R, Land JA, van Wely M, van der Post JA, Goddijn M, Bisschop PH | title = Significance of (sub)clinical thyroid dysfunction and thyroid autoimmunity before conception and in early pregnancy: a systematic review | journal = Human Reproduction Update | volume = 17 | issue = 5 | pages = 605–619 | year = 2011 | pmid = 21622978 | doi = 10.1093/humupd/dmr024 | doi-access = free }}</ref>

Intimate violence against the mother is another risk factor for preterm birth.<ref>{{cite journal | vauthors = Boy A, Salihu HM | title = Intimate partner violence and birth outcomes: a systematic review | journal = International Journal of Fertility and Women's Medicine | volume = 49 | issue = 4 | pages = 159–164 | year = 2004 | pmid = 15481481 }}</ref>

Physical trauma may case a preterm birth. The Nigerian cultural method of abdominal massage has been shown to result in 19% preterm birth among women in [[Nigeria]], plus many other adverse outcomes for the mother and baby.<ref>
{{cite journal | vauthors = Ugboma HA, Akani CI | title = Abdominal massage: another cause of maternal mortality | journal = Nigerian Journal of Medicine | volume = 13 | issue = 3 | pages = 259–262 | year = 2004 | pmid = 15532228 }}</ref> This ought not be confused with massage therapy conducted by a fully trained and certified/licensed massage therapist or by significant others trained to provide massage during pregnancy, which—in a study involving pregnant females with prenatal depression—has been shown to have numerous positive results during pregnancy, including the reduction of preterm birth, less depression, lower cortisol, and reduced anxiety.<ref>{{cite journal | vauthors = Field T, Deeds O, Diego M, Hernandez-Reif M, Gauler A, Sullivan S, Wilson D, Nearing G | display-authors = 6 | title = Benefits of combining massage therapy with group interpersonal psychotherapy in prenatally depressed women | journal = Journal of Bodywork and Movement Therapies | volume = 13 | issue = 4 | pages = 297–303 | date = October 2009 | pmid = 19761951 | pmc = 2785018 | doi = 10.1016/j.jbmt.2008.10.002 }}</ref> In healthy women, however, no effects have been demonstrated in a controlled study.

====Infection====
The frequency of infection in preterm birth is inversely related to the gestational age. ''[[Mycoplasma genitalium]]'' infection is associated with increased risk of preterm birth, and spontaneous abortion.<ref name="LisRowhani-Rahbar2015">{{cite journal | vauthors = Lis R, Rowhani-Rahbar A, Manhart LE | title = Mycoplasma genitalium infection and female reproductive tract disease: a meta-analysis | journal = Clinical Infectious Diseases | volume = 61 | issue = 3 | pages = 418–426 | date = August 2015 | pmid = 25900174 | doi = 10.1093/cid/civ312 | doi-access = free | hdl = 1773/26479 | hdl-access = free }}</ref>

Infectious microorganisms can be ascending, hematogenous, iatrogenic by a procedure, or retrograde through the fallopian tubes. From the deciduae they may reach the space between the [[amnion]] and [[chorion]], the [[amniotic fluid]], and the fetus. A [[chorioamnionitis]] also may lead to [[sepsis]] of the mother. Fetal infection is linked to preterm birth and to significant long-term disability including [[cerebral palsy]].<ref>{{cite journal | vauthors = Schendel DE | title = Infection in pregnancy and cerebral palsy | journal = Journal of the American Medical Women's Association | volume = 56 | issue = 3 | pages = 105–108 | year = 2001 | pmid = 11506145 }}</ref>

It has been reported that asymptomatic [[colonization]] of the decidua occurs in up to 70% of women at term using a [[DNA probe]] suggesting that the presence of micro-organism alone may be insufficient to initiate the infectious response.

As the condition is more prevalent in black women in the U.S. and the UK, it has been suggested to be an explanation for the higher rate of preterm birth in these populations. It is opined that bacterial vaginosis before or during pregnancy may affect the decidual inflammatory response that leads to preterm birth. The condition known as [[aerobic vaginitis]] can be a serious risk factor for preterm labor; several previous studies failed to acknowledge the difference between aerobic vaginitis and bacterial vaginosis, which may explain some of the contradiction in the results.<ref>{{cite journal | vauthors = Donders G, Bellen G, Rezeberga D | title = Aerobic vaginitis in pregnancy | journal = BJOG | volume = 118 | issue = 10 | pages = 1163–1170 | date = September 2011 | pmid = 21668769 | doi = 10.1111/j.1471-0528.2011.03020.x | s2cid = 7789770 | doi-access = free }}</ref>

Untreated [[Candidiasis|yeast]] infections are associated with preterm birth.<ref name=rob2015>{{cite journal | vauthors = Roberts CL, Algert CS, Rickard KL, Morris JM | title = Treatment of vaginal candidiasis for the prevention of preterm birth: a systematic review and meta-analysis | journal = Systematic Reviews | volume = 4 | issue = 1 | pages = 31 | date = March 2015 | pmid = 25874659 | pmc = 4373465 | doi = 10.1186/s13643-015-0018-2 | doi-access = free }}</ref>

A review into prophylactic antibiotics (given to prevent infection) in the second and third trimester of pregnancy (13–42 weeks of pregnancy) found a reduction in the number of preterm births in women with bacterial vaginosis. These antibiotics also reduced the number of waters breaking before labor in full-term pregnancies, reduced the risk of infection of the lining of the womb after delivery (endometritis), and rates of gonococcal infection. However, the women without bacterial vaginosis did not have any reduction in preterm births or pre-labor preterm waters breaking. Much of the research included in this review lost participants during follow-up so did not report the long-term effects of the antibiotics on mothers or babies. More research in this area is needed to find the full effects of giving antibiotics throughout the second and third trimesters of pregnancy.<ref name="ReferenceC">{{cite journal | vauthors = Thinkhamrop J, Hofmeyr GJ, Adetoro O, Lumbiganon P, Ota E | title = Antibiotic prophylaxis during the second and third trimester to reduce adverse pregnancy outcomes and morbidity | journal = The Cochrane Database of Systematic Reviews | volume = 2015 | issue = 6 | pages = CD002250 | date = June 2015 | pmid = 26092137 | pmc = 7154219 | doi = 10.1002/14651858.CD002250.pub3 | veditors = Thinkhamrop J }}</ref>

A number of maternal bacterial infections are associated with preterm birth including [[pyelonephritis]], asymptomatic [[bacteriuria]], [[pneumonia]], and [[appendicitis]]. A review into giving antibiotics in pregnancy for asymptomatic bacteriuria (urine infection with no symptoms) found the research was of very low quality but that it did suggest that taking antibiotics reduced the numbers of preterm births and babies with low birth weight.<ref name="Smaill_2019">{{cite journal | vauthors = Smaill FM, Vazquez JC | title = Antibiotics for asymptomatic bacteriuria in pregnancy | journal = The Cochrane Database of Systematic Reviews | volume = 2019 | issue = 11 | date = November 2019 | pmid = 31765489 | pmc = 6953361 | doi = 10.1002/14651858.CD000490.pub4 }}</ref> Another review found that one dose of antibiotics did not seem as effective as a course of antibiotics but fewer women reported side effects from one dose.<ref>{{cite journal | vauthors = Widmer M, Lopez I, Gülmezoglu AM, Mignini L, Roganti A | title = Duration of treatment for asymptomatic bacteriuria during pregnancy | journal = The Cochrane Database of Systematic Reviews | volume = 2015 | issue = 11 | pages = CD000491 | date = November 2015 | pmid = 26560337 | pmc = 7043273 | doi = 10.1002/14651858.CD000491.pub3 }}</ref> This review recommended that more research is needed to discover the best way of treating asymptomatic bacteriuria.<ref name="Smaill_2019"/>

A different review found that preterm births happened less for pregnant women who had routine testing for low genital tract infections than for women who only had testing when they showed symptoms of low genital tract infections.<ref name="ReferenceD">{{cite journal | vauthors = Sangkomkamhang US, Lumbiganon P, Prasertcharoensuk W, Laopaiboon M | title = Antenatal lower genital tract infection screening and treatment programs for preventing preterm delivery | journal = The Cochrane Database of Systematic Reviews | volume = 2015 | issue = 2 | pages = CD006178 | date = February 2015 | pmid = 25922860 | pmc = 8498019 | doi = 10.1002/14651858.CD006178.pub3 }}</ref> The women being routinely tested also gave birth to fewer babies with a low birth weight. Even though these results look promising, the review was only based on one study so more research is needed into routine screening for low genital tract infections.<ref name="ReferenceD"/>

Also [[periodontal disease]] has been shown repeatedly to be linked to preterm birth.<ref>
{{cite journal | vauthors = Jeffcoat MK, Geurs NC, Reddy MS, Cliver SP, Goldenberg RL, Hauth JC | title = Periodontal infection and preterm birth: results of a prospective study | journal = Journal of the American Dental Association | volume = 132 | issue = 7 | pages = 875–880 | date = July 2001 | pmid = 11480640 | doi = 10.14219/jada.archive.2001.0299 }}</ref><ref>{{cite web |url=https://www.unitedconcordia.com/dental-insurance/dental/conditions/pregnancy-oral-health/ |title=Pregnancy and Oral Health - United Concordia Dental |access-date=2015-01-19 |url-status=live |archive-url=https://web.archive.org/web/20150120074310/https://www.unitedconcordia.com/dental-insurance/dental/conditions/pregnancy-oral-health/ |archive-date=20 January 2015}}</ref> In contrast, viral infections, unless accompanied by a significant febrile response, are considered not to be a major factor in relation to preterm birth.<ref name=Goldenberg2008/>

====Genetics====
There is believed to be a maternal genetic component in preterm birth.<ref name=Kistka2008>
{{cite journal | vauthors = Kistka ZA, DeFranco EA, Ligthart L, Willemsen G, Plunkett J, Muglia LJ, Boomsma DI | title = Heritability of parturition timing: an extended twin design analysis | journal = American Journal of Obstetrics and Gynecology | volume = 199 | issue = 1 | pages = 43.e1–43.e5 | date = July 2008 | pmid = 18295169 | doi = 10.1016/j.ajog.2007.12.014 }}</ref> Estimated heritability of timing-of-birth in women was 34%. However, the occurrence of preterm birth in families does not follow a clear inheritance pattern, thus supporting the idea that preterm birth is a non-Mendelian trait with a polygenic nature.<ref name="Zhang2017">{{cite journal | vauthors = Zhang G, Feenstra B, Bacelis J, Liu X, Muglia LM, Juodakis J, Miller DE, Litterman N, Jiang PP, Russell L, Hinds DA, Hu Y, Weirauch MT, Chen X, Chavan AR, Wagner GP, Pavličev M, Nnamani MC, Maziarz J, Karjalainen MK, Rämet M, Sengpiel V, Geller F, Boyd HA, Palotie A, Momany A, Bedell B, Ryckman KK, Huusko JM, Forney CR, Kottyan LC, Hallman M, Teramo K, Nohr EA, Davey Smith G, Melbye M, Jacobsson B, Muglia LJ | display-authors = 6 | title = Genetic Associations with Gestational Duration and Spontaneous Preterm Birth | journal = The New England Journal of Medicine | volume = 377 | issue = 12 | pages = 1156–1167 | date = September 2017 | pmid = 28877031 | pmc = 5561422 | doi = 10.1056/NEJMoa1612665 }}</ref>

====Prenatal care====
The absence of prenatal care has been associated with higher rates of preterm births. Analysis of 15,627,407 live births in the United States in 1995–1998 concluded that the absence of prenatal care carried a 2.9 (95%CI 2.8, 3.0) times higher risk of preterm births.<ref name="sciencedirect1254"/> This same study found statistically significant relative risks of maternal anemia, intrapartum fever, unknown bleeding, renal disease, placental previa, hydramnios, placenta abruption, and pregnancy-induced hypertension with the absence of prenatal care. All these prenatal risks were controlled for other high-risk conditions, maternal age, gravidity, marital status, and maternal education. The absence of prenatal care prior to and during the pregnancy is primarily a function of socioeconomic factors (low family income and education), access to medical consultations (large distance from the place of residence to the healthcare unit and transportation costs), quality of healthcare, and social support.<ref>{{cite journal | vauthors = Rosa CQ, Silveira DS, Costa JS | title = Factors associated with lack of prenatal care in a large municipality | journal = Revista de Saude Publica | volume = 48 | issue = 6 | pages = 977–984 | date = December 2014 | pmid = 26039401 | pmc = 4285828 | doi = 10.1590/S0034-8910.2014048005283 }}</ref> Efforts to decrease rates of preterm birth should aim to increase the deficits posed by the aforementioned barriers and to increase access to prenatal care.