Editing Gestational age

{{short description|Measure of the age of a pregnancy}}
{{for|actual age of gestation|Human fertilization#Fertilization age}}

In [[obstetrics]], '''gestational age''' is a measure of the age of a [[pregnancy]] taken from the beginning of the woman's [[Menstruation#Onset and frequency|last menstrual period]] (LMP),<ref name="medline">{{cite web |title=Gestational age: MedlinePlus Medical Encyclopedia |url=https://medlineplus.gov/ency/article/002367.htm |website=medlineplus.gov |language=en}}</ref> or the corresponding age of the gestation as estimated by a more accurate method, if available. Such methods include adding 14 days to a known duration since [[Human fertilization|fertilization]] (as is possible in [[in vitro fertilization]]), or by [[obstetric ultrasonography]]. The popularity of using this measure of pregnancy is largely due to convenience:  menstruation is usually noticed, while there is generally no convenient way to discern when fertilization or [[implantation (embryology)|implantation]] occurred.

Gestational age is contrasted with [[fertilization age]], which takes the date of fertilization as the start date of [[gestation]]. There are [[Beginning of pregnancy controversy|different approaches to defining the start of a pregnancy]]. This definition is unusual in that it describes women as becoming "pregnant" about two weeks before they even had sex. The definition of pregnancy and the calculation of gestational age are also relevant [[Beginning of pregnancy controversy|in the context of the abortion debate]] and the philosophical debate over the [[beginning of human personhood]].

==Methods==
According to [[American College of Obstetricians and Gynecologists]], the main methods to calculate gestational age are:<ref name=acog2012>[http://www.acog.org/About_ACOG/ACOG_Departments/Patient_Safety_and_Quality_Improvement/~/media/Departments/Patient%20Safety%20and%20Quality%20Improvement/201213IssuesandRationale-GestationalAgeTerm.pdf Obstetric Data Definitions Issues and Rationale for Change - Gestational Age & Term] {{webarchive|url=https://web.archive.org/web/20131106111500/http://www.acog.org/About_ACOG/ACOG_Departments/Patient_Safety_and_Quality_Improvement/~/media/Departments/Patient%20Safety%20and%20Quality%20Improvement/201213IssuesandRationale-GestationalAgeTerm.pdf |date=2013-11-06 }} from Patient Safety and Quality Improvement at [[American Congress of Obstetricians and Gynecologists]]. Created November 2012.</ref>
* Directly calculating the days since the beginning of the last menstrual period
* Early [[obstetric ultrasound]], comparing the size of an [[human embryo|embryo]] or [[fetus]] to that of a [[reference group]] of pregnancies of known gestational age (such as calculated from last menstrual periods) and using the mean gestational age of other embryos or fetuses of the same size. If the gestational age as calculated from an early ultrasound is contradictory to the one calculated directly from the last menstrual period, it is still the one from the early ultrasound that is used for the rest of the pregnancy.<ref name=acog2012/>
* In case of [[in vitro fertilization]], calculating days since [[oocyte retrieval]] or [[co-incubation]] and adding 14 days.<ref>{{Cite journal | last1 = Tunon | first1 = K. | last2 = Eik-Nes | first2 = S. H. | last3 = Grøttum | first3 = P. | last4 = Von Düring | first4 = V. | last5 = Kahn | first5 = J. A. | title = Gestational age in pregnancies conceived after in vitro fertilization: A comparison between age assessed from oocyte retrieval, crown-rump length and biparietal diameter | doi = 10.1046/j.1469-0705.2000.00004.x | journal = Ultrasound in Obstetrics and Gynecology | volume = 15 | issue = 1 | pages = 41–46 | year = 2000 | pmid = 10776011 | s2cid = 20029116 | doi-access = free }}</ref>

Gestational age can also be estimated by calculating days from [[ovulation]] if it was estimated from related signs or [[ovulation test]]s, and adding 14 days by convention.<ref>{{Cite journal | last1 = Robinson | first1 = H. P. | last2 = Fleming | first2 = J. E. E. | doi = 10.1111/j.1471-0528.1975.tb00710.x | title = A Critical Evaluation of Sonar "crown-Rump Length" Measurements | journal = BJOG: An International Journal of Obstetrics and Gynaecology | volume = 82 | issue = 9 | pages = 702–10 | year = 1975 | pmid =  1182090| s2cid = 31663686 }}</ref>

A more complete listing of methods is given in following table:<ref name=Hunter2010>[http://www.medscape.com/viewarticle/703501_4 A Simple Solution to Dating Discrepancies: The Rule of Eights] {{Cite journal | last1 = Hunter | first1 = L. A. | title = Issues in Pregnancy Dating: Revisiting the Evidence | doi = 10.1016/j.jmwh.2008.11.003 | journal = Journal of Midwifery & Women's Health | volume = 54 | issue = 3 | pages = 184–190 | year = 2009 | pmid =  19410210}}</ref>
{|class="wikitable"
! Method of estimating gestational age !! [[Statistical dispersion|Variability]] (2 [[standard deviation]]s)<ref name=Hunter2010/>
|-
| Days from [[oocyte retrieval]] or [[co-incubation]] in [[in vitro fertilisation]] + 14 days || ±1 day
|-
| Days from estimated ovulation in [[ovulation induction]] + 14 days || ±3 days
|-
| Days from [[artificial insemination]] + 14 days || ±3 days
|-
| Days from known single [[sexual intercourse]] + 14 days || ±3 days
|-
| Days from estimated ovulation by basal body temperature record + 14 days || ±4 days
|-
| First-trimester physical examination || ±2 weeks
|-
| Second-trimester physical examination || ±4 weeks
|-
| Third-trimester physical examination || ±6 weeks
|-
| First-trimester [[obstetric ultrasonography]] ([[crown-rump length]]) || ±8% of the estimate
|-
| Second-trimester obstetric ultrasonography (head circumference, [[femur]] length) || ±8% of the estimate
|-
| Third-trimester obstetric ultrasonography (head circumference, femur length) || ±8% of the estimate
|}

As a general rule, the official gestational age should be based on the actual beginning of the last menstrual period, unless any of the above methods gives an estimated date that differs more than the variability for the method, in which case the difference cannot probably be explained by that variability alone.<ref name=Hunter2010/> For example, if there is a gestational age based on the beginning of the last menstrual period of 9.0 weeks, and a first-trimester obstetric ultrasonography gives an estimated gestational age of 10.0 weeks (with a 2 [[standard deviation|SD]] variability of ±8% of the estimate, thereby giving a variability of ±0.8 weeks), the difference of 1.0 weeks between the tests is larger than the 2 SD variability of the ultrasonography estimate, indicating that the gestational age estimated by ultrasonography should be used as the official gestational age.<ref name=Hunter2010/>

Once the estimated due date (EDD) is established, it should rarely be changed, as the determination of gestational age is most accurate earlier in the pregnancy.<ref name=pmid25244460>{{cite journal |doi=10.1097/01.AOG.0000454932.15177.be |pmid=25244460 |title=Committee Opinion No 611 |journal=Obstetrics & Gynecology |volume=124 |issue=4 |pages=863–866 |year=2014 }}</ref>

Assessment of gestational age can be made based on selected head and trunk parameters.<ref>{{Cite book |last1=Kędzia |first1=Alicja |title=The computer-aided scientific research |last2=Woźniak |first2=Jowita |last3=Ziajkiewicz |first3=Marcin |last4=Dudek |first4=Krzysztof |last5=Derkowski |first5=Wojciech |date=2009 |publisher=Wrocław Scientific Society |isbn=978-83-7374-060-0 |editor-first= |series= |volume=16 |location= |publication-date=2009 |pages=241–246 |chapter=Foetal age assessment on the basis of selected parameters of head and trunk}}</ref> Following are diagrams for estimating gestational age from [[obstetric ultrasound]], by various target parameters:
<gallery>
File:Gestational sac diameter by gestational age.png|By [[gestational sac]] diameter
File:Crown-rump length by gestational age.png |By [[crown-rump length]] (CRL)
File:Biparietal diameter by gestational age.png|By [[biparietal diameter]] (BPD)
</gallery>

==Comparison to fertilization age==
The [[Human fertilization#Fertilization age|fertilization or conceptional age]] (also called ''embryonic age'' and later ''fetal age'') is the time from the [[human fertilization|fertilization]]. It usually occurs within a day of [[ovulation]], which, in turn, occurs on average 14.6 days after the beginning of the preceding menstruation (LMP).<ref name=Geirsson1991>{{cite journal |author=Geirsson RT |title=Ultrasound instead of last menstrual period as the basis of gestational age assignment |journal=Ultrasound Obstet Gynecol |volume=1 |issue=3 |pages=212–9 |date=May 1991 |pmid=12797075 |doi=10.1046/j.1469-0705.1991.01030212.x |s2cid=29063110 }}</ref> There is also considerable variability in this interval, with a 95% [[prediction interval]] of the ovulation of 9 to 20 days after menstruation even for an average woman who has a mean LMP-to-ovulation time of 14.6.<ref name=fehring2006>Derived from a [[standard deviation]] in this interval of 2.6, as given in: {{cite journal |vauthors=Fehring RJ, Schneider M, Raviele K |title=Variability in the phases of the menstrual cycle |journal=J Obstet Gynecol Neonatal Nurs |volume=35 |issue=3 |pages=376–84 |year=2006 |pmid=16700687 |doi=10.1111/j.1552-6909.2006.00051.x |s2cid=30317703 |url=https://epublications.marquette.edu/cgi/viewcontent.cgi?article=1010&context=nursing_fac}}</ref> In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days.<ref name=Geirsson1991/> The actual variability between gestational age as estimated from the beginning of the last menstrual period (without the use of any additional method mentioned in previous section) is substantially larger because of uncertainty which menstrual cycle gave rise to the pregnancy. For example, the menstruation may be scarce enough to give the false appearance that an earlier menstruation gave rise to the pregnancy, potentially giving an estimated gestational age that is approximately one month too large. Also, vaginal bleeding occurs during 15–25% of first trimester [[Pregnancy|pregnancies]],<ref name=Preg09>{{cite journal|last=Snell|first=BJ|title=Assessment and management of bleeding in the first trimester of pregnancy.|journal=Journal of Midwifery & Women's Health|date=Nov–Dec 2009|volume=54|issue=6|pages=483–91|pmid=19879521|doi=10.1016/j.jmwh.2009.08.007}}</ref> and may be mistaken as menstruation, potentially giving an estimated gestational age that is too low.

==Uses==
Gestational age is used for example for:{{citation needed|date=June 2020}}
* The events of [[prenatal development]], which usually occur at specific gestational ages. Hence, the gestational timing of a [[Environmental toxicants and fetal development|fetal toxin exposure]], [[Drugs in pregnancy|fetal drug exposure]] or [[vertically transmitted infection]] can be used to predict the potential consequences to the fetus.
* [[Estimated date of delivery]]
* Scheduling [[prenatal care]]
* Estimation of [[fetal viability]]
* Calculating the results of various [[Prenatal diagnosis|prenatal tests]], (for example, in the [[triple test]]).
* Birth classification into for example preterm, term or postterm.
* Classification of infant deaths and stillbirths
* Postnatally (after birth) to estimate various risk factors

[[File:Pregnancy timeline.png|center|thumb|upright=3|Timeline of pregnancy by gestational age]]

===Estimation of due date===
{{Main|Estimated date of delivery}}
[[File:Distribution of gestational age at childbirth.jpg|thumb|Distribution of gestational age at childbirth among singleton live births, given both when gestational age is estimated by first trimester ultrasound and directly by last menstrual period<ref name=hoffman2008>{{cite journal|last1=Hoffman|first1=Caroline S.|last2=Messer|first2=Lynne C.|last3=Mendola|first3=Pauline|last4=Savitz|first4=David A.|last5=Herring|author5-link= Amy H. Herring |first5=Amy H.|last6=Hartmann|first6=Katherine E.|title=Comparison of gestational age at birth based on last menstrual period and ultrasound during the first trimester|journal=Paediatric and Perinatal Epidemiology|volume=22|issue=6|year=2008|pages=587–596|issn=0269-5022|doi=10.1111/j.1365-3016.2008.00965.x|pmid=19000297}}</ref>]]

The mean pregnancy length has been estimated to be 283.4 days of gestational age as timed from the first day of the [[last menstrual period]] and 280.6 days when retrospectively estimated by [[obstetric ultrasound]] measurement of the [[fetal biparietal diameter]] (BPD) in the second trimester.<ref name=pmid8590208>{{cite journal |doi=10.1046/j.1469-0705.1995.06050353.x |pmid=8590208 |title=The length of human pregnancy as calculated by ultrasonographic measurement of the fetal biparietal diameter |journal=Ultrasound in Obstetrics and Gynecology |volume=6 |issue=5 |pages=353–7 |year=1995 |last1=Kieler |first1=H |last2=Axelsson |first2=O |last3=Nilsson |first3=S |last4=Waldenströ |first4=U |s2cid=39447672 |doi-access=free }}</ref> Other algorithms take into account other variables, such as whether this is the first or subsequent child, the mother's race, age, length of menstrual cycle, and menstrual regularity.  In order to have a standard reference point, the normal pregnancy duration is assumed by medical professionals to be 280 days (or 40 weeks) of gestational age. Furthermore, actual childbirth has only a certain probability of occurring within the limits of the estimated due date. A study of singleton live births determined that childbirth has a [[standard deviation]] of 14 days when gestational age is estimated by first-trimester [[obstetric ultrasonography|ultrasound]] and 16 days when estimated directly by last menstrual period.<ref name=hoffman2008/>

The most common system used among healthcare professionals is [[Naegele's rule]], which estimates the expected date of delivery (EDD) by adding a year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period (LMP) or corresponding date as estimated from other means.

===Medical fetal viability===
There is no sharp limit of development, gestational age, or weight at which a human fetus automatically becomes viable.<ref name=developinghuman>Moore, Keith and Persaud, T. [https://books.google.com/books?id=dbRpAAAAMAAJ&q=%22Prematurity+is+one+of+the+most+common+causes+of+morbidity%22 ''The Developing Human: Clinically Oriented Embryology''], p. 103 (Saunders 2003).</ref> According to studies between 2003 and 2005, 20 to 35 percent of babies born at 23 [[Gestational age (obstetrics)|weeks of gestation]] survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive.<ref name="MoD">[http://www.marchofdimes.org/loss/neonatal-death.aspx March of Dimes --> Neonatal Death] {{Webarchive|url=https://web.archive.org/web/20141024114718/http://www.marchofdimes.org/loss/neonatal-death.aspx |date=2014-10-24 }} Retrieved on November 10, 2014. In turn citing:
* {{cite journal |vauthors=Tyson JE, Parikh NA, Langer J, Green C, Higgins RD |title=Intensive care for extreme prematurity--moving beyond gestational age |journal=N. Engl. J. Med. |volume=358 |issue=16 |pages=1672–81 |date=April 2008 |pmid=18420500 |pmc=2597069 |doi=10.1056/NEJMoa073059 }}
* {{cite journal |vauthors=Luke B, Brown MB |title=The changing risk of infant mortality by gestation, plurality, and race: 1989-1991 versus 1999-2001 |journal=Pediatrics |volume=118 |issue=6 |pages=2488–97 |date=December 2006 |pmid=17142535 |pmc=3623686 |doi=10.1542/peds.2006-1824 }}
* {{cite journal |author=The American College of Obstetricians and Gynecologists |title=ACOG Practice Bulletin: Clinical Management Guidelines for Obstetrcian-Gynecologists: Number 38, September 2002. Perinatal care at the threshold of viability |journal=Obstet Gynecol |volume=100 |issue=3 |pages=617–24 |date=September 2002 |pmid=12220792 |doi= 10.1016/S0029-7844(02)02260-3}}</ref> It is rare for a baby weighing less than 500&nbsp;g (17.6&nbsp;ounces) to survive.<ref name=developinghuman/> A baby's chances for survival increases 3–4% per day between 23 and 24 weeks of gestation and about 2–3% per day between 24 and 26 weeks of gestation. After 26 weeks the rate of survival increases at a much slower rate because survival is high already.<ref name="spensershope.org">(). What are the chances that my baby will survive?. [ONLINE] Available at: http://www.spensershope.org/chances_for_survival.htm {{Webarchive|url=https://web.archive.org/web/20180809230110/http://www.spensershope.org/chances_for_survival.htm |date=2018-08-09 }}. [Last Accessed 14 November 2012].</ref> Prognosis depends also on medical protocols on whether to resuscitate and aggressively treat a very premature newborn, or whether to provide only [[palliative care]], in view of the high risk of severe disability of very preterm babies.<ref name=pmid15032380>{{cite journal |doi=10.1177/088307380401900106011 |pmid=15032380 |title=Guidelines for Resuscitation in the Delivery Room of Extremely Preterm Infants |journal=Journal of Child Neurology |volume=19 |issue=1 |pages=31–4 |year=2016 |last1=Verlato |first1=Giovanna |last2=Gobber |first2=Daniela |last3=Drago |first3=Donatella |last4=Chiandetti |first4=Lino |last5=Drigo |first5=Paola |author6=Working Group of Intensive Care in the Delivery Room of Extremely Premature Newborns |s2cid=20200767 }}</ref>

[[File:Prenatal development table.svg|thumb|center|upright=3|Stages in [[prenatal development]], showing ''viability'' and point of 50% chance of survival (''limit of viability'') at bottom. Weeks and months numbered by gestation.]]

{| class="wikitable"
|-
! Completed weeks of gestation at birth
|| 21 and less || 22 || 23 || 24 || 25 || 26 || 27 || 30 || 34
|-
! Chance of survival<ref name="spensershope.org"/>
|| <1%<ref>{{Cite web |title=World's most premature baby defies sub-1% survival odds to break record |url=https://guinnessworldrecords.com/news/2021/11/worlds-most-premature-baby-defies-sub-1-survival-odds-to-break-record-681851 |access-date=2022-03-15 |website=[[Guinness World Records]]|date=10 November 2021 }}</ref>|| 0–10% || 10–35% || 40–70% || 50–80% || 80–90% || >90% || >95% || >98%
|}

===Birth classification===
Using gestational age, births can be classified into broad categories:

{| class="wikitable" align="right"
|-
! Gestational Age in Weeks !! Classification
|-
| < 37 0/7 || Preterm
|-
| 34 0/7 - 36 6/7 || Late preterm<ref>[http://www.acog.org/Resources_And_Publications/Committee_Opinions/Committee_on_Obstetric_Practice/Late-Preterm_Infants Late-Preterm Infants] {{webarchive|url=https://web.archive.org/web/20120502070318/http://www.acog.org/Resources_And_Publications/Committee_Opinions/Committee_on_Obstetric_Practice/Late-Preterm_Infants |date=2012-05-02 }} ACOG Committee Opinion 404</ref>
|-
| 37 0/7 - 38 6/7 || Early Term<ref name=acog2013>[http://www.acog.org/About_ACOG/News_Room/News_Releases/2013/Ob-Gyns_Redefine_Meaning_of_Term_Pregnancy Ob-Gyns Redefine Meaning of "Term Pregnancy"] {{Webarchive|url=https://web.archive.org/web/20170503020915/http://www.acog.org/About_ACOG/News_Room/News_Releases/2013/Ob-Gyns_Redefine_Meaning_of_Term_Pregnancy |date=2017-05-03 }}, from [[American College of Obstetricians and Gynecologists]]. October 22, 2013</ref>
|-
| 39 0/7 - 40 6/7 || Full Term<ref name=acog2013/>
|-
| 41 0/7 - 41 6/7 || Late Term<ref name=acog2013/>
|-
| > 42 0/7 || Postterm
|}

Using the LMP (last menstrual period) method, a full-term human pregnancy is considered to be 40 weeks (280 days), though pregnancy lengths between 38 and 42 weeks are considered normal. A fetus born prior to the 37th week of gestation is considered to be ''preterm''.  A preterm baby is likely to be [[premature birth|premature]] and consequently faces increased risk of [[morbidity]] and [[death|mortality]]. An estimated due date is given by [[Naegele's rule]].

According to the WHO, a preterm birth is defined as "babies born alive before 37 weeks of pregnancy are completed."<ref name="WHOfs363">{{cite web | url=https://www.who.int/mediacentre/factsheets/fs363/en/ | title=Preterm birth }}</ref> According to this classification, there are three sub-categories of preterm birth, based on gestational age: extremely preterm (fewer than 28 weeks), very preterm (28 to 32 weeks), moderate to [[Late preterm infant|late preterm]] (32 to 37 weeks).<ref name=WHOfs363/> Various jurisdictions may use different classifications.

===In classifying perinatal deaths, stillbirths  and infant deaths===
For most of the 20th century, official definitions of a [[Live birth (human)|live birth]] and [[infant mortality|infant death]] in the [[Soviet Union]] and [[Russia]] differed from common international standards, such as those established by the [[World Health Organization]] in the latter part of the century.<ref>{{cite journal |doi=10.2307/1973432 |jstor=1973432 |title=Infant Mortality in the Soviet Union: Regional Differences and Measurement Issues |journal=Population and Development Review |volume=12 |issue=4 |pages=705–38 |year=1986 |last1=Anderson |first1=Barbara A |last2=Silver |first2=Brian D }}</ref><ref>{{cite journal |last1=Anderson |first1=Barbara A. |first2=Brian D. |last2=Silver |title=The Geodemography of Infant Mortality in the Soviet Union, 1950-1990 |journal=PSC Research Report No. 94-316 |pages=8 |year=1994 |url=https://www.psc.isr.umich.edu/pubs/abs/958 }}</ref> Babies who were fewer than 28 weeks of gestational age, or weighed fewer than 1000&nbsp;grams, or fewer than 35&nbsp;cm in length&nbsp;– even if they showed some sign of life (breathing, heartbeat, voluntary muscle movement)&nbsp;– were classified as "live fetuses" rather than "live births." Only if such newborns survived seven days (168 hours) were they then classified as live births. If, however, they died within that interval, they were classified as stillbirths.  If they survived that interval but died within the first 365 days they were classified as infant deaths.

More recently, thresholds for "[[fetal death]]" continue to vary widely internationally, sometimes incorporating weight as well as gestational age. The gestational age for statistical recording of fetal deaths ranges from 16 weeks in Norway, to 20 weeks in the US and Australia, 24 weeks in the UK, and 26 weeks in Italy and Spain.<ref name=pmid23700489>{{cite journal |doi=10.1371/journal.pone.0064869 |pmid=23700489 |pmc=3658983 |title=International Comparisons of Fetal and Neonatal Mortality Rates in High-Income Countries: Should Exclusion Thresholds Be Based on Birth Weight or Gestational Age? |journal=PLOS ONE |volume=8 |issue=5 |pages=e64869 |year=2013 |last1=Mohangoo |first1=Ashna D |last2=Blondel |first2=Béatrice |last3=Gissler |first3=Mika |last4=Velebil |first4=Petr |last5=MacFarlane |first5=Alison |last6=Zeitlin |first6=Jennifer |bibcode=2013PLoSO...864869M |doi-access=free }}</ref><ref name=AIHW-2012>{{cite web|last=Li|first=Z|title=Australia's Mothers and Babies 2010|url=http://www.aihw.gov.au/WorkArea/DownloadAsset.aspx?id=60129542372|work=Perinatal statistics series no. 27. Cat. no. PER 57|publisher=Australian Institute of Health and Welfare National Perinatal Statistics Unit, Australian Government|access-date=4 July 2013|author2=Zeki, R |author3=Hilder, L |author4= Sullivan, EA |year=2012}}</ref><ref name=RCOG-late-abortion>{{cite web|author1=Royal College of Obstetricians|author2=Gynaecologists UK|title=Further Issues Relating to Late Abortion, Fetal Viability and Registration of Births and Deaths|url=http://www.rcog.org.uk/womens-health/clinical-guidance/further-issues-relating-late-abortion-fetal-viability-and-registrati|publisher=Royal College of Obstetricians and Gynaecologists UK|access-date=4 July 2013|date=April 2001|url-status=dead|archive-url=https://web.archive.org/web/20131105042348/http://www.rcog.org.uk/womens-health/clinical-guidance/further-issues-relating-late-abortion-fetal-viability-and-registrati|archive-date=5 November 2013}}</ref>

The WHO defines the [[perinatal period]] as "The perinatal period commences at 22 completed weeks (154 days) of gestation and ends seven completed days after birth."<ref>[https://web.archive.org/web/20131203064838/http://www.who.int/maternal_child_adolescent/topics/maternal/maternal_perinatal/en/]{{full citation needed|date=September 2018}}</ref> Perinatal mortality is the death of fetuses or neonates during the perinatal period. A 2013 study found that "While only a small proportion of births occur before 24 completed weeks of gestation (about 1 per 1000), survival is rare and most of them are either fetal deaths or live births followed by a neonatal death."<ref name=pmid23700489/>

===Postnatal use===
Gestational age (as well as ''fertilization age'') is sometimes used postnatally (after birth) to estimate various risk factors. For example, it is a better predictor than postnatal age for risk of [[intraventricular hemorrhage]] in [[Preterm birth|premature babies]] treated with [[extracorporeal membrane oxygenation]].<ref>{{cite journal |doi=10.1016/j.jpeds.2004.07.010 |title=Post-conceptional age and IVH in ECMO patients |journal=The Journal of Pediatrics |volume=145 |issue=2 |pages=A2 |year=2004 |last1=Jobe |first1=Alan H }}</ref>

==Factors affecting pregnancy length==
Child's gestational age at birth (pregnancy length) is associated with various likely causal maternal non-genetic factors: stress during pregnancy,<ref>{{Cite journal|last1=Dole|first1=N.|last2=Savitz|first2=D. A.|last3=Hertz-Picciotto|first3=I.|last4=Siega-Riz|first4=A. M.|last5=McMahon|first5=M. J.|last6=Buekens|first6=P.|date=2003-01-01|title=Maternal stress and preterm birth|journal=American Journal of Epidemiology|volume=157|issue=1|pages=14–24|issn=0002-9262|pmid=12505886|doi=10.1093/aje/kwf176|doi-access=free}}</ref> age, parity, smoking, infection and inflammation, [[Body mass index|BMI]]. Also, preexisting maternal medical conditions with genetic component, e.g., [[diabetes mellitus type 1|diabetes mellitus type&nbsp;1]], [[systemic lupus erythematosus]], [[Anemia|anaemia]]. Parental ancestral background (race) also plays a role in pregnancy duration. Gestational age at birth is on average shortened by various pregnancy aspects: twin pregnancy, [[Prelabor rupture of membranes|prelabor rupture of (fetal) membranes]], [[pre-eclampsia]], [[eclampsia]], [[intrauterine growth restriction]].<ref>{{Cite journal|last1=Goldenberg|first1=Robert L.|last2=Culhane|first2=Jennifer F.|last3=Iams|first3=Jay D.|last4=Romero|first4=Roberto|date=2008-01-05|title=Epidemiology and causes of preterm birth|journal=Lancet|volume=371|issue=9606|pages=75–84|doi=10.1016/S0140-6736(08)60074-4|issn=1474-547X|pmid=18177778|pmc=7134569}}</ref> The ratio between fetal growth rate and uterine size (reflecting uterine distension) is suspected to partially determine the pregnancy length.<ref>{{Cite journal|last1=Bacelis|first1=Jonas|last2=Juodakis|first2=Julius|last3=Adams Waldorf|first3=Kristina M.|last4=Sengpiel|first4=Verena|last5=Muglia|first5=Louis J.|last6=Zhang|first6=Ge|last7=Jacobsson|first7=Bo|date=2018-10-01|title=Uterine distention as a factor in birth timing: retrospective nationwide cohort study in Sweden|url= |journal=BMJ Open|language=en|volume=8|issue=10|pages=e022929|doi=10.1136/bmjopen-2018-022929|issn=2044-6055|pmid=30385442|pmc=6252709}}</ref>

==Heritability of pregnancy length==
Family-based studies showed that gestational age at birth is partially (25–40%) determined by genetic factors.<ref name=pmid10740335>{{cite journal |doi=10.1111/j.1471-0528.2000.tb13234.x |pmid=10740335 |title=Genetic influence on birthweight and gestational length determined by studies in offspring of twins |journal=BJOG |volume=107 |issue=3 |pages=375–81 |year=2000 |last1=Clausson |first1=Britt |last2=Lichtenstein |first2=Paul |last3=Cnattingius |first3=Sven |s2cid=43470321 |doi-access=}}</ref>

==See also==
* [[Pregnancy]]
* [[Maternity]]
* [[Prenatal development]]
* [[Pregnancy (mammals)#Gestation periods|Gestation periods in mammals]]
* [[Abortion law]]
* [[Reproductive rights]]
* [[Fetal rights]]

==References==
{{reflist|2}}

{{Pregnancy}}
{{Human development}}
{{Infants and their care}}

[[Category:Obstetrics]]
[[Category:Neonatology]]
[[Category:Demography]]
[[Category:Midwifery]]