Editing Miscarriage (section)

====First trimester====
{| class="wikitable" style = "float: right; margin-left:15px; text-align:center"
|+ Chromosomal abnormalities found in first trimester miscarriages
! Description
! Proportion of total
|-
| [[XY sex-determination system|Normal]]
| 45–55%
|-
| [[Trisomy|Autosomal trisomy]]
| 22–32%
|-
| [[Turner syndrome|Monosomy X (45, X)]]
| 5–20%
|-
| [[Triploid syndrome|Triploidy]]
| 6–8%
|-
| [[Chromosome abnormality#Structural abnormalities|Structural abnormality]] of<br>the chromosome
| 2%
|-
| Double or triple trisomy
| 0.7–2.0%{{sfn|Hoffman|page=171}}
|-
|[[Chromosomal translocation|Translocation]]
| Unknown<ref>{{cite web |url=https://www.acog.org/-/media/For-Patients/faq100.pdf?dmc=1&ts=20150820T1255284207 |title=Archived copy |access-date=September 14, 2017 |archive-url=https://web.archive.org/web/20170914125252/https://www.acog.org/-/media/For-Patients/faq100.pdf?dmc=1&ts=20150820T1255284207 |archive-date=September 14, 2017 |url-status=dead }}</ref>
|}

Most clinically apparent miscarriages (two-thirds to three-quarters in various studies) occur during the first trimester.<ref name="John20122" /><ref name="NHS-Miscarriage-Causes"/><ref name="webmd">{{cite web | last = Rosenthal | first = M. Sara | title = The Second Trimester | website = The Gynecological Sourcebook | publisher = WebMD | year = 1999 | url = http://www.webmd.com/content/article/4/1680_51802.htm | access-date = December 18, 2006 | url-status = live | archive-url = https://web.archive.org/web/20061201085538/http://www.webmd.com/content/article/4/1680_51802.htm | archive-date = December 1, 2006 | df = mdy-all }}</ref><ref name=Will2013>{{cite book|title=Williams Obstetrics| vauthors = Cunningham F, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS|publisher=McGraw-Hill|year=2013|pages=5|chapter=Abortion}}</ref> About 30% to 40% of all fertilised eggs miscarry, often before the pregnancy is known.<ref name="John20122" /> The embryo typically dies before the pregnancy is expelled; bleeding into the [[decidua basalis]] and [[tissue necrosis]] cause uterine contractions to expel the pregnancy.<ref name=Will2013 /> Early miscarriages can be due to a developmental abnormality of the placenta or other embryonic tissues. In some instances, an embryo does not form but other tissues do. This has been called a "[[blighted ovum]]".<ref>{{cite news|url=http://americanpregnancy.org/pregnancy-complications/blighted-ovum/|title=Blighted Ovum: Symptoms, Causes and Prevention|date=2012-04-26|work=American Pregnancy Association|access-date=2017-09-09|language=en|url-status=live|archive-url=https://web.archive.org/web/20170725113127/http://americanpregnancy.org/pregnancy-complications/blighted-ovum/|archive-date=July 25, 2017|df=mdy-all}}</ref><ref>{{cite news|url=http://www.mayoclinic.org/diseases-conditions/pregnancy-loss-miscarriage/expert-answers/blighted-ovum/faq-20057783|title=Blighted ovum: What causes it?|work=Mayo Clinic|access-date=2017-09-09|url-status=live|archive-url=https://web.archive.org/web/20170720141145/http://www.mayoclinic.org/diseases-conditions/pregnancy-loss-miscarriage/expert-answers/blighted-ovum/faq-20057783|archive-date=July 20, 2017|df=mdy-all}}</ref>{{sfn|Hoffman|page=171}}

Successful implantation of the [[zygote]] into the [[uterus]] is most likely eight to ten days after fertilization. If the zygote has not been implanted by day ten, implantation becomes increasingly unlikely in subsequent days.<ref name="pmid10362823">{{cite journal | vauthors = Wilcox AJ, Baird DD, [[Clarice Weinberg|Weinberg CR]] | title = Time of implantation of the conceptus and loss of pregnancy | journal = The New England Journal of Medicine | volume = 340 | issue = 23 | pages = 1796–9 | date = June 1999 | pmid = 10362823 | doi = 10.1056/NEJM199906103402304 | df = mdy-all | doi-access = free }}</ref>

A chemical pregnancy is a pregnancy that was detected by testing but ends in miscarriage before or around the time of the next expected period.<ref>{{cite book|editor1-last=Condous |editor1-first=George |editor2=Tom Bourne |date=2006 |title=Handbook of early pregnancy care |url=https://books.google.com/books?id=_Y3PyNvc6dcC&pg=PA28 |url-status=live |archive-url=https://web.archive.org/web/20170910181311/https://books.google.com/books?id=_Y3PyNvc6dcC&pg=PA29 |archive-date=September 10, 2017 |location=London |publisher=Informa Healthcare |isbn=978-0203016213 |pages=28–29 |df=mdy-all}}</ref>

Chromosomal abnormalities are found in more than half of embryos miscarried in the first 13 weeks. Half of embryonic miscarriages (25% of all miscarriages) have an [[aneuploidy]] (abnormal number of chromosomes).<ref>{{cite journal | vauthors = Kajii T, Ferrier A, Niikawa N, Takahara H, Ohama K, Avirachan S | title = Anatomic and chromosomal anomalies in 639 spontaneous abortuses | journal = Human Genetics | volume = 55 | issue = 1 | pages = 87–98 | year = 1980 | pmid = 7450760 | doi = 10.1007/BF00329132 | s2cid = 2133855 }}</ref> Common chromosome abnormalities found in miscarriages include an [[Trisomy|autosomal trisomy]] (22–32%), [[Turner syndrome|monosomy X]] (5–20%), [[Triploid syndrome|triploidy]] (6–8%), [[tetraploidy]] (2–4%), or other structural chromosomal abnormalities (2%).<ref name=Will2013 /> Genetic problems are more likely to occur with older parents; this may account for the higher rates observed in older women.<ref>{{cite web | title = Pregnancy Over Age 30 | website=MUSC Children's Hospital | url = http://www.musckids.com/health_library/hrpregnant/over30.htm | access-date = December 18, 2006 |archive-url = https://web.archive.org/web/20061113233603/http://www.musckids.com/health_library/hrpregnant/over30.htm <!-- Bot retrieved archive --> |archive-date = November 13, 2006}}</ref>

[[Luteal phase]] progesterone deficiency may or may not be a contributing factor to miscarriage.<ref>{{cite journal | vauthors = Bukulmez O, Arici A | title = Luteal phase defect: myth or reality | journal = Obstetrics and Gynecology Clinics of North America | volume = 31 | issue = 4 | pages = 727–44, ix | date = December 2004 | pmid = 15550332 | doi = 10.1016/j.ogc.2004.08.007 }}</ref>