Editing Twin (section)

==Complications during pregnancy==

===Vanishing twins===
{{Main|Vanishing twin}}

Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other fetus has died very early in the pregnancy and has not been detected or recorded.<ref name="Keith-2">{{Cite book
|title=Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome
|first=Luigi
|last=Gedda
|editor=Louis G. Keith |editor2=Emile Papiernik |editor3=Donald M. Keith |editor4=Barbara Luke
|isbn=978-1-85070-666-3
|publisher=[[Taylor and Francis]]
|chapter=The role of research in twin medicine
|year=1995
|pages=[https://books.google.com/books?id=9SGiQsVevlwC 4]
}}</ref> Early [[obstetric ultrasonography]] exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes in the uterus. There are several reasons for the "vanishing" fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. This is known as [[vanishing twin]] syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single [[Chimera (genetics)|chimeric]] embryo, and, later, fetus.

===Conjoined twins===
{{Main|Conjoined twins}}
[[File:Chang-eng-bunker-PD.png|thumb|upright|[[Chang and Eng Bunker]], born in Siam (now [[Thailand]]) in 1811, were the origin of the term "Siamese twins".]]

Conjoined twins (or the once-commonly used term "siamese") are monozygotic twins whose bodies are joined during pregnancy. This occurs when the zygote starts to split after day 12<ref name="Shulman"/> following fertilization and fails to separate completely. This condition occurs in about 1 in 50,000 human pregnancies.
Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the [[brain]], [[heart]], [[liver]] or [[lung]]s.

===Chimerism===
{{Main|Chimera (genetics)}}
A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic [[mosaicism]] skin coloration in human chimeras. A chimera may be [[intersex]], composed of cells from a male twin and a female twin. In one case [[DNA test]]s determined that a woman, [[Lydia Fairchild]], mystifyingly, was not the mother of two of her three children; she was found to be a chimera, and the two children were conceived from eggs derived from cells of their mother's twin.<ref>{{cite web |last=Ainsworth |first=Chris |work=New scientist |title=The stranger within |date=15 November 2003 |url=https://www.newscientist.com/article/mg18024215.100-the-stranger-within.html?full=true |url-access=subscription |access-date=17 September 2017 |archive-date=20 June 2015 |archive-url=https://web.archive.org/web/20150620124749/http://www.newscientist.com/article/mg18024215.100-the-stranger-within.html?full=true |url-status=live}}</ref>

===Parasitic twins===
{{Main|Parasitic twin}}

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a [[parasite]] towards the other. Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be treated medically.

===Partial molar twins===
A very rare type of parasitic twinning is one where a single viable twin is endangered when the other zygote becomes cancerous, or "molar". This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either [[triploidy]] or complete paternal [[uniparental disomy]], resulting in little or no fetus and a cancerous, overgrown placenta, resembling a [[bunch of grapes]].

===Miscarried twin===
Occasionally, a woman will suffer a [[miscarriage]] early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the [[vanishing twin]] syndrome, but typically occurs later, as the twin is not reabsorbed.

===Low birth weight===
It is very common for twins to be born at a low [[birth weight]]. More than half of twins are born weighing less than {{convert|5.5|lb}}, while the average birth weight of a healthy baby should be around {{convert|6|-|8|lb|0}}.<ref>{{cite web|title = Risks Linked With Twin Births|url = https://www.webmd.com/baby/risks-linked-with-twin-births|website = WebMD|access-date = 2015-10-23|archive-date = 2019-04-05|archive-url = https://web.archive.org/web/20190405205039/https://www.webmd.com/baby/risks-linked-with-twin-births|url-status = live}}</ref> This is largely due to the fact that twins are typically born [[Premature labour|premature]]. Premature birth and low birth weights, especially when under {{convert|3.5|lb}}, can increase the risk of several health-related issues, such as vision and hearing loss, mental disabilities, and [[cerebral palsy]].<ref>{{cite web|title = Very Low Birthweight – Online Medical Encyclopedia – University of Rochester Medical Center|url = https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=90&ContentID=P02424|website = www.urmc.rochester.edu|access-date = 2015-10-23|archive-date = 2018-01-19|archive-url = https://web.archive.org/web/20180119120157/https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=90&ContentID=P02424|url-status = live}}</ref> There is an increased possibility of potential complications as the birth weight of the baby decreases.

===Twin-to-twin transfusion syndrome===
{{Main|Twin-to-twin transfusion syndrome}}
[[File:Tweeling transfusiesyndroom.jpg|thumb|Twin-to-twin transfusion syndrome (TTTS) illustration of twins showing one fetus with exposure to more amniotic fluid while the other is "stuck" with the membrane tightly around itself]]
Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the 'donor' twin, is small and [[anemic]], the other, the 'recipient' twin, is large and [[polycythemic]]. The lives of both twins are endangered by this condition.

===Stillbirths===
[[Stillbirth]]s occurs when a fetus dies after 20 weeks of gestation. There are two types of stillbirth, including intrauterine death and intrapartum death. Intrauterine death occurs when a baby dies during late pregnancy. Intrapartum death, which is more common, occurs when a baby dies while the mother is giving birth. The cause of stillbirth is often unknown, but the rate of babies who are stillborn is higher in twins and multiple births. Caesareans or inductions are advised after 38 weeks of pregnancy for twins, because the risk of stillbirth increases after this time.<ref>{{cite web|title = Pregnant with twins: potential complications|url = https://www.babycentre.co.uk/a3584/pregnant-with-twins-potential-complications|website = BabyCentre|access-date = 2015-10-26|archive-date = 2015-11-09|archive-url = https://web.archive.org/web/20151109123503/http://www.babycentre.co.uk/a3584/pregnant-with-twins-potential-complications|url-status = live}}</ref>

===Heterotopic pregnancy===
[[Heterotopic pregnancy]] is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an [[ectopic pregnancy]]. Ectopic pregnancies must be resolved because they can be life-threatening to the mother. However, in most cases, the intrauterine pregnancy can be salvaged.{{Citation needed|date=September 2015}}