Editing Twin (section)

===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.