Why Do Monoamniotic Twins Form?
No one is really sure why monoamniotic twins develop. Identical twins (also known as monozygotic twins) develop from a single fertilized egg that splits at some point after fertilization. Typically, this split occurs a few days after fertilization, just after the placenta forms but before the development of the amniotic sacs. These twins share a placenta but have individual amniotic sacs. However, sometimes the fertilized embryo does not split until after the formation of the amniotic sac. As a result, these identical twins share both a placenta and an amniotic sac. Recent research has shown a correlation between the number of yolk sacs present in pregnancy and the number of amniotic sacs twins will have. The yolk sac forms just after conception occurs and is thought to play a role in nourishing the developing embryo. With twins, there can be one yolk sac or two yolk sacs. It has been found that twins with two yolk sacs are much more likely to have two separate amniotic sacs. On the ot