The following describes the various developmental se-quences possible when the monozygotic conceptus sepa-rates into twins (also called chorionicity) [Fig. 17.1]:
· Diamnionic/Dichorionic: If division of the conceptusoccurs within 3 days of fertilization, each fetus will be surrounded by an amnion and chorion.
· Diamnionic/Monochorionic: If division occurs be-tween the 4th and 8th day following fertilization, the chorion has already begun to develop, whereas the amnion has not. Therefore, each fetus will later be sur-rounded by an amnion, but a single chorion will surround both twins.
· Monoamnionic/Monochorionic: In 1% of monozy-gotic gestations, division occurs between days 9 and 12, after development of both the amnion and the chorion, and the twins will share a common sac. Division there-after is incomplete, resulting in the development of con-joined twins. The fetuses may be fused in a number ofways, with the most common involving the chest and/or abdomen. This rare condition is seen in approximately 1 in 70,000 deliveries. This condition is associated with a mortality rate of up to 50%.