various types of chorionicity , amniosity (how baby s sac looks) in monozygotic (one egg/identical) twins result of when fertilized egg divides
the degree of separation of twins in utero depends on if , when split 2 zygotes. dizygotic twins 2 zygotes. monozygotic twins split 2 zygotes @ time in pregnancy. timing of separation determines chorionicity (the number of placentae) , amniocity (the number of sacs) of pregnancy. dichorionic twins either never divided (i.e.: dizygotic) or divided within first 4 days. monoamnionic twins divide after first week.
in rare cases, twins become conjoined twins. non-conjoined monozygotic twins form day 14 of embryonic development, when twinning occurs after 14 days, twins conjoined. furthermore, there can various degrees of shared environment of twins in womb, potentially leading pregnancy complications.
it common misconception 2 placentas means twins dizygotic. if monozygotic twins separate enough, arrangement of sacs , placentas in utero indistinguishable dizygotic twins.
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