Identical twins occur when a single sperm fertilizes a single egg. The egg splits, and each half develops into a baby that is genetically identical to the other. Identical twins are always the same sex. Dependingon when the egg splits and implants, each fetus may have its own placenta and amniotic sac, or the two may share one or the other. Both sharing a placenta and having separate amniotic sacs and the opposite pose an increased risk.
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