Illustration of various types of uniovular gemellary pregnancies (true twins); these pregnancies result from a single egg and a single sperm, the fertilized egg divides. Depending on the date of division: Before 4 days: dichorionic pregnancy (2 placentas), diamniotic (two amniotic sacs). From 4 to 8 days: monochorionic pregnancy (1 placenta for 2 fetuses), diamniotic (two amniotic sacs). From 8 to 14 days: monochorionic pregnancy (1 placenta), monoamniotic (one amniotic sac). After 14 days: conjoined twins, 1 umbilical cord (Siamese).