monozygotic twins, two offspring born of the same pregnancy and developed from a single fertilized ovum that splits into equal halves during an early cleavage phase in embryonic development, giving rise to separate fetuses. Such twins are always of the same sex, have the same genetic constitution, possess identical blood groups, and closely resemble each other in physical, psychological, and mental characteristics. Monozygotic twins may have single or separate placentas and membranes, depending on the time during development when division occurs. Monozygotic twinning occurs with relatively uniform frequency in all races, is unaffected by heredity, and represents approximately one third of all twin births. Also called enzygotic twins, identical twins, true twins, uniovular twins. Compare dizygotic twins. See also Siamese twins.
