a. Multi-fetal pregnancy is a pregnancy involving two or more fetuses.

b. Twin fetuses may originate several ways (see figure 2-11).

(1) Identical twins (monozygotic) originate from the same ovum and are always of the same sex. They share a single placenta.

(2) Fraternal twins (dizygotic) originates from two separate ova and sperm and may be of different sexes. They each have their own placenta.

c. Pregnancies involving more than two fetuses (that is, triplets, quadruplets) may occur by either situation.

(1) Monozygotic–all will be identical.

(2) Multi-zygotic–often associated with fertility drugs in which the ovary matured and released many eggs in the same cycle.