(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.
Figure 2-11. Development of twin fetuses.
c. Pregnancies involving more than two fetuses (that is, triplets, quadruplets)
may occur by either situation.
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.