Multiple gestation does occur spontaneously, as mentioned above, but fertility techniques have certainly increased the rates. Whether multiple gestation is obtained spontaneously or through infertility treatment, twins can be identical or fraternal. Identical twins start as one embryo that splits into two. These are always the same sex but may have one or two placentas and one or two sacs. In the best situation identical twins each have their own placenta and sac.
Fraternal twins start as two eggs and two sperm and are no more alike than two brothers or sisters.
Fraternal twins each have their own placenta and sac, and may be two boys, two girls or one of each. The most serious complication of multiple gestation is preterm delivery. Full term delivery is from 37 weeks to 42 weeks. The placenta contains arteries and veins that transport the babies' blood back and forth to the placenta. With a shared placenta, the blood vessels can wander to the other twin's placental territory and will frequently hook up with the arteries or veins of the other twin.