Multiple pregnancy is a pregnancy where more than one fetus develops in the womb. Twins happen naturally about one in every 100 births. There are two types of twinning--identical and fraternal. Identical twins represent the splitting of a single fertilized zygote (union of a male sperm and a female egg that produce a developing fetus) into two separate individuals. They normally have identical genes. When they do not separate completely, the result is Siamese (or conjoined) twins.

Fraternal twins are three times more common than identical twins. They occur when two eggs are fertilized by separate sperm. Each has a different selection of its parents' genes. Fraternal twins and are genetically no more closely related than any siblings with the same parents.

The natural incidence of multiple pregnancy has been upset by advances in fertility treatments that may cause the simultaneous release of multiple eggs. The result...