Recent advances in infertility treatments have created many more multiple gestations than in the past when it was rare. Without the aid of infertility treatment, twins occur once in 83 deliveries, triplets occur in every 6,900 deliveries and quadruplets occur in every 575,000 deliveries. To give perspective, there are about four million deliveries each year in the United States, which would mean eight spontaneous quadruplet pregnancies and 580 triplets. 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.