Twins from one egg occur about once in every 250 births around the world. Twins from two eggs occur in 1 out of every 100 births in white women and in 1 out of 79 births in black women. In certain areas in Africa, twins occur once in every 20 births! The occurrence of twins in Asian populations is less common—about 1 in every 150 births. Hispanics also have a higher incidence of twin births.
Triplets are much less common; they occur only once in every 8,000 deliveries. I've been lucky to deliver two sets of triplets, but most doctors never deliver a set of triplets in their entire careers.
Fertility drugs and the growing number of women who are having babies at an older age are two major reasons for the increased incidence of twins. Fertility drugs can stimulate the ovaries to release more than one egg, increasing the chance of a multiple pregnancy.
The incidence of twins is highest among women between 35 and 39. This increase is attributed to higher levels of gonadotropin, the hormone that stimulates the ovaries to develop and to release eggs. As a woman gets older, the level of gonadotropin increases, and she is more likely to produce two eggs during one menstrual cycle.
Conjoined twins, until recently also called Siamese twins, are twins whose bodies are connected. It is a serious complication because these babies may share important internal organs, such as the heart, lungs or liver. Sometimes they can be separated; often they cannot. The birth of conjoined twins is very rare. When it happens, it is usually necessary to deliver conjoined twins by Cesarean section.
Tendency Runs in Families
It's true that twin births can run in the family—on the mother's side. One study showed that if a woman was a twin, the chance of her giving birth to twins was about 1 in 58! If a woman is the daughter of a twin, she also has a higher chance of having twins. Another study reported that 1 out of 24 twins' mothers (4%) was also a twin, but only 1 out of 60 (1.7%) of the fathers was a twin.
More Common in Large Families
Research shows that the more pregnancies a woman has, the greater the chance of having twins. I know of one woman who had three single births, then twins, then triplets!
With in-vitro fertilization, multiple embryos are placed in the uterine cavity in hopes that at least one will implant and grow to maturity. Sometimes more than one embryo implants, resulting in a multiple birth.