A multiple pregnancy is a pregnancy with 2 or more fetuses. Some names for these are:

• Twins for 2 fetuses

• Triplets for 3 fetuses

• Quadruplets for 4 fetuses

• Quintuplets for 5 fetuses

• Sextuplets for 6 fetuses

• Septuplets for 7 fetuses

Multiples make up only about 3 in 100 births, but the multiple birth rate is rising. According to the National Center for Health Statistics, the twin birth rate has risen 70% since 1980. It is now 32.6 per 1,000 live births. The birth rate for triplets and other higher-order multiples rose dramatically. But it has slowed since 1998.

Many factors are linked to having a multiple pregnancy. Naturally occurring factors are:

• Heredity. A family history of multiple pregnancy raises the chances of having twins.

• Older age. Women older than 30 have a greater chance of multiple conception. Many women today are waiting to have children until later in life. They may have twins as a result.

• High parity. Having 1 or more previous pregnancies, especially a multiple pregnancy, raises the chances of having multiples.

• Race. African-American women are more likely to have twins than any other race. Asian and Native Americans have the lowest twinning rates. White women, especially those older than 35, have the highest rate of higher-order multiple births (triplets or more).

Other factors that have greatly raised the multiple birth rate in recent years are reproductive technologies such as:

• Ovulation-stimulating medicines, such as clomiphene citrate and follicle stimulating hormone (FSH). These help produce many eggs. If fertilized, they can result in multiple babies.

• Assisted reproductive technologies. In vitro fertilization (IVF) and other methods may help couples get pregnant. These technologies often use ovulation-stimulating medicines to produce multiple eggs. These are then fertilized in the lab and returned to the uterus to grow.

Multiple pregnancy often happens when more than 1 egg is fertilized and implants in the uterus. This is called fraternal twinning. It can produce boys, girls, or a combination of both. Fraternal multiples are simply siblings conceived at the same time. But just as siblings often look alike, fraternal multiples may look very similar. Fraternal multiples each have a separate placenta and amniotic sac.

Sometimes, 1 egg is fertilized and then splits into 2 or more embryos. This is called identical twinning. It makes all boys, or all girls. Identical multiples are genetically the same. They often look so much alike that even parents have a hard time telling them apart. But these children have different personalities and are distinct people. Identical multiples may have individual placentas and amniotic sacs. Most share a placenta with separate sacs. Rarely, identical twins share 1 placenta and a single amniotic sac.