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Overview of Multiple Pregnancy

What is multiple pregnancy?

Multiple pregnancy is a pregnancy with 2 or more fetuses. Names for these include the following:

  • Twins are 2 fetuses

  • Triplets are 3 fetuses

  • Quadruplets are 4 fetuses

  • Quintuplets are 5 fetuses

  • Sextuplets are 6 fetuses

  • Septuplets are 7 fetuses

While multiples account for only a small percentage of all births (about 3%), 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 currently 32.6 per 1,000 live births. The birth rate for triplets and other higher-order multiples rose dramatically, but has slowed since 1998.

What causes multiple pregnancy?

There are many factors related to having a multiple pregnancy. Naturally occurring factors include the following:

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

  • Older age. Women over 30 have a greater chance of multiple conception. Many women today are delaying childbearing until later in life. They may have twins as a result.

  • High parity. Having 1 or more previous pregnancies, especially a multiple pregnancy, increases 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 over age 35, have the highest rate of higher-order multiple births (triplets or more).

Other factors that have greatly increased the multiple birth rate in recent years include reproductive technologies, including the following:

  • Ovulation-stimulating medicines, like clomiphene citrate and follicle stimulating hormone (FSH) 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 conceive. These technologies often use ovulation-stimulating medicines to produce multiple eggs. These are then fertilized in the lab and returned to the uterus to develop.

How does multiple pregnancy happen?

Multiple pregnancy usually 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. However, just as siblings often look alike, fraternal multiples may look very similar. Fraternal multiples each have a separate placenta and amniotic sac.

Illustration of a twin birth, head down/head down
Click Image to Enlarge

Sometimes, 1 egg is fertilized and then divides into 2 or more embryos. This is called identical twinning. It produces all boys, or all girls. Identical multiples are genetically identical. They usually look so much alike that even parents have a hard time telling them apart. However, these children have different personalities and are distinct individuals. 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.

Illustration of a twin birth, head down/head down; 1 placenta, 2 cords
Click Image to Enlarge

Online Medical Reviewer: Burd, Irina, MD, PhD
Online Medical Reviewer: Kanipe, Jennifer, RN, BSN
Date Last Reviewed: 8/16/2015
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