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Topic Overview
What is a multiple pregnancy?
In the United States, about 31 per 1,000 births are multiple pregnancies, mostly bearing twins or triplets. In fact, about 29 per 1,000 American births produce twins.
Multiple fetuses can be identical or fraternal.
Identical (monozygotic) twins or triplets come from a single egg that has been fertilized by one sperm. For unknown reasons, the fertilized egg splits into 2 or more embryos during the first stage of development. Identical twins may share the same placenta. However, they are usually contained within separate amniotic sacs in the uterus. In rare cases, identical twins share one amniotic sac (monoamniotic twins).
Identical twins:
- Are always the same sex and blood type.
- Do not always look alike, but sometimes look like mirror images of each other. One child may be right-handed while the other is left-handed.
- Occur in about 3 to 4 out of 1,000 births worldwide. Race and heredity do not seem to affect the occurrence of identical twins.
- Fraternal (dizygotic) twins, triplets, or more develop when separate eggs are fertilized by separate sperm. Fraternal fetuses have separate placentas and amniotic sacs. Fraternal twin births have greatly increased since 1980, as infertility treatment has become more common. Multiple fetuses conceived with assisted reproductive technology are always fraternal.
Fraternal twins:
- Tend to run in families.
- Are born most frequently to women who are white or of African descent.
- Can be of different sexes and have different blood types.
- May look very different from one another, with different-colored hair and eyes; they may also look alike, as siblings tend to do.
What are common causes of multiple pregnancy?
Infertility treatment. Since 1980, infertility treatment has doubled the American twin birth rate. In that same time period, the number of pregnancies with triplets or more has quadrupled.3 You have an increased risk for having a multiple pregnancy if you use:
- Fertility drugs to stimulate your ovaries to release multiple eggs, which can be fertilized at once.
- Assisted reproductive technology (ART) to transfer multiple embryos directly into the uterus to increase your chances of pregnancy.4 Overall, 25% to 30% of pregnancies from ART are twins; 5% result are triplets; less than 1% are quadruplets or more.1 For more information, see the topic Infertility.
Natural causes. You have an increased risk for having a multiple pregnancy if you:
- Are in your thirties.5 Naturally occurring fraternal twins are most common among women aged 35 to 40.2
- Are black, non-Hispanic, particularly between age 35 and 45.5, 6
- Are white, non-Hispanic.
- Have already had a multiple pregnancy. Women who have carried fraternal twins have double the usual risk of having twins.
- Have a family history of fraternal twins or triplets. (A history of multiple pregnancy on your partner's side of the family does not increase your risk of multiple pregnancy.)
- Become pregnant in the first menstrual cycle after stopping birth control pills.
- Have been taking opiate drugs (such as morphine or heroin).
What are the risks of multiple pregnancy?
A multiple pregnancy increases your and your fetuses' risks of complications.
Each additional fetus increases your risk of miscarriage, premature birth, gestational diabetes, preeclampsia, placenta abruptio, placenta previa, urinary tract infections, anemia, cesarean delivery, and heavy postpartum bleeding.
Multiple fetuses have an increased risk of birth defects and genetic disorders. Because they are usually born prematurely, multiples also have a higher-than-usual rate of complications after birth. For more information, see the topic Premature Infant.
How can I tell that I have a multiple pregnancy? What exams and tests am I likely to have?
You are likely to have more early pregnancy symptoms with a multiple pregnancy than with a singleton pregnancy.
Your health professional can test your blood for higher levels of pregnancy hormones, a common sign of multiple pregnancy. A fetal ultrasound can confirm whether you have more than one fetus, and is used throughout a multiple pregnancy to monitor fetal well-being. In the late first to early second trimesters, you can choose to use chorionic villus sampling or amniocentesis to check for genetic disorders and birth defects.
How is a multiple pregnancy treated?
All multiple pregnancies are considered high-risk. If you are carrying more than one fetus, you will have more frequent medical checks than you would with a singleton pregnancy. In all likelihood, you will deliver your babies prematurely, so make sure you and your health professional review the signs of preterm labor and what you should do when you go into labor.
