How do multiple pregnancies occur?

A multiple pregnancy occurs when one egg (ovum) splits before implanting or when separate eggs are each fertilized by a different sperm.

Identical twins or triplets occur with the fertilization of a single egg that later divides into two or three identical embryos. Identical twins or triplets have the same genetic identity, are always the same sex, and look almost exactly the same.

Fraternal multiples develop from separate eggs that are each fertilized by a different sperm. Fraternal twins might or might not be of the same sex and might not necessarily resemble each other any more than two siblings from the same parents might.

In a pregnancy with triplets or more, the babies can be all identical, all fraternal, or a mixture of both. This can happen when multiple eggs are released by the mother and fertilized. If one or more of these fertilized eggs divides into two or more embryos, a mixture of identical and fraternal multiples will occur.

What increases the chance of a multiple pregnancy?

The chance that a woman will have fraternal multiples is higher if a woman is older, taller, and heavier. In addition, twins are more likely if a woman is herself a twin, or if having twins runs in the maternal (mother’s) side of the family.

The use of fertility drugs increases a woman’s chance of having a multiple birth. Infertility procedures such as in vitro fertilization (IVF), gamete intrafallopian transfer (GIFT), and zygote intrafallopian transfer (ZIFT) also increase the chance of a multiple pregnancy. These procedures often involve transferring more than one fertilized egg into the mother's womb to increase the odds of a pregnancy.

How common are multiple births?

Multiple births have become more common in recent years because more couples are using fertility drugs and treatments such as IVF to help them conceive. In about 95 percent of multiple pregnancies, the mother is carrying twins.

What complications are linked to multiple births?

Most doctors consider multiple pregnancies higher risk than single pregnancies, but this does not mean that women who are pregnant with multiples automatically have problems. Some complications that can occur in multiple births include:

  • Premature labor and birth: The most common complication of multiple births is premature labor. Mothers carrying multiple babies go into premature labor (usually defined as before 37 weeks) more often than women carrying only one baby. Many women strive to reach 38 weeks, considered full-term in a twin pregnancy, to increase the odds that their babies will be born healthy and at a good weight.
  • Low birth weight due to preterm delivery
  • Pre-eclampsia, or gestational hypertension (high blood pressure): This complication occurs at twice the rate in women carrying multiples than in women pregnant with one baby. The condition also tends to develop earlier and be more severe in women carrying two or more babies.
  • Gestational diabetes: due to increased hormones from the placenta, as well as placental size. If there are two placentas, an increased resistance to insulin.
  • Twin-twin transfusion syndrome (TTTS) may develop due to identical twins sharing a placenta. Blood flow between the twins can become unbalanced.
  • Placenta abruption:This condition occurs when the placenta detaches (separates) from the uterine wall before delivery. Placenta abruption is also more common when a woman is carrying more than one baby.
  • Fetal growth restriction, also called small for gestational age (SGA) or intrauterine growth restriction (IUGR): This condition can occur when one or both twins is not growing at the proper rate. The condition might cause the babies to be born prematurely or at a low birth-weight. Up to nearly half of pregnancies with more than one baby have this problem, compared with slightly more than 10 percent of single pregnancies.

What can a woman do to stay healthy during a multiple pregnancy?

Eating nutritious foods, getting enough rest, and visiting the doctor regularly are important steps for any pregnant woman to take toward a healthy pregnancy. These steps are especially important during multiple pregnancies.

Women who are expecting multiple births have additional dietary needs to meet. Getting enough protein and enough hydration (fluids) is important, as is getting extra calories for the developing fetuses. One rule of thumb is to eat an extra 300 calories a day per baby that is expected. According to The American Congress of Obstetricians and Gynecologists (ACOG), women who are pregnant with multiples generally should take in around 2700 calories per day, but they should check with their health care providers for specific amounts.

Women who are expecting multiples can experience the typical discomforts of pregnancy more intensely. Good self-care and getting plenty of rest can help to ease the stress of pregnancy.

Finally, women expecting multiples should find health care professionals who have experience with multiple births. Specialized health care can help ensure that mother and babies receive the best care available. The need for frequent, intensive prenatal care is very important in a multiple pregnancy. A family doctor or obstetrician/gynecologist can recommend a facility that specializes in multiple births.

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