Learn important facts and risk factors for twin pregnancy.

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pregnant woman
Credit: Greace Huang

The twin birth rate in the United States is about 33 in 1000, compared to about 19 in 1000 during the early 1980s. That's because there are growing numbers of older moms (hormonal changes are believed to be responsible for the release of more than one egg at ovulation) and more successful fertility treatments. Here are some other facts about twin pregnancies that you might not know, from Pregnancy Day by Day editor-in-chief Maggie Blott, M.B., B.S.; and consultant editor Paula Amato, M.D. (Dorling Kindersley):

  • Research shows that 10 percent to 15 percent of all singleton births may have started off as twins; often one is lost early in pregnancy in a phenomenon known as "vanishing twin syndrome."

  • There are two types of twins: fraternal and identical. Identical twins are less common than fraternal because identical twins require the splitting of a single fertilized egg, which is rare. Fraternal twins occur when two separate eggs are fertilized by two separate sperm and implant in the womb at the same time.

  • The odds of having identical twins are about 1 in 250.

  • If you've had nonidentical, or fraternal, twins (the result when two eggs are released at ovulation and both are fertilized) without having taken fertility drugs, your chance of having a second set is about 1 in 3,000.

  • More than half of twins are born before 37 weeks; plan your maternity leave accordingly so you don't leave your employer in the lurch.

  • The likelihood that you'll have twins is greater if there is a history of twins in your family. Fertility drugs also increase the incidence of twins because they often cause the release of more than one egg during ovulation.

  • Twins conceived naturally (i.e., without fertility treatment) are most common in the African-American population, followed by Caucasians, Hispanics and Asians. Taller women are also more likely to have twins.

  • For most women pregnant with twins, gaining 24 pounds by week 24 is advised; a total weight gain of 35-45 pounds is generally desirable.

  • When you're pregnant with twins, you'll need to eat at least 2,700 calories each day. You'll probably be prescribed extra vitamins and minerals to take. Women pregnant with twins are more susceptible to anemia because more iron is needed to support the healthy growth of two fetuses. Your doctor will also prescribe enough folic acid for the healthy development of your twins.

  • Your doctor will diagnose a twin pregnancy by listening with a stethoscope to determine if there is more than one heartbeat, or by using ultrasound.

  • The twin who is developing closest to the exit (your cervix) is called Baby A. In 75 percent of cases, Baby A is head down before delivery; Baby B may be either head down or breech.

  • Though vaginal delivery is usually an option, you're more likely to give birth via Cesarean section than if you were having one baby.

  • Sometimes Baby A is born vaginally, then Baby B is delivered by C-section. This is often the case if Baby B is breech or shows signs of distress because he's gone through two rounds of uterine contractions.

  • The average birth weight of full-term twins (37 weeks or later, compared to 39-40 weeks for singletons) is around 5 ½ pounds each, though one baby often weighs more than the other.

  • Because of the additional weight, twin pregnancies tend to be more uncomfortable. Get plenty of rest and put your feet up frequently to reduce swelling and leg pain.

  • Twin pregnancies have an increased risk of preterm labor, premature rupture of membranes, uneven growth, high blood pressure, preeclampsia, and diabetes. Your doctor will carefully monitor you for these conditions throughout your pregnancy.

  • Labor is sometimes longer than usual with twins, but the actual delivery is not much longer. Twins are usually born only minutes apart.

  • You can certainly breastfeed twins, but you'll need at least 500 additional calories each day to keep up with the demands of producing the necessary breast milk. If you've had twins or are expecting them, be sure to meet with a lactation consultant for special support and advice.

  • RELATED: Parenting Twins: Questions About Life with Multiples