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New CDC report: Twin birth rate soaring in U.S.

Robin Wulffson MD's picture
twins, pregnancy complications, cesarean section, prematurity, low birth weight

ATLANTA, GA - On January 4, the Centers for Disease Control and Prevention (CDC) reported that the number of twins born in the United States has soared over the last three decades. The reason: fertility treatment and women postponing childbearing until they are older. Both medications that increase ovulation such as clomiphene citrate (Clomid) and in vitro fertilization (IVF) are responsible for the upsurge. According to the Data Brief prepared by the CDC's National Center for Health Statistics, in 2009, 1 in every 30 births was a twin pregnancy; in comparison, the rate was 1 in every 53 births.

The announcement of a twin pregnancy often brings smiles to the parents as well as their friends and relatives. However, the diagnosis of twins places the pregnancy in a high risk category. A variety of obstetrical combinations are more common in a twin pregnancy; these include preeclampsia (toxemia), premature births, low birth weight infants, intrauterine death of one or both twins, and cerebral palsy. The cesarean section rate is also higher with twin gestations. More than half of twins born from 1980 to 2009 were low birth weight, or less than 5 1/2 pounds, according to the report. One in 10 was very low birth weight, or less than 3 pounds, 5 ounces. By way of comparison, only 1 in 100 single infants is very low birth weight.

According to Joyce Martin, MPH, a CDC epidemiologist who co-authored the report, some increase was expected as more women are delaying starting a family until they are over age 30. Women who conceive in their 30s are more likely to carry a twin pregnancy than younger or older women. The reason is unknown; however, before fertility treatments existed, about 2.5% of the babies born to women in their late 30s were twins, compared to less than 2% for younger and older women. Ms. Martin noted that as much as a third of the increase can be attributed to that group of women; however, the rest of the increase is due to fertility medications and IVF treatment.

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In recent years, fertility specialists have become aware of the downside of a multiple gestation. The risk soars with each increase over two. In the past, fertility specialists transferred multiple embryos in the hope that at least one would survive. However, as IVF technology improved, multiple gestations increased. Currently, many fertility specialists are transferring only a single embryo and report a high rate of a successful pregnancy outcome. This transfer of a single embryo protocol should result in a decreased twin rate during this decade.

According to the CDC report, the twin birth rate rose by more than 2% a year, on average, from 1980 through 2004. It then leveled off to less than 1% for several years; however, from 2008 to 2009 it increased to almost 2%. Although the increase occurred in all 50 states, the twin rate varied by region. The increases were highest in New Jersey, the lower New England states, and Hawaii. Furthermore, in Connecticut, twins currently account for almost 5% of births. Ms. Martin noted that the triplet birth rate has stabilized somewhat; however, it is still much higher than it was before fertility treatments became available.

Over the last three decades, twin rates increased for Caucasians, African Americans, and Hispanic women; however, the increases were not uniform. Rates doubled for Caucasians, rose by half as much for African Americans, and by about a third as much for Hispanics. Without fertility treatment, African Americans have a higher rate of twin pregnancies than Caucasians or Hispanics. The baseline twin rate is lower for Asians than Caucasians.

The greatest increase in twin rates was reported by the CDC for women 40 and older. These women are more likely to require fertility treatment. Approximately 7% of all births for women 40 and older were twins, compared to 5% of women in their late 30s and 2% of women age 24 or younger.

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