What do former first lady Laura Bush and entertainers Julia Roberts, Jennifer Lopez, Celine Dion and Mariah Carey have in common?
They have all contributed to the United States' soaring twin birthrate, which grew a stunning 76 percent in the past 30 years.
Twins now account for 1 in 30 infants, the National Center for Health Statistics (NCHS) said in a report released Wednesday. In contrast, only 1 in 53 babies were twins from 1915 to 1980.
For women older than 40, the number of twin deliveries for every 1,000 births in the country more than tripled since 1980.
There are two likely reasons for this surge, said Joyce A. Martin, a statistician and lead author of the NCHS report.
"About a third of the increase is the result of older age at childbearing - that is, more births to moms in their 30s in 2009 compared with 1980," Ms. Martin said.
Late childbearing affects the twin birthrates because women in their 30s historically have had higher "spontaneous" - i.e., without fertility therapies - rates of twins and multiples, compared to younger women, she explained.
The other two-thirds of the increase in twinning rates is the result of women's increased use of fertility therapies, such as drugs and assisted reproductive technology (ART), Ms. Martin said.
This is because fertility drugs that stimulate ovulation can result in multiple eggs being released for fertilization at the same time, raising the likelihood for twin pregnancies. The same result is possible with ART, because it is possible to transfer more than one embryo in a woman at a time.
Another new development is that women in their 40s now represent the age group most likely to have twins, Ms. Martin added.
Historically, twin birthrates peaked among women in the 35-39 age group and then declined, she said.
But by 2009, probably thanks to ART, 7 percent of births to women 40 and older were in a twin delivery. In comparison, 5 percent of births to women 35-39 and just 2 percent of births to women younger than age 25 were in a twin delivery.
The NCHS data referred only to live births in twin pregnancies and did not include babies born in triplet or other higher-order pregnancies.
The federal data also didn't distinguish between identical twins (those born from a single fertilized egg that split in two) or fraternal twins (those born from two eggs fertilized by two different sperm). Fraternal twins, however, are believed to be the most common, accounting for about two-thirds of twin sets.
"Multiple pregnancies" are automatically high-risk pregnancies and require extra monitoring and doctor visits due to concerns for miscarriage, gestational diabetes, preeclampsia, premature labor, cesarean section delivery and low birth weight.
For instance, of the estimated 865,000 additional twins born between 1980 and 2009, more than half had a low birth weight and 1 in 10 had a very low birth weight.
Will the U.S. twinning rates continue their relatively steep climb?
Probably not, said Dr. Michael Katz, senior advisor and interim medical director at the March of Dimes.
In the past, he said, it was not unusual for women to have multiple embryos transferred in the in-vitro fertilization process, both to ensure at least one pregnancy and because the IVF procedure was costly.
Now, in many countries, particularly in Europe, there is a legal limit on the number of embryos that can be transferred in ART - "usually two, or even one," Dr. Katz said.
The U.S. uses a voluntary system regarding embryo transfers, but even here, "There is an effort to implant fewer" embryos at a time because of recognition of the consequences of transferring too many, he said.
NCHS data also indicate a slowdown: From 1980 to 2004, the twin birthrate grew more than 2 percent a year. From 2005 to 2009, however, the rate rose by less than 1 percent annually.
Other highlights of the report, "Three Decades of Twin Births in the United States, 1980-2009," include:
• The twin birthrate rose from 18.9 per 1,000 births in 1980 to 33.2 per 1,000 in 2009.
• Twinning rates rose in all 50 states and the District, but doubled in Hawaii and four East Coast states
(Massachusetts, Rhode Island, Connecticut and New Jersey.)
• Twin birthrates rose in all racial and ethnic groups but doubled among white women.
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