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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.