The researchers used information already collected by a large project paid
for by the government, the National Birth Defects Prevention study. Dr. Reefhuis
and her colleagues compared 9,584 women who had children with birth defects, and
4,792 control women whose children were born without defects. In the control
group, 1.1 percent of the women (51) had undergone fertility procedures; in
those whose children had birth defects, the figure was 2.4 percent (230). The
increased risk among those who had the procedures was not related to their age,
income, the number of children they had or to whether or not their babies were
born prematurely.

“Now is the time for all of us to rethink what is the paradigm of a
successful I.V.F. pregnancy,”
said Dr. Aaron K. Styer, a reproductive endocrinologist at the Massachusetts
General Hospital Fertility Center in Boston. “Is it a pregnancy without
regard to the number of gestations or a pregnancy with a singleton live birth?”

The number of I.V.F. cycles in which four or more embryos were transferred
has dropped sharply, to 21 percent in 2004 from 62 percent in 1996. Although the
efforts have substantially lowered the rates of triplets born through in vitro
fertilization, they have not made a dent in the twin rate. That is because many
doctors and patients are reluctant to take the final step to ensure a single
birth, a process called S.E.T., for single embryo transfer. From 1996 to 2004,
the rate of such procedures rose modestly, to 8 percent from 6 percent.

The American Society of Reproductive Medicine now recommends that women
younger than 35 with a good prognosis have just one embryo transferred. Women
under 35 make up 44 percent of I.V.F. cycles.

In women older than 37, who have a higher incidence of embryos with
chromosomal defects, three to five embryos are still recommended, depending on
the woman’s age.

Other researchers are looking at the traits of women at high risk of having
multiples. In research presented at the reproductive society’s annual meeting
last October, Dr. Stern linked a higher number of oocytes, or eggs retrieved
from ovaries, with higher rates of single and multiple pregnancies.