How Many Is Too Many?

Of all triplet or higher births in 2000, 43 percent came from advanced reproductive technologies, primarily IVF. (In this process ripened eggs are removed from the woman's ovary, fertilized with semen and allowed to incubate in a laboratory dish. One or more developing embryos are then surgically transferred back into the woman's uterus.) Another 40 percent were likely the result of other fertility treatments such as ovulation-inducing drugs, and only 18 percent occurred naturally, according to a 2003 CDC report.

For a woman who gets pregnant the old-fashioned way, the odds of giving birth to twins are about 1 in 80, says Rockville perinatologist Thomas Pinckert, who specializes in multiple births. The odds of having triplets is 1 in 6,400.

For a woman who became pregnant by IVF in 2001, the odds of having twins were about 1 in 3, according to a study published in April in the New England Journal of Medicine. Her odds of having triplets or more were about 1 in 13. While that's a relatively high likelihood, it's down from 1 in 9 just four years earlier, thanks to concerted efforts by reproductive specialists.

The very high multiples that make the news -- six, seven and even eight babies at a time -- are virtually all the product of fertility drugs plus intrauterine insemination (a procedure in which sperm are injected directly into the uterine cavity, bypassing the cervix), says Michael Levy, a reproductive endocrinologist at Shady Grove Fertility in Rockville.

Applying the Brakes

Many experts attribute the recent downturn in triplets in large part to guidelines issued by SART in 1998. For IVF patients with above-average prognoses for successful IVF (women under 35 who were using fresh -- not frozen -- embryos), SART recommended transferring only three embryos per pregnancy attempt. Four embryos was the recommendation for women with average prognoses, and five for those with below-average chances of getting pregnant. A year later, SART amended the guidelines to recommend only two embryos be transferred in women with a "most favorable" prognosis -- those under 35, with enough good-quality embryos that there would be leftover ones to be frozen.

After its success at reducing the frequency of triplets, SART now wants to take aim at twins, said the group's president, Owen Davis, an associate professor of reproductive medicine at Cornell University's Center for Reproductive Medicine and Infertility in New York. Next month, SART will therefore revise its guidelines again. For most women under 35, SART will recommend that doctors transfer no more than two embryos and will suggest that a single embryo transfer be considered for those with the most favorable prognoses.

IntegraMed, the country's largest network of fertility specialists, announced in May that its Council of Physicians and Scientists has decided to develop a nationwide trial of elective single embryo transfer (eSET). The group expects hundreds of women undergoing IVF treatment to agree to have a single fertilized egg transferred, rather than two or more. What IntegraMed wants to learn is how the transfer of single embryos affects IVF outcome.

The basis for IntegraMed's plan is a recent Belgian study in which 367 women under age 38 with top-quality embryos underwent IVF treatment for the first time. Half had two embryos transferred, half had one. Both groups had a 37 percent birth rate, but 31 percent of the double-embryo-transfer pregnancies resulted in twins, while none of the single embryos transfers did.

Shady Grove Fertility, one of the IntegraMed clinics participating in the study, has collected some preliminary data of its own. In its practice to date, said Levy, it has found that with young healthy women with strong embryos, transferring two embryos has produced a 65 percent chance of pregnancy -- but that comes with a 50-50 chance of twins and a 1 to 2 percent chance of triplets (which results from one of the embryos splitting after being transferred to the mother's womb). With single transfer, the same woman, he said, was found to have a 40 percent chance of pregnancy, with only a 1 to 2 percent chance of twins.

One challenge: persuading clients to accept single implantation. Shady Grove has found, says Levy, that about 45 percent of couples would prefer twins, so they won't have to go through fertility procedures again. "We constantly have to reeducate these couples" about the risks associated with multiple births, he said. With a single baby, the chances of its being born healthy are 97 percent. With twins, that drops to 92 percent, according to Levy.