Two embryos aren’t twice as nice for IVF patients

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When couples with fertility problems turn to IVF, they often assume that they can double their chances for a healthy baby by transferring two embryos to the womb instead of just one.

But new data published in Thursday’s edition of the New England Journal of Medicine shows that what they’re really doing is increasing their odds of having twins – an outcome that is riskier for the mother and babies alike.


In the early days of in vitro fertilization, doctors routinely transferred half a dozen – or more – embryos into a patient’s uterus to boost the odds that at least one would implant and grow into a healthy baby. As the treatment improved, doctors were able to get better results using fewer embryos.

Debate lingers about the pros and cons of transferring two embryos instead of just one. As my colleague Shari Roan reported in the Health section in 2007, pregnancies involving multiples come with a greater risk of gestational diabetes, bleeding and preeclampsia for the mother and cerebral palsy, birth defects, developmental delays and death for the babies. Twins usually survive, but incur much higher medical expenses over the first five years of life.

A team of Swedish researchers is trying to assess the optimum number of embryos to transfer during a cycle of in vitro fertilization. In their study of 661 IVF patients, 331 women got two embryos on their first try, and 142 of them (or 43%) had a live birth. The other 330 women got a single embryo, and if that didn’t work, they made a second attempt with another embryo that had been frozen and thawed. Of these women, 128 (or 39%) had a live birth.

The difference between 43% and 39% wasn’t statistically significant. But one-third of the patients in the double-embryo group had twins or triplets, compared with only 1% of patients in the single-embryo group. Those results were published in the New England Journal of Medicine in 2004.

The research team, from Gothenburg, Sweden, continued to follow those 661 patients through as many as four additional IVF cycles (with no restrictions on how many thawed embryos were transferred at a time). In the latest issue of the journal, they offered the final tally.

Their key statistic is the cumulative live birth rate, which measures the number of patients who wound up with at least one live birth divided by the total number of patients. In the group that started out with a double-embryo transfer, the cumulative live birth rate was 51%. In the group that started out with a single transfer, the rate was 44%. As before, the difference wasn’t statistically significant.


Also as before, the difference in multiples was huge. There were four sets of twins in the single-embryo group (for an overall rate of 2.3%) versus 51 sets of twins and one set of triplets in the double-embryo group (for an overall rate of 27.5%).

The researchers conclude that for IVF patients, there’s little downside and plenty of upside to transferring one embryo at a time instead of two.

-- Karen Kaplan