In the US, more than six million couples are affected by infertility. Infertility is usually diagnosed after a couple has been trying to conceive for one year. For women 35 and older, the time is decreased to six months. More than nine million women experiencing infertility have attempted pregnancy though the help of assisted reproductive technologies, such as in-vitro fertilization, or IVF.

IVF has resulted in the birth of an estimated 1.2 million children worldwide. Since its introduction in the US in 1981, more than 114,000 babies have been born as a result of IVF. During the procedure, a man’s sperm and a woman’s eggs are combined outside of the body in a laboratory dish. If fertilization occurs, the resulting embryos are transferred to the woman’s uterus, where one or more may implant in the uterine lining and develop. With the introduction of more than one fertilized egg, the chances of multiple births are increased. In fact, 32% of IVF-assisted births result in twins, and 5% result in triplets and higher.

Multiple births, especially triplets and higher, are associated with a significantly increased risk of adverse outcomes, such as prematurity and low birth weight. Additionally, the risk of neonatal death is increased four times for twins as compared to single births.

In 1993, Swedish physicians voluntarily reduced the number of transferred embryos from three to two, which virtually eliminated the conception of triplets in that country. However, IVF-assisted birth rates stayed constant, as did the rate of twin births. A study published in the December 2, 2004 issue of the New England Journal of Medicine looked at how transferring a single embryo, as compared to two, affects the rates of single and multiple births.

About the Study

The study included 661 women, all under the age of 36, undergoing their first or second IVF cycle. The women were randomized into one of two groups:

  • 330 women underwent the transfer of a single fresh embryo and, if there was no live birth, subsequent transfer of a single, frozen-and-thawed embryo. (Following the transfer of an embryo to the uterus, any remaining viable embryos are frozen for later transfer.)
  • 331 women underwent the single transfer of two fresh embryos

The researchers were primarily interested in the rate of pregnancies resulting in at least one live birth. Additionally, they looked at the rates of multiple births.

The Findings

Transferring a single-fresh-embryo resulted in 91 (27.6%) live births. This rate of live births was significantly lower than the double-embryo-transfer group, in which 142 (42.9%) live births were recorded. However, after single, thawed embryos were transferred in the first group, in the end a total of 128 (38.8%) women in the single-embryo group had a pregnancy that resulted in at least one live birth. Although this still was a lower success rate than the double-embryo group, it was certainly comparable.

The rate of multiple births was as follows:

  • 47 of the 142 live births in the double-embryo-transfer group (46 pairs of twins and one set of triplets)
  • One of the 128 live births in the single-embryo-transfer group (one pair of twins)

The study’s authors concluded that a single fresh-embryo transfer, followed by single frozen-and-thawed embryo if necessary, resulted in a marked decrease in the rate of multiple births, but not a substantial reduction in the rate of successful pregnancies.

How Does This Affect You?

The participants of this study had the most favorable in-vitro prognosis. Specifically, the women were under 36 years of age, were undergoing their first or second cycle of IVF, and had at least two available, good embryos. Most women seeking IVF do not meet these criteria.

The two approaches clearly have their advantages and disadvantages. While the single-embryo-transfer essentially avoids the risks associated with multiple births, the success rate is far lower the first time around. Even though a second attempt with a single, thawed embryo significantly improved the odds, this additional step would presumably increase the already considerable cost and anxiety associated with IVF.

Balancing the risks and benefits of single versus multiple-embryo transfer is a deeply personal issue facing couples. If you are considering IVF, this is an important discussion to have with your doctor at the onset of your treatment.