NEW YORK (Reuters Health) - The transfer of a single embryo into the uterus decreases the number of twin pregnancies without adversely affecting pregnancy outcomes, according to a report in the current issue of Fertility and Sterility.

Recent efforts have focused on transferring fewer embryos as a means of reducing the rising rate of twin pregnancies, the authors explain. Several studies have shown similar pregnancy outcomes, but a lower rate of twin births, after the transfer of a single embryo in women under the age of 37 years.

Dr. Aaron K. Styer and colleagues from Massachusetts General Hospital, Boston, compared the pregnancy outcomes of 52 women who underwent in vitro fertilization (IVF) with a single-embryo transfer versus 187 women who underwent a double-embryo transfer. All of the subjects were 37 years old or younger.

IVF cycle characteristics did not differ between the single- and double-embryo transfer groups, the report indicates.

Single-embryo and double-embryo cycles also did not differ in the rates of pregnancy - 61 percent versus 63.4 percent - or in rates of pregnancy loss - 20 percent versus 18.6 percent, retrospectively, the authors report. However the rate of implantation was significantly greater for single-embryo transfer cycles (70.5 percent) than for double-embryo cycles (47.8 percent).

No single-embryo transfer cycles resulted in ectopic pregnancies. An ectopic pregnancy occurs when the embryo does not implant on the lining of the uterus, but somewhere else, usually the fallopian tubes. If not detected early, the consequences can be serious. In contrast, two percent of double-embryo transfer cycles did result in ectopic pregnancies, despite the lack of risk factors.

More double-embryo transfer cycles (51 percent) than single-embryo transfer cycles (3.1 percent) resulted in twin pregnancies, the researchers note.

Moreover, there was no difference in live-birth rates between the single-embryo-transfer group (53.8 percent) and the double-embryo transfer group (54.4 percent).

"This study provides further evidence that elective (single-embryo) transfer is not only a feasible but a realistic option in the young patient who has a favourable reproductive profile and several good-quality embryos available for transfer and cryopreservation," the authors conclude.

Candidates for the single-embryo transfer may include patients who don't want to risk multiple pregnancies or who have a pre-existing medical condition for which a multiple pregnancy would not be safe, the researchers explain.

The most important factor in the decision to proceed with elective single-embryo transfer, they add, may be for women with a "favourable reproductive profile."