NEW YORK (Reuters Health) - In agreement with what had been seen in laboratory studies, blastocyst embryo transfer results in a sex-ratio imbalance strongly favoring male offspring, U.S. researchers report.

During in vitro fertilization, the most advanced embryos are typically selected for transfer. Findings from laboratory studies have indicated that these embryos, at both the cleavage and blastocyst stages, are usually male. Whether this difference actually results in sex-ratio imbalance in the offspring was unclear.

In a new study reported in the March issue of Fertility and Sterility, Dr. Alan B. Copperman, from the Reproductive Medicine Associates of New York, and colleagues assessed the sex-ratio of 1284 offspring derived from either embryo transfer at day 3 or from blastocyst stage transfer.

Male offspring dominated regardless of the stage of embryo development, the report indicates. However, the difference was more pronounced with the use of blastocyst stage embryos rather than with embryos obtained at day 3 of culture. Overall, with day 3 embryo transfer, 48.8% of offspring were female and 51.2% were male. With blastocyst embryo transfer, the corresponding percentages were 42.3% and 57.7%.

Even greater sex differences were seen with use of autologous oocytes and in singleton deliveries. With autologous oocyte use and blastocyst embryo transfer, females accounted for 41.7% of offspring, while males accounted for 58.3%. With singleton deliveries, just 36.3% of offspring were female and 63.7% were male.

The authors believe that a revision of blastocyst selection criteria could help reduce the sex-ratio imbalance identified in this study.