In a paper posted on Fertility and Sterility on-line, researchers at the Mount Sinai School of Medicine and Reproductive Medicine Associates in New York analyze data from their center and report that blastocyst transfer results in a significant sex-ratio imbalance towards male offspring. They found that almost 58% of babies born as a result of blastocyst transfers after a fresh IVF cycle were male. Approximately 51% of all babies born in the US are male.
The analysis included 1,284 babies from 937 deliveries that took place from August 2003 to August 2005 as a result of fresh IVF cycles. Sex ratios were determined for deliveries resulting from embryos transferred on the third day of culture as well as for blastocysts transferred after five days of culture. In addition, the researchers looked at singleton births to help determine whether the practice of single blastocyst transfer might impact sex ratio.
Deliveries from embryos transferred on day three of culture exhibited a much more balanced sex-ratio, close to that of all US births: 48.8% female to 51.2% male. For singleton deliveries from day three transfers, there was no significant change in sex ratio.
However, when the data from singleton births after blastocyst transfer were analyzed, a sex ratio of 36.3% female to 63.7% male emerged.
The authors attribute the higher proportion of male offspring from blastocyst transfer to the faster preimplantation development of male embryos. Criteria for selecting blastocysts for transfer favor faster developing embryos with more cells. Most of the patients in the study who underwent embryo transfer transferred more than one embryo, increasing the chances for a more balanced sex ratio. However, the greater disparity between male and female birth rates among singletons born after blastocyst transfer indicates that the practice of selecting the single, most fully developed embryo for a single embryo transfer could lead to further and greater imbalances.
David Grainger, President of SART remarked, "This is a fascinating finding. Further research in this area will help distinguish between the effects of culture techniques and the effects of embryo selection criteria on the sex ratio of children born after blastocyst transfer. In the meantime, doctors might want to examine the sex ratios of children born from their procedures in order to be able to advise patients in case they care."
(Luna et al, Blastocyst embryo transfer is associated with a sex-ration imbalance in favor of male offspring, Fertility and Sterility, published on-line at www.fertstert.org .)
