Blastocyst embryo transfer is associated with a sex-ratio imbalance in favor of male offspring.

OBJECTIVE To evaluate the sex ratio of offspring born after blastocyst transfers. DESIGN Retrospective data analysis. SETTING A large assisted reproductive technology center. PATIENT(S) We included 1,284 offspring from 937 deliveries during the period August 2003-August 2005. INTERVENTION(S) Tabulation and statistical analysis of all births resulting from fresh IVF cycles. The sex of resulting offspring was compared in both day 3 and blastocyst transfers for all births and for singleton deliveries. In addition, the sex of children conceived with the use of autologous oocytes and donor oocytes was evaluated. MAIN OUTCOME MEASURE Sex ratio of offspring born following embryo transfers (ETs) after day 3 of culture and sequential blastocycst culture. RESULT(S) The overall sex ratio was significantly shifted toward males when blastocyst transfers were performed. Blastocyst transfers with only the use of autologous oocytes resulted again in a significantly higher proportion of male offspring. An even greater proportional difference was encountered in singleton offspring from donor oocytes. However, significance was not reached because of the limited number of offspring in the subgroup. CONCLUSION(S) This is the first individual-center report of a significant sex-ratio imbalance after the sequential media culture of blastocysts. The large imbalance in singleton births associated with the use of donor oocytes, although not significant, is cautionary in regard to the use of elective single ETs. Observation and publication of phenomena such as the effects of extended culture on the sex ratio of live-borns will allow us a better understanding of early differences in sexual dimorphism of the embryo, and will allow us to counsel our patients more appropriately.