研究開発・学会発表

【Study question】
This study aimed to examine the effect of developmental stage of embryos on human sex ratio at birth by assisted reproductive technology.

【Summary answer】
Whether cleavage stage embryo transfer or blastocyst transfer has nothing to do with sex ratio at birth, but transfers of faster growing blastocysts are more likely to result in male births.

【What is known already】
Generally, male embryos are thought to grow faster than female embryos. Some studies observed a trend of higher male births following blastocyst transfer.

【Stydy design, size, duration】
In this retrospective case control study, we analyzed a total of 836 single blastocyst transfers from January 2008 to December 2009 at our clinic.

【Participants/materials, setting, methods】
We collected data from patients whose pregnancy resulted in a live birth after single blastocyst transfer. We defined sex ratio at birth as the proportion of males in all live births and used logistic regression to model the effect of the developmental stage of embryos on sex ratio at birth.

【Main results and the role of chance】
In the present study, there was no significant increase in sex ratio at birth after blastocyst transfer (50.7%), when compared with cleavage stage embryo transfer (46.0%) (OR 1.20, 95%CI: 0.79-1.83). However, in transfers of faster growing blasytocysts (i.e., highly graded blastocysts according to the grading system proposed by Gardner and Schoolcraft), there was a statistically significant increase in sex ratio at birth (Grade 1: 33.3%, Grade 2: 41.4%, Grade 3: 44.7%, Grade 4: 55.4%) (OR 1.36, 95%CI: 1.11-1.65).

【Limitations, reason for caution】
We compared sex ratio at birth, not at conception nor at embryo development.

【Wider implications of the findings】
To the best of our knowledge, this is the first study to report the effect of developmental stage of embryos on sex ratio at birth with a large number of single embryo transfers.

【Study funding/competing interest(s)】
None.

【Trial registration number】
None.