Male gender explains increased birthweight in children born after transfer of blastocysts.

STUDY QUESTION: Does extended embryo culture have a different effect on the birthweight of girls and boys?

SUMMARY ANSWER: The mean birthweight of boys born after fresh and frozen-thawed blastocyst transfer was increased compared with those born after cleavage stage embryo transfer. This effect was not detected among girls.

WHAT IS KNOWN ALREADY: Previous studies indicate that newborns from frozen-thawed cleavage stage embryos may present with a higher weight than newborns from fresh embryo transfers. With regard to fresh embryos, newborns after a blastocyst transfer have been reported as having higher birthweights than newborns from cleavage stage embryos.

STUDY DESIGN, SIZE, DURATION: Retrospective multicentre case-control cohort study. All IVF/ICSI treatments were performed in the time-period from January 2008 to March 2014.

PARTICIPANTS/MATERIALS, SETTING, METHODS: Birthweight of singletons born at full-term (≥37 weeks), after fresh or frozen blastocyst embryo transfers (n = 277), were compared with weights of children born after fresh or frozen cleavage stage embryo transfers (Day 2-3) (n = 277). The cases and controls were matched by delivery week, and by gender. Data of IVF/ICSI treatments, and the treatments' outcomes were collected and analysed.

MAIN RESULTS AND THE ROLE OF CHANCE: The birthweight after a fresh blastocyst transfer was significantly higher (mean 3530.6 g) than that after a transfer of cleavage stage embryos (mean 3418.8 g; weight difference 111.8 g, P = 0.047). The weights of newborns after frozen-thawed blastocyst transfers (mean 3647.5 g) and the frozen-thawed cleavage stage embryo transfers (mean 3650.9 g), were similar (weight difference 3.4 g, P = 0.95). The boys born after transfer of frozen-thawed blastocysts had a significantly higher birthweight (mean 3767.9 g) than girls (3525.2 g; weight difference 242.7 g, P = 0.002), whereas the difference of birthweights between genders was only 13.5 g in cleavage stage (P = 0.863). The same effect was seen after fresh blastocyst transfers (weight difference 211.5 g, P = 0.011), but not after fresh Day 2-3 embryo transfers (weight difference 53.6 g, P = 0.478).

LIMITATIONS, REASONS FOR CAUTION: The study material was large enough to detect differences between birthweights as a whole, but a larger study group would confirm these new findings. To avoid selection bias, the next possible control candidate, fulfilling the selection criteria, was included for matching cases and controls. We have matched the cases and controls by gender and gestational week at birth, with an aim to reduce their impact as confounding factors.

WIDER IMPLICATIONS OF THE FINDINGS: Our findings of a similar weight at birth of newborns after frozen-thawed blastocysts and frozen-thawed cleavage stage embryos, when matching for age and duration of pregnancy, are novel. The gender of the newborn has an impact on the birthweight, and the extended embryo culture increases the weight difference between the genders, which is a new finding as well.

STUDY FUNDING/COMPETING INTERESTS: The study was funded by the Fertility Society of Finland.