Effect of in vitro culture period on birth weight after vitrified-warmed transfer cycles: analysis of 4,201 singleton newborns.

OBJECTIVE: To investigate the effect of embryo culture duration on birth weight in vitrified-warmed cycles.

DESIGN: Retrospective cohort study.

SETTING: Tertiary-care academic medical center.

PATIENT(S): A total of 4,201 women who gave birth to 3,520, 215, and 466 live-born singletons after frozen-thawed cleavage-stage (day 3) and day 5 and day 6 blastocyst transfer, respectively.

INTERVENTION(S): None.

MAIN OUTCOME MEASURE(S): Neonatal birth weight.

RESULT(S): The mean birth weight did not differ between the three study groups. However, the gestational age- and sex-adjusted birth weight (Z-scores) of singletons and the proportion of large-for-gestational-age (LGA) babies were significantly higher after day 5 and day 6 transfer than after transfer of day 3 embryos. Furthermore, multiple linear regression analysis indicated that gestational age, parental body mass index, neonatal sex, and length of the culture period all had significant and strong impacts on birth weight of singleton newborns.

CONCLUSION(S): In the vitrified-warmed transfer cycles, birth weight Z-scores and the proportion of LGA infants were both higher in singletons born after blastocyst transfer than after transfer of cleavage-stage embryos. This finding suggests that the effect of culture duration was not overcome by transfer of embryos into a more physiologic uterine environment.