Birthweights and Down syndrome in neonates that were delivered after frozen-thawed embryo transfer: The 2007-2012 Japan Society of Obstetrics and Gynecology National Registry data in Japan

Aim: To evaluate the use of frozen embryos on the outcome of assisted reproductive technology (ART), a retrospective study of the Japanese Assisted Reproductive Technology Registry data during the years 2007-2012 was conducted. Methods: A total of 124 946 singleton neonates who reached term gestation following ART from 2007-2012, with 80 660 achieved through frozen-thawed embryo transfer (ET) and 44 286 being achieved through fresh ET, were analyzed for their birthweights and chromosomal abnormalities. Results: The birthweight of the neonates from the frozen-thawed ETs was significantly higher than that of those from the fresh ETs throughout all the study years. The frequency of Down syndrome was 0.17% for the fresh ETs and 0.13% for the frozen-thawed ETs in the period 2007-2012. This study showed that frozen-thawed ETs result in a constant increase of the average birthweight between 37 and 41 weeks gestational age and lower frequencies of Down syndrome. Conclusion: Frozen-thawed ETs were comparable to the fresh ET method, with the exceptions of higher birthweights and a lower frequency of Down syndrome in the neonates that were born from frozen-thawed ET. The increase in birthweights was not proportional to the gestational ages. This cannot be explained with any well-known mechanism. The frequency of chromosomal abnormalities needs detailed data for analysis.