Comparison of birth weights in patients randomly assigned to fresh or frozen-thawed embryo transfer.

Objective: To estimate birth weight differences between patients randomized to fresh or thawed ET.

Design: Post hoc analysis of results from two similar randomized trials.

Setting: Private fertility center.

Patient(s): One hundred thirty-four first-time IVF patients, ages 18-40 years at oocyte retrieval, who had live birth.

Intervention(s): Patients were randomly assigned to have either fresh blastocyst transfer or all bipronuclear oocytes frozen followed by thaw, extended culture, and blastocyst transfer in a subsequent cycle. Preimplantation genetic screening was not allowed.

Main Outcome Measure(s): Mean birth weight.

Result(s): After allowing for the contributions of multiple significant variables (gestational age at birth, the presence of a vanished twin, number of infants delivered) in multiple linear regression, the adjusted mean birth weight was 166 g (95% confidence interval, 43-290 g) lower after fresh blastocyst transfer when compared with transfer of blastocysts derived from thawed bipronuclear oocytes.

Conclusion(s): Birth weights are lower in cycles with fresh blastocyst transfer after controlled ovarian stimulation than in transfers of frozen-thawed embryos in the absence of ovarian stimulation. This finding confirms similar results reported in many retrospective studies.

Clinical Trial Registration Numbers: NCT00963625 and NCT00963079.