Embryo re-expansion does not affect clinical pregnancy rates in frozen embryo transfer cycles: a retrospective study

A retrospective study examining the effects of embryo re-expansion before transfer on pregnancy outcomes for frozen embryo transfers (FET).

A total of 486 FET cycles from November 2017 through December 2019 were studied. These cycles included patients using autologous, donor oocytes, and donor embryo with patients ranging from ages 23 to 48 years with infertility diagnoses. Programmed FET priming was performed with exogenous estrogen and progesterone. All blastocysts were cultured in trigas incubators for 20 min to 4 h and 42 min. Pictures of each blastocyst after thaw and before transfer were taken utilizing the Hamilton Thorne Zilos laser software (Beverly, MA). The longest portion of the embryo was measured in µm. Pregnancy was defined by a positive hCG, and ongoing clinical pregnancy was defined by the presence of fetal cardiac activity. Wilcoxon rank sum tests were used to access differences in change parameters.

There is no significant difference in the amount of embryo expansion or contraction to achieve an ongoing pregnancy. The difference remained non-significant when stratified by embryo expansion or contraction. The amount of change over time and percent change from the first measurement were also not associated with achieving an ongoing pregnancy. This remained true after adjustment for patient age and whether or not a biopsy was performed.

Embryos that do not re-expand after warming appear to have a similar chance of achieving a successful pregnancy as those that do re-expand.