To determine if weight or body mass index (BMI) affects the serum progesterone level at the time of the pregnancy test in cryopreserved blastocyst transfer cycles and to determine if those serum progesterone levels affect live births.

Retrospective cohort study.

US academic medical center.

Six hundred thirty-three patients undergoing their first cryopreserved embryo transfer cycle.

None.

The primary outcome was the serum progesterone level on the day of the pregnancy test by patient weight and BMI. Our secondary analysis assessed the serum progesterone effect on live birth rate (LBR) in a clinic where progesterone supplementation was increased if the progesterone level was <15 ng/mL on the day of the pregnancy test.

There was a strong negative correlation between serum progesterone level and both BMI and weight, with BMI accounting for 27% and weight accounting for 29% of the variance in progesterone level. Serum progesterone level on the day of the pregnancy test was <15 ng/mL in 3% of women weighing <68 kg compared with 29% of women weighing ≥90.7 kg. Among women weighing ≥90.7 kg, live birth occurred in 47% whose serum progesterone level was <15 ng/mL on the day of the pregnancy test compared with 49% in those with serum progesterone level of 15–19 ng/mL and 44% in those with serum progesterone level of ≥20 ng/mL.

Body weight was a significant factor in serum progesterone level at the time of the pregnancy test, with nearly 30% of patients weighing ≥90.7 kg having serum progesterone level of <15 ng/mL, a value associated with lower LBRs in prior studies. However, we found no effect of low progesterone levels on LBR after cryopreserved embryo transfer cycles in a clinic where progesterone dosing was increased if serum progesterone levels were <15 ng/mL.