Association between endometrial thickness and cumulative live birth rate in one oocyte-retrieval cycle: a retrospective cohort study including 26,127 patients.

BACKGROUND

This retrospective study aimed to evaluate the association between endometrial thickness (EMT) on the day of human chorionic gonadotropin (hCG) trigger and the cumulative live birth rate (CLBR) across one oocyte-retrieval cycle.

METHODS

This single-center retrospective cohort study included 26,127 women undergoing their first cycles of in vitro fertilization with or without intracytoplasmic sperm injection with autologous oocytes between January 2013 and December 2019. EMT on the hCG trigger day was analyzed both categorically (Group 1: <8.0, Group 2: 8.0–10.9, Group 3: 11.0–13.9, Group 4: ≥14.0 mm) and as a continuous variable in 1–2 mm increments. The primary outcome was the CLBR per retrieval cycle. Subgroup analyses were stratified by embryo number and stage at fresh transfer, maternal age, and the number of oocytes retrieved. Multivariable logistic regression analyses assessed the association between EMT and pregnancy outcomes.

RESULTS

The CLBR increased with each millimeter of EMT, reaching a maximum at 15 mm before slightly declining. Across EMT categories, the CLBR rose from 45.4% (< 8.0 mm) to 74.6% (≥ 14.0 mm). Live birth rates following fresh embryo transfer were 27.0%, 44.0%, 53.8%, and 58.0%for groups 1 to 4, respectively. Conversely, the absolute gains from subsequent frozen embryo transfers remained relatively constant, at 18.4%, 19.5%, 17.6%, and 16.6%. A positive association between EMT and CLBR was evident in all subgroups. In fresh cycles, thicker EMT was associated with lower rates of biochemical miscarriage and ectopic pregnancy (P < .001). And a significant interaction was observed between EMT and embryo transfer strategy on live birth rate (P < .001).

CONCLUSIONS

EMT on the day of hCG trigger is positively associated with CLBR within one oocyte- retrieval cycle, with optimal outcomes around 15 mm. This association is predominantly driven by fresh-transfer outcomes, whereas the absolute contribution from subsequent frozen transfers is relatively stable.