Live birth rates increases with increasing endometrial thickness in fresh embryo transfer: Study

In this retrospective cohort study, authors analyzed all autologous cycles from the CARTR Plus database that resulted in an embryo transfer from 2013 to 2019. During this period, data were available from 33 Canadian ART clinics. The endometrial thickness in fresh IVF-ET cycles was recorded on the day of trigger, while for FETs it was recorded before the initiation of progesterone or documentation of luteinizing hormone surge or HCG administration. Cleavage stage and blastocyst transfers were included, and blastocyst transfers on days 5 and 6 were analyzed together.

Thirty-three Canadians clinics participated in voluntary reporting of IVF and pregnancy outcomes to the Canadian Assisted Reproductive Technology Registry Plus database, and a total of 43,383 fresh and 53,377 frozen transfers were included. Clinical pregnancy, pregnancy loss, and live birth rates were noted.

In fresh IVF-embryo transfer cycles, increasing endometrial thickness is associated with significant increases in the mean number of oocytes retrieved, peak estradiol levels, number of usable embryos, clinical pregnancy rates, live birth rates, and mean term singleton birth weights, and a decrease in pregnancy loss rates. However, live birth rates plateau after 10–12 mm. In contrast, in FET cycles live birth rates plateau after the endometrium measures 7–10 mm. The improvement in live birth rates with increasing endometrial thickness was independent of patient age, timing of embryo transfer (e.g., cleavage stage vs. blastocyst stage), or the number of oocytes at retrieval.

An endometrial thickness <6 mm is associated clearly with a dramatic reduction in the chance of live birth. With fresh embryo transfers, authors found that the live birth rates increased significantly as the endometrium thickened until an endometrial thickness of 10–12 mm.

In contrast, with FET cycles, live birth rates increased significantly as the endometrium thickened until an endometrial thickness of 7–10 mm. However, after these thresholds, live birth rates plateaued and they were unable to find an endometrial thickness beyond which live birth rates significantly declined. In fresh cycles, increasing endometrial thickness was associated with significantly higher mean numbers of oocytes retrieved, mean peak estradiol levels, and mean numbers of usable embryos. This raises the possibility that the improvement in outcomes with thicker endometrial linings could simply reflect confounding by patients with better ovarian reserve and, therefore, a better prognosis for pregnancy.

In cycles with a fresh embryo transfer, live birth rates increase with increasing endometrial thickness until 10–12 mm, and in FET cycles live birth rates plateau after 7–10 mm. There does not appear to be a thickness above which pregnancy outcomes worsen.

Source: Neal Mahutte, Michael Hartman, Lynn Meng; Fertility and Sterility® Vol. 117, No. 4, April 2022 0015-0282

https://doi.org/10.1016/j.fertnstert.2021.12.025