The Association of Endometrial Thickness with Singleton Infant Birth Weight and Preterm Birth

To study the relationship of endometrial thickness (EMT) with singleton infant birth weights and gestational age at delivery in fresh and frozen embryo transfer with and without preimplantation genetic testing (PGT).

Retrospective cohort study using the Society for Assisted Reproductive Technology Clinic Outcome Reporting System.

Autologous in vitro fertilization fresh and frozen embryo transfer cycles initiated in 2019-2020 which led to singleton live birth.

Endometrial thickness measured in millimeters.

Low birth weight, early and late preterm birth, macrosomia, birth weight, birth weight z-scores, and gestational age at delivery.

98,657 cycles met inclusion criteria: 50,530 frozen embryo transfers (FET) with PGT, 34,602 FET without PGT, and 13,525 fresh embryo transfers. Odds of low birth weight significantly decreased with each 1mm increase in EMT until the endometrium reached 10mm (adjusted ORs per 1mm increase of 0.83 (95% CI: 0.78-0.89), 0.87 (95% CI: 0.83-0.90), and 0.84 (95% CI: 0.82-0.87) for blastocyst transfer in fresh, frozen without PGT, and frozen with PGT cycles, respectively). After 10mm, these associations were not significant. There was a decrease in the odds of late PTB with each 1mm increase in EMT until a threshold of 10mm in frozen blastocyst transfer (aORs of 0.92 (95% CI: 0.89-0.95) and 0.91 (95% CI: 0.88-0.94) for cycles without and with PGT) and with no threshold in fresh blastocyst transfer (aOR 0.94 (95% CI: 0.91-0.97)). Early PTB decreased with each 1mm increase in EMT until 12mm for fresh blastocyst transfer and frozen blastocyst transfer without PGT (aORs: 0.85 (95% CI: 0.79-0.92) and 0.89 (95% CI:0.86-0.93)) and without a threshold for frozen blastocyst cycles with PGT (aOR: 0.89 (95% CI: 0.87-0.92)).

In this population, increasing EMT was associated with a decreased odds of low birth weight until a threshold of 10mm, after which the association was not significant. Increasing EMT was also associated with a decrease in odds of early and late preterm birth. The results of this study can be used in patient counseling as well as to inform obstetric providers, but should not preclude embryo transfer in those with thin endometrial lining.