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.

EMT measured in millimeters.

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

A total of 98,657 cycles met inclusion criteria: 50,530 frozen embryo transfers with PGT, 34,602 frozen embryo transfers without PGT, and 13,525 fresh embryo transfers. Odds of LBW significantly decreased with each 1 mm increase in EMT until the endometrium reached 10 mm (adjusted ORs per 1 mm increase of 0.83 (95% confidence interval [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 10 mm, these associations were not significant. There was a decrease in the odds of late PTB with each 1 mm increase in EMT until a threshold of 10 mm in frozen blastocyst transfer (adjusted odds ratios (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 1 mm increase in EMT until 12 mm 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 decreased odds of LBW until a threshold of 10 mm, after which the association was not significant. Increasing EMT was also associated with a decrease in odds of early and late PTB. The results of this study can be used in patient counseling as well as to inform obstetric providers; however, they should not preclude embryo transfer in those with thin endometrial lining.