How endometrial thickness affects pregnancy and perinatal outcomes: a retrospective study of 16,566 frozen-thawed single blastocyst transfer cycles

Endometrial thickness (EMT) is widely monitored in assisted reproductive technology as a marker of implantation potential, yet its precise association with pregnancy and perinatal outcomes in frozen embryo transfer (FET) cycles remains controversial.

This retrospective cohort study evaluated 16,566 single blastocyst FET cycles from a single center. Cycles were categorized into five distinct EMT groups: ≤7 mm, (7,8] mm, (8,9] mm, (9,10] mm, and > 10 mm. Generalized linear mixed-effects models with inverse probability treatment weighting were used to assess pregnancy and perinatal outcomes for singleton live births (LBs). Additionally, the modifying effects of maternal age and body mass index (BMI) on the associations between EMT and outcomes were evaluated through statistical interactions.

Compared to the reference group of (8–9] mm, an EMT of ≤ 7 mm was significantly associated with lower odds of live birth (LB) (aOR: 0.428, 95% CI: 0.353–0.519, p < 0.001), higher odds of miscarriage (aOR: 1.660, 95% CI: 1.231–2.238, p < 0.001) and preterm birth (PTB) (aOR: 1.919, 95% CI: 1.075–3.426, p < 0.05), and shortened gestational age (adjusted β: −0.505 weeks, 95% CI: −0.743 to − 0.266, p < 0.001). Conversely, an EMT > 10 mm was significantly associated with improved odds of LB (aOR: 1.254, 95% CI: 1.108–1.420, p < 0.001) without an increase in adverse perinatal outcomes. Tests for statistical interactions revealed that an EMT of (9,10] mm was associated with an increased risk of miscarriage in women aged ≥ 35 years (aOR = 1.47, 95% CI 1.12–1.94, p < 0.01), and maternal BMI appeared to modify the impact of EMT on birth weight.

In FET cycles, an EMT ≤ 7 mm was associated with lower odds of LB and increased odds of PTB, whereas an EMT approaching 10 mm was associated with higher odds of LB. Clinically, our findings suggest that a thin endometrium alone may not necessarily warrant the cancellation of an embryo transfer. Given that maternal age and BMI potentially modulate the effects of EMT on clinical outcomes, individualized FET protocols might be beneficial.