Definition of thin endometrium from the perspective of miscarriage following frozen embryo transfer: a retrospective cohort study of 15,979 pregnancies

This study aimed to explore whether an endometrial thickness (EMT) cutoff of 7 mm or 8 mm was more appropriate to define thin endometrium (TE) from the perspective of miscarriage and obstetric-neonatal complications. We conducted a retrospective cohort study of 15,979 patients who had a positive serum β-human chorionic gonadotropin test after undergoing their first frozen-thaw embryo transfer. The main outcomes were early pregnancy loss, late pregnancy loss, and the rates of obstetric and neonatal complications. When TE was defined by EMT <8 mm, the TE group exhibited a similar risk of early miscarriage (17.2% versus 15.7%, P=0.086) to the control group (EMT ≥8 mm). However, the TE group defined by EMT <7 mm had significantly higher rates of early miscarriage (21.5% versus 15.6%, P<0.001) than the control group (EMT ≥7 mm). The conclusion remained consistent after adjustment for age and other confounders in logistic regression. In addition, the adjusted odds ratio (aOR) for preterm birth increased from 1.281 (95% CI: 1.022-1.606) at an EMT <8 mm to 2.050 (95% CI: 1.427-2.945) at an EMT <7 mm, and the aOR for low birth weight increased from 1.394 (95% CI: 1.041-1.867) at an EMT <8 mm to 2.488 (95% CI: 1.611-3.843) at an EMT <7 mm. When thin endometrium was defined as EMT <7 mm, the TE group had significantly higher risks of early miscarriage, preterm birth, and low birth weight than when thin endometrium was defined as EMT <8 mm. Because dilation and curettage after miscarriage may further damage the endometrium in patients with thin endometrium, pregnancy loss should be considered when debating the definition of thin endometrium.