Frozen embryo transfer outcomes in patients with prior cesarean delivery: a focus on endometrial lining parameters

To evaluate the association between prior mode of delivery (cesarean vs. vaginal) and endometrial thickness in the subsequent frozen embryo transfer (FET) cycle. The secondary aim was to evaluate FET cycle outcomes.

Retrospective cohort study.

This multicenter study included patients with one previous live birth who initiated a subsequent, autologous FET cycle from January 2012 to December 2024. Patients were grouped by mode of prior delivery: cesarean or vaginal.

Cesarean or vaginal delivery.

The primary outcome was endometrial thickness at final lining assessment. Secondary outcomes included FET cycle cancellation rate due to thin or fluid-filled endometrium, pregnancy, and live birth rates. Subgroup analyses evaluated outcomes of euploid FET cycles only.

A total of 11,131 FET cycles consisting of 5,149 from patients with prior cesarean and 5,982 from patients with prior vaginal delivery were included. Endometrial thickness in the subsequent FET cycle was similar between cohorts (cesarean: 10.4 mm [10.3-10.5 mm]; vaginal: 10.4 mm [10.3-10.4 mm]). The FET cycle cancellation rate due to thin endometrium was comparable (cesarean: 1.3%, vaginal: 1.2%; adjusted relative risk [aRR] 1.02 [0.72-1.43]); cancellation due to endometrial fluid accumulation was higher in the cesarean cohort (1.0% vs. 0.5%; aRR 2.22 [1.40-3.58]). Cesarean delivery was associated with lower live birth (49.5% vs. 53.0%; aRR 0.95 [0.90-0.98]) and clinical pregnancy (60.5% vs. 64.1%; aRR 0.95 [0.90-0.99]), with similar rates of clinical pregnancy loss (cesarean: 10.6%, vaginal: 10.6%; aRR 0.95 [0.84-1.07]). Similar trends were observed in patients undergoing euploid FET, with lower live birth demonstrated in those with a prior cesarean (55.3% vs. 60.3%; aRR 0.91 [0.84-0.98]), but similar chance of clinical pregnancy loss (cesarean: 9.1%, vaginal: 9.4%; aRR 0.95 [0.77-1.17]).

Although patients can be reassured that a prior cesarean does not appear to be associated with reduced endometrial thickness, receptivity in FET cycles may be diminished, as reflected by lower subsequent live birth rates. One possible explanation is an altered endometrial environment, given the increased risk of cycle cancellation due to endometrial fluid accumulation in patients with prior cesarean delivery.