No difference in live birth rates observed between alternative endometrial preparation regimens in thin endometrium

Does the type of endometrial preparation protocol influence reproductive outcomes in women with thin endometrium (<7 mm) undergoing single blastocyst embryo transfer (SET).

Retrospective cohort study including infertile women aged 25-48 years with thin trilaminar endometrium (<7 mm) who underwent blastocyst SET between January 2021 and December 2024. Patients were allocated to the modified natural cycle (mNC) or artificial cycle groups according to endometrial preparation protocol. Only patients undergoing SET with euploid blastocysts from autologous PGT-A or donor oocyte cycles were included. Primary outcome was live birth rate (LBR); secondary outcomes included positive beta-HCG test, clinical pregnancy and miscarriage rates.

A total of 762 SETs were carried out in 619 patients (mNC [n = 106], artificial cycle [n = 656). Patients who underwent artificial cycles were significantly older (P < 0.001); endometrial thickness and preparation time were comparable. The artificial cycle group showed more day-5 blastocyst transfers (P = 0.03). Reproductive outcomes, including LBR, did not differ between mNC and artificial cycles in the overall cohort and in subgroups (autologous and oocyte donation). Multivariate analysis confirmed no effect of endometrial preparation; day-6 transfers and low blastocyst grade were independently associated with decreased outcomes.

Reproductive outcomes in women with thin endometrium (<7 mm) were comparable and sustainable after mNC and artificial cycle preparation, suggesting that this condition is not an absolute contraindication to embryo transfer. The practice of cancelling embryo transfers in cases of thin endometrium and shifting to an alternative preparation protocol is not supported.