Abstract: Objective To investigate the association of the number and quality of embryos transferred in frozen-thawed embryo transfer (FET) cycle with the occurrence of early pregnancy loss in infertile women at an advanced age. Methods We collected the data of a total of 2622 infertile women aged 36 years or older receiving FET between January, 2015 and December, 2018. The occurrence of early pregnancy loss was analyzed in these women, and the relation of age, the number and quality of embryos transferred with early pregnancy loss was explored in the overall cohort and in different age (36-37, 38-39, 40, 41, 42, 43, 44 and 45-48 years) groups. Results Of the 2622 women, 976 (37.2%) achieved clinical pregnancy after FET, among whom 241 (24.7%) experienced early pregnancy loss. The clinical pregnancy rate and live birth rate decreased significantly and early pregnancy loss rate increased markedly with age (P<0.001). The age at both FET and oocyte pick-up was significantly higher in early pregnancy loss group than ongoing pregnancy group. Compared with those with ongoing pregnancy, the women with early pregnancy loss had a significantly greater number of D3 embryos transferred (0.97 vs 0.81) and a lower number of implanted embryos (1.09 vs 1.25). The early pregnancy loss rates was not significantly correlated with the number and quality of embryo transferred across all the age groups. Conclusions In infertile women at an advanced age, the early pregnancy loss rate in FET cycle increases with age. An advanced age is an inherent risk factor for early pregnancy loss, therefore ART should be implemented as soon as possible for these women. To reduce early pregnancy loss rate and balance the risk of multiple pregnancies, the number of embryos transferred should be carefully determined on the basis of embryo quality.

Key words: assisted reproductive technology; in vitro fertilization-embryo transfer; frozen-thawed embryo transfer; advanced-aged women; embryo quality; early pregnancy loss