Objective:To explore the relationship between different gestational weeks of transvaginal ultrasound-guided fetal reduction and abortion in patients with multiple pregnancies after embryo transfer, and to seek the best gestational age for fetal reduction.Methods:The datas of 486 pregnant women with multiple pregnancies after embryo transfer in the Second Hospital of Hebei Medical University from January 2012 to December 2020 were retrospectively analyzed. The relationship between gestational weeks of fetal reduction and abortion rate was analyzed by curve fitting, threshold effect and multivariate logistic regression analysis.Results:After adjusting for age, infertility type, infertility years, number of births, abortion times, body mass index(BMI), various infertility and sterility factors, endometrial thickness on the day of transformation, monozygotic twins and reduction methods, when the gestational age was less than 8.43 weeks, the abortion rate increased significantly with the increase of reduction gestational age, and the abortion rate increased by 221% ( OR=3.21, 95% CI=1.47-6.99, P=0.003 3). When the gestational age of reduction ≥8.43 weeks, the abortion rate tended to be stable and did not increase ( OR=0.81, 95% CI=0.54-1.22, P=0.317 7); meanwhile, in stratified analysis, the OR value of the BMI ≥24 kg/m 2 was 12.38, and that of BMI <24kg/m 2 was 1.91, P=0.053 9. Conclusions:There is a non-linear relationship between gestational age and abortion rate of ultrasound-guided multiple pregnancy reduction in patients with embryo transfer. The abortion rate increases significantly with the increase of gestational age before 8.43 weeks of gestation. It is recommended to carry out the operation as early as possible before 8 weeks of pregnancy. The effect of BMI on the abortion rate of patients with fetal reduction needs further study.