Association with pre-pregnancy BMI on early pregnancy loss in infertility patients in first HRT-FET cycles: A retrospective cohort study

Objective: To investigate the association between pre-pregnancy body mass index (BMI) and the early pregnancy loss rate in patients in first hormone replacement therapy-frozen-thawed embryo transfer (HRT-FET) cycles and find the threshold. Methods: A retrospective cohort study was conducted using a total of 14030 HRT-FET cycles at the Reproductive Center from January 2017 to December 2021. The association of pre-pregnancy BMI on early pregnancy loss rate in patients in HRT-FET cycles was assessed by performing univariate analysis, multivariable logistic regression, curve fitting and threshold effect analysis. Results: There were 2076 cycles of early pregnancy loss, and the pregnancy loss rate was 14.80%. After adjusting for confounding factors, the early pregnancy loss rate of the obese group was significantly higher than that of the normal weight group (P < 0.05). The threshold effect analysis showed that as the pre-pregnancy BMI ranged from 21.2 to 25.8 kg/m(2), the early pregnancy loss rate came to the plateau phase at the low level. In addition, when the BMI was >= 25.8 kg/m(2), the early pregnancy loss rate increased by 3% (aOR = 1.03, P = 0.01) with each 1 kg/m(2) increment of BMI. Conclusion: The early pregnancy loss rate might achieve a low level when the pre-pregnancy BMI was within the range of 21.2- 25.8 kg/m(2). The early pregnancy loss rate would increase when pre-pregnancy BMI is more than 25.8 kg/m(2). For patients in HRT-FET cycles, adjusting their pre-pregnancy BMI to the optimal level by following a healthy diet and daily exercise may help to reduce the early pregnancy loss.