To investigate the association between body mass index (BMI) and ectopic pregnancy (EP) following embryo transfer (ET).
Retrospective cohort study.
University affiliated hospital.
A total of 16378 pregnancies derived from either fresh ET or frozen-thawed embryo transfer (FET) between Jan 2008 and Dec 2017.
We used the generalized estimating equation (GEE) to analyze the association between BMI categories and EP, as one patient may contribute to more than one pregnancy. Generalized additive models (GAMs) were also used to demonstrate the non-linear association. Models were adjusted for age, parity, gravidity, previous history of ectopic pregnancy, duration of infertility, PCOS, endometriosis, diagnosis of tubal problem, ovarian reserve markers, ovarian stimulation parameters, insemination protocol, endometrial thickness, and embryo transfer policies.
Ectopic pregnancy.
According to the WHO criteria, the number of cycles with low (<18.5 kg/m ), normal (18.5-24.9 kg/m ) and high BMI (≥25 kg/m ) were 2155, 13447 and 776 respectively. In comparison with the normal BMI group, the rate of EP was significantly increased in the low BMI group (2.92% vs 2.02%, relative risk 1.45, 95%CI: 1.11-1.90), but not in the high BMI group (2.84%, relative risk 1.41, 95%CI: 0.92-2.20). Adjusted for confounding factors, the odds ratio for EP comparing low BMI versus normal BMI was 1.61(95%CI: 1.19-2.16) and that comparing high BMI versus normal BMI was 1.12(95%CI: 0.72-1.76).
Low BMI is associated with an increased risk of EP.

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