A non-linear dose-response relation of female body mass index and in vitro fertilization outcomes

Obesity, measured by body mass index (BMI), is implicated in adverse pregnancy outcomes for women seeking in vitro fertilization (IVF) care. However, the shape of the dose-response relationship between BMI and IVF outcomes remains unclear.

We therefore conducted a dose-response meta-analysis using a random effects model to estimate summary relative risk (RR) for clinical pregnancy (CPR), live birth (LBR), and miscarriage risk (MR) after IVF.

A total of 18 cohort-based studies involving 975,889 cycles were included. For each 5-unit increase in BMI, the summary RR was 0.95 (95% CI: 0.94-0.97) for CPR, 0.93 (95% CI: 0.92-0.95) for LBR, and 1.09 (95% CI: 1.05-1.12) for MR. There was evidence of a non-linear association between BMI and CPR (P_{non-linearity} < 10^-5) with CPR decreasing sharply among obese women (BMI > 30). Non-linear dose-response meta-analysis showed a relatively flat curve over a broad range of BMI from 16 to 30 for LBR (P_{non-linearity} = 0.0009). In addition, we observed a J-shaped association between BMI and MR (P_{non-linearity} = 0.006) with the lowest miscarriage risk observed with a BMI of 22-25.

In conclusion, obesity contributed to increased risk of adverse IVF outcomes in a non-linear dose-response manner. More prospective trials in evaluating the effect of body weight control are necessary.