Combined effects of pre-pregnancy BMI and gestational weight gain on preterm birth: comparison between spontaneous and ART conception

Background Inappropriate pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) are both linked to preterm birth (PTB); however, which one plays a dominant role in PTB risk is not yet sure. We aimed to evaluate the combined effect of pre-pregnancy BMI and GWG on the risk of PTB in singleton pregnancies conceived both spontaneously and through assisted reproductive technology (ART). Methods The data included all mothers ( n = 17,540,977) who had a live singleton birth from the US National Vital Statistics System (NVSS) 2015–2019. Logistic regression models, quantile-g-computation, and generalized additive model were used to analyze the combined association of pre-pregnancy BMI and GWG with PTB. Results The singleton PTB rate was significantly higher in ART pregnancies (11.5%) than in non-ART pregnancies (7.9%). When compared to those women with pre-pregnancy normal weight and GWG within Institute of Medicine (IOM) guidelines, the highest PTB risk was observed in non-ART women with pre-pregnancy underweight and GWG below IOM guidelines (aOR 2.56; 95% CI 2.53–2.60) and in ART women with pre-pregnancy obese and GWG below IOM guidelines (aOR 2.56; 95%CI 2.36–2.78). GWG dominated the combined effect with its joint effect coefficient of − 0.281 ( P < 0.05) in non-ART women and − 0.108 ( P < 0.05) in ART women. Conclusions Inappropriate GWG played a dominant role in increasing the risk of PTB in both non-ART and ART populations. Counseling regarding pre-pregnancy BMI and especially GWG appears to be even more crucial for pregnancies conceived via ART, given their impact on PTB.