Parental preconception BMI and risk of adverse birth outcomes in 8 million parent-child triads: a nationwide population-based cohort study in China.

Background: Maternal obesity has been identified as a risk factor for adverse birth outcomes, but the role of paternal obesity (separately and in combination with maternal obesity) on perinatal outcomes remains to be determined. Therefore, this study aims to investigate the independent and joint associations of paternal and maternal BMI with adverse birth outcomes in the general population in China.

Methods: This population-based cohort study used data from the National Free Preconception Checkups Project between 2010 and 2020. Parent-child triads with available data on parental preconception BMI and birth outcomes were included. The outcomes included spontaneous abortion, medically induced abortion, preterm birth, small for gestational age (SGA), large for gestational age (LGA), birth defects, and perinatal death. Restricted cubic spline analyses were performed to assess the dose-response relationship and robust Poisson regression models were applied to quantify the risk ratios (RRs) and 95% CIs.

Findings: We included 8 787 096 parent-child triads, with an average age of 26·18 (SD 4·33) years for mothers and 27·83 (4·73) years for fathers. Restricted cubic spline analyses showed robust monotonic associations for both maternal and paternal BMI with risk of LGA (positive association) and SGA (inverse association). Both maternal and paternal BMI showed J-shaped associations with all other adverse birth outcomes examined. Compared with a healthy BMI in both parents, we found that preconception BMI categories of underweight, overweight, or obesity in either or both parents were associated with a higher risk of spontaneous abortion, medically induced abortion, perinatal death, preterm birth, birth defects, and composite adverse birth outcome. For instance, the RR of perinatal death was 1·17 (95% CI 1·03-1·33) when both parents had obesity and 1·12 (1·01-1·23) when both parents had underweight. The RR of LGA was 1·70 (1·65-1·74) when both parents had obesity and 0·67 (0·66-0·69) when both parents had underweight. The RR of SGA was 0·73 (0·69-0·76) when both parents had obesity and 1·57 (1·53-1·60) when both parents had underweight. However, regarding all these outcomes as a composite, the risk increased by 10% (RR 1·10 [1·08-1·11]) when the mother had obesity and the father had a healthy BMI, by 4% (1·04 [1·04-1·05]) when the mother had a healthy BMI and the father had obesity, and by 16% (1·16 [1·14-1·18]) when both parents had obesity; the RR when both parents had underweight was 1·08 (1·06-1·09).

Interpretation: In this large, nationwide population-based cohort study, a preconception BMI in the category of underweight, overweight, or obesity in either or both parents was associated with a higher risk of various adverse birth outcomes, including perinatal death and birth defects. Therefore, maintaining a healthy weight for both parents before pregnancy is key to perinatal wellbeing.

Funding: National Key Research and Development Program of China, National Natural Science Foundation of China, USTC Research Funds of the Double First-Class Initiative, and Anhui Provincial Natural Science Foundation.

Translation: For the Chinese translation of the abstract see Supplementary Materials section.