Abstract

Abstract Objective Maternal obesity is a highly suggestive risk factor of offspring congenital heart diseases (CHD). However, the risk of offspring CHD associated with maternal underweight has rarely been mentioned. Therefore, this study aimed to explore the effect of preconception underweight on offspring CHD. Methods From November 2017 to August 2021, 132 386 pregnant women were enrolled in a birth cohort study in China in early pregnancy, and completed follow‐up until delivery (or miscarriage/termination). Offspring CHD was diagnosed by prenatal ultrasound examination in both live births and stillbirths. Log‐binomial regression and restricted cubic spline were used to estimate the risk of offspring CHD associated with preconception body mass index (BMI). A generalized additive model was used to explore the modification effect of maternal age on the association between preconception BMI and offspring CHD. Results A total of 129 096 pregnant women were included in the analysis. The incidence of CHD in the underweight, normal weight, overweight, and obesity groups were 117/17 313 (0.68%), 556/85 695 (0.65%), 128/19 936 (0.64%), 47/6152 (0.76%), respectively. Both underweight and obesity before pregnancy marginally increased the risk of offspring CHD. The association between preconception BMI and offspring CHD varied by maternal age, with low preconception BMI associated with a significantly higher risk of offspring CHD in women <24 years (RR 2.32, 95% CI: 1.07–5.01 for 17 vs 21 kg/m 2 ). Conclusion Preconception underweight was associated with an increased risk of offspring CHD in young pregnant women. Therefore, weight gain is important to prevent offspring CHD, especially for young women with low preconception BMI.