Association between BMI and increased time-to-pregnancy in planned pregnancy couples: a cohort study in Guangzhou, China

This study examines the relationship between overweight and obesity and fertility in the context of China's fertility. Given the inconsistent results in previous research, which mostly focused on females, our study targets couples in Guangzhou. We investigate the relationship between Body Mass Index (BMI) and time-to-pregnancy (TTP) to provide evidence-based strategies for enhancing reproductive outcomes in China. This cohort study, utilizing the National Free Pre-pregnancy Checkups Project (NFPCP), employs a Cox regression model to assess the associations between different BMI categories and TTP. Heatmaps are utilized to investigate the association between various BMI combinations of couples and TTP. Additionally, restricted cubic spline (RCS) curves were used to explore the impact of different ranges of male and female BMI on TTP. The results showed that females and males classified as overweight and obese (fecundability ratios (FR) 0.78, 95% CI 0.64, 0.95 for females; FR 0.86, 95% CI 0.76, 0.97 for males) had longer TTP compared with those in the normal weight category, regardless of controlled covariates, while those classified as underweight also had longer TTP, but the difference was not statistically significant (P > 0.05). Across all BMI combinations, couples in the overweight and obese groups exhibited the longest TTP, experiencing a 34% increase in TTP compared to couples in the normal BMI combination (FR 0.66; 95% CI 0.50, 0.85). After adjusting for all covariates in the RCS model, a male BMI within the range of 23.40 to 29.44 was significantly associated with an increase in TTP (FR