Couples' prepregnancy body mass index and time to pregnancy among those attempting to conceive their first pregnancy.

Objective To explore the relationship between couples’ prepregnancy body mass index (BMI) and time to pregnancy (TTP) among those with no prior gravidity. Design Retrospective cohort study. Setting Local maternal/child health care or family-planning service institutions. Patient(s) A total of 2,301,782 eligible couples participating in the National Free Preconception Check-up Projects from January 1, 2015 to December 31, 2017. Intervention(s) None. Main Outcome Measure(s) Time to pregnancy. Result(s) Underweight, overweight, or obese status in women and underweight status in men were associated with couples’ prolonged TTP. The optimal BMI levels were 20.61–23.06 kg/m2 and 22.69–27.74 kg/m2 for women and men, respectively. Compared with couples in whom both partners had a normal BMI, the pregnancy rates were reduced by 10% (fecundability odds ratio [FOR] 0.90; 95% confidence interval [CI], 0.88–0.92) and 19% (FOR 0.81; 95% CI, 0.78–0.84) for couples in whom both partners were underweight or obese, respectively. The combination of women with a normal BMI and overweight men had the greatest fertility (FOR 1.03; 95% CI, 1.02–1.03), and the combination of obese women and underweight men had the lowest fertility (FOR 0.70; 95% CI, 0.65–0.76). Conclusion(s) Abnormal prepregnancy BMIs in either women or men were associated with prolonged TTP among couples with no prior gravidity. Scientific management of couples’ prepregnancy BMI could improve their fertility.