Association Between Women's Birth Weight and Reproductive Characteristics in Adulthood: The JPHC-NEXT Study.

BACKGROUND: We aimed to investigate the association between women's birth weight and their reproductive characteristics. METHODS: We used data from the Japan Public Health Center-based Prospective Study for the Next Generation (JPHC-NEXT), a population-based cohort in Japan. The main analysis included 40,796 women aged 40 to 68 years. Outcomes of interest were age at menarche, age at menopause, history of menstrual irregularity, and nulliparity. Associations between self-reported birth weight categories and outcomes were assessed using either a linear regression or a modified Poisson regression adjusted for potential confounders. RESULTS: Among participants with complete data, those with lower birth weights (<1,500 g and 1,500-2,499 g) compared to women with a birth weight of 3,000-3,999 g had a later age at menarche (adjusted mean difference [aMD]: 2.4 months and 2.0 months, respectively), earlier age at menopause (aMD: -6.7 months and -2.7 months, respectively), and therefore a shorter reproductive span (aMD: -7.7 months and -4.5 months, respectively). They also had a higher risk of menstrual irregularity (adjusted relative risk [aRR]: 1.19 and 1.11, respectively) and a higher likelihood of nulliparity (aRR: 1.25 and 1.19, respectively). CONCLUSION: We observed that Japanese women's birth weight was significantly associated with reproductive characteristics. Specifically, those with a low birth weight had a shorter reproductive span and a higher risk of irregular menses and nulliparity compared to those with a normal birth weight.