Why do studies show different associations between intrauterine exposure to maternal smoking and age at menarche?

PURPOSE: Studies suggests that intrauterine exposure to maternal smoking both accelerates or delays age at menarche. We hypothesize that these opposing findings relate to different infant and childhood growth patterns across cohorts.

METHODS: Using data from an adult follow-up study of the Child Health and Development Studies and the National Collaborative Perinatal Project, we examined, using generalized estimating linear regression models, whether intrauterine exposure to maternal smoking was associated with age at menarche in 1090 daughters before and after accounting for growth in weight.

RESULTS: Compared to the nonexposed, intrauterine exposure to maternal smoking was associated with a 4-month acceleration in menarche in the National Collaborative Perinatal Project (β = -0.35 years; 95% confidence interval [CI]: -0.63, -0.08), but a 6-month delay in menarche in the Child Health and Development Studies (β = 0.48 years; 95% CI: 0.13, 0.83), despite having a similar reduction in birth weight in both cohorts (∼300 g). The results were more consistent across cohorts when we stratified by postnatal growth patterns. For example, in those with rapid weight gain (increasing two growth references from 0 to 4 years), intrauterine exposure to maternal smoking was related to a 7-month acceleration in menarche (β = -0.56 years; 95% CI: -0.95, -0.17).

CONCLUSIONS: These findings suggest that the association of intrauterine exposure to maternal smoking on age at menarche depends on postnatal growth patterns.