Abstract
Background: We examined the associations of maternal age with low birthweight (LBW) and preterm birth in four cohorts from a middleand a high-income country, where the patterning of maternal age by socio-economic position (SEP) is likely to differ. Methods: Population-based birth cohort studies were carried out in the city of Pelotas, Brazil in 1982, 1993, and 2004, and in Avon, UK in 1991 [Avon Longitudinal Study of Parents and Children (ALSPAC)]. Adjustment for multiple indicators of SEP were applied. Results: Low SEP was associated with younger age at childbearing in all cohorts, but the magnitudes of these associations were stronger in ALSPAC. Inverse associations of SEP with LBW and preterm birth were observed in all cohorts. U-shaped associations were observed between maternal age and odds of LBW in all cohorts. After adjustment for SEP, increased odds of LBW for young mothers (<20 years) attenuated to the null but remained or increased for older mothers (≥35 years). Very young (<16 years) maternal age was also associated with both outcomes even after full SEP adjustment. SEP adjusted odds ratio of having a LBW infant in women <16 years and ≥35 years, compared with 25–29 years, were 1.48 [95% confidence interval (CI) 1.00, 2.20] and 1.66 [95% CI 1.36, 2.02], respectively. The corresponding results for preterm birth were 1.80 [95% CI 1.23, 2.64)] and 1.38 [95% CI 1.15, 1.67], respectively. Conclusion: Confounding by SEP explains much of the excess risk of LBW and preterm among babies born to teenage mothers as a whole, but not for mothers aged <16 or ≥35 years. Given that the proportion of women becoming pregnant at <16 years is smaller than for those ≥35 years, the population burden is greater for older age.
Highlights
We examined the associations of maternal age with low birthweight (LBW) and preterm birth in four cohorts from a middle- and a high-income country, where the patterning of maternal age by socio-economic position (SEP) is likely to differ
We have focused on LBW and preterm birth because of the robust associations of both of these with infant morbidity and mortality[19,20] and with lifelong adverse health outcomes,[21] and because these are the two commonest outcomes examined in previous studies of maternal age with adverse perinatal outcomes.[8]
The prevalence of LBW tended to be highest among adolescents in the Pelotas cohorts and among mothers older than 34 years in Avon Longitudinal Study of Parents and Children (ALSPAC)
Summary
We examined the associations of maternal age with low birthweight (LBW) and preterm birth in four cohorts from a middle- and a high-income country, where the patterning of maternal age by socio-economic position (SEP) is likely to differ. In comparison with adjusting for all potential SEP indicators in our studies, when we adjusted only for confounding by maternal education, the association of young maternal age with LBW attenuated less and that of older maternal age with LBW increased less in the Pelotas 1982 cohort, but for all of the other cohorts, there was no marked difference between the two models (Table 2).
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