The reversing association between advanced maternal age and child cognitive ability: evidence from three UK birth cohorts.

Background: Studies on advanced maternal age-defined here as age 35 or older-and children's cognitive ability report mixed evidence. Previous studies have not analysed how the time period considered in existing studies influences the association.

Methods: We analysed trends in the association between maternal age and cognitive ability using data from the 1958 National Child Development Study ( n  = 10 969), the 1970 British Cohort Study ( n  = 9362) and the 2000-2002 Millennium Cohort Study ( n  = 11 600). The dependent variable measures cognitive ability at age 10/11 years. Cognitive scores were standardised to a mean of zero and a standard deviation of one.

Results: For the 1958-70 cohort studies, maternal ages 35 -39 were negatively associated with children's cognitive ability compared with maternal ages 25-29 (1958 cohort β = -0.06 standard deviations (SD) 95% confidence interval (CI): -0.13, -0.00; 1970 cohort β = -0.12 SD 95% CI: -0.20, -0.03). By contrast, for the 2000-2002 cohort study maternal ages 35-39 were positively associated with cognitive ability (β = 0.16 SD 95% CI: 0.09, 0.23). For maternal ages 40+, the pattern was qualitatively similar. These cross-cohort differences were explained by the fact that in the earlier cohorts advanced maternal age was associated with high parity, whereas in the 2000-2002 cohort it was associated with socioeconomically advantaged family background.

Conclusions: The association between advanced maternal age and children's cognitive ability changed from negative in the 1958 and 1970 cohorts to positive in the 2000-2002 cohort because of changing parental characteristics. The time period considered can constitute an important factor in determining the association between maternal age and cognitive ability.