Gestational age, kindergarten-level literacy, and effect modification by maternal socio-economic and demographic factors.

Background: Shorter gestational age at birth is associated with worse academic performance in childhood. Socio-economic and demographic factors that affect a child's development may modify the relationship between gestational age and later academic performance.

Objective: The purpose of this study was to investigate socio-economic and demographic effect modification of gestational age's association with kindergarten-level literacy skills in a longitudinal Wisconsin birth cohort.

Methods: We sampled 153 145 singleton births (2007-2010) that linked to Phonological Awareness Literacy Screening-Kindergarten (PALS-K) scores (2012-2016 school years). PALS-K outcomes included meeting the screening benchmark (≥28 points, range 0-102 points) and the standardised score. Multivariable linear regressions of PALS-K outcomes on gestational age (completed weeks) included individual interactions for five maternal attributes measured at delivery: Medicaid coverage, education, age, race/ethnicity, and marital status.

Results: Each additional completed gestational week was associated with a 0.5 percentage point increase in the probability of meeting the PALS-K literacy benchmark. The benefit of an additional week of gestational age was 0.5 percentage points (95% confidence interval 0.3, 0.7 percentage points) greater for Medicaid-covered births (0.8 percentage points) relative to non-Medicaid births (0.3 percentage points). Relative to only completing high school, having college education weakened this association by 0.3-0.6 percentage points, depending on years in college. Similar but modest relations emerged with standardised scores.

Conclusions: Socio-economic advantage as indicated by non-Medicaid coverage or higher levels of completed maternal education may diminish the cost of preterm birth on a child's kindergarten-level literacy skills.