Maternal nutritional status mediates the association between maternal age and birth outcomes

Young maternal age during pregnancy is linked with adverse birth outcomes. This study examined the role of maternal nutritional status in the association between maternal age and small for gestational age (SGA) delivery and birth length. We used data from a birth cohort study in Ethiopia, involving women who were 15-24 years of age and their newborns. A mediation analysis was fitted in a sample of 1,422 mother infant dyads for whom data on birth length were available, and 777 dyads for whom gestational age and birth weight was measured. We used commands, medeff for the mediation analysis and medsens for sensitivity analysis in STATA 14. Maternal nutritional status, measured by mid-upper arm circumference, mediated 21% of the association between maternal age and birth length and 14% of the association with SGA delivery. The average direct effect (ADE) of maternal age on birth length was (β = 0.45, 95% CI [0.17, 0.99]) and the average causal mediated effect (ACME) was (β = 0.12, 95% CI [0.02, 0.15]). We also found an ADE (β = 0.31, 95% CI [0.09, 0.47]) and an ACME of (β = 0.05, 95% CI [0.003, 0.205]) of maternal age on SGA delivery. The sensitivity analysis suggests an unmeasured confounder with a positive correlation of 0.15 and 0.20 between the mediator and the outcome could explain the observed ACME for birth length and SGA, respectively. We cannot make strong causal assertions as the findings suggest the mediator partly explained the total effect of maternal age on both outcomes.