To evaluate in a rural Tanzanian birth cohort the association between birth timing in relation to the preharvest lean season and early-life growth and cognitive development.

Children were enrolled within 14 days of birth and followed up for 18 months. Child anthropometry was measured every 3 months. The Malawi Developmental Assessment Test was administered at the end of follow-up. We estimated the association between timing of birth in the context of other early childhood risk factors and both growth and Malawi Developmental Assessment Test scores.

Children born in the preharvest months September and October had the lowest cognitive scores at 18 months, compared with birth in July and August (−1.05 change in overall Malawi Developmental Assessment Test development-for-age Z score, 95% CI: −1.23, −0.86). This association was observed for the language (−1.67 change in development-for-age Z score; 95% CI: −1.93, −1.40) and fine motor subcomponent scores (−1.67; 95% CI: −1.96, −1.38) but not for gross motor (−0.07; 95% CI: −0.23, 0.10) or social subcomponents (−0.07; 95% CI: −0.23, 0.10). Children born in September and October were the longest at birth but had the largest declines in growth Z scores during the first 6 months.

There was a strong association between birth at the beginning of the preharvest season and poor growth and cognitive development. If these associations were mediated by the preharvest postnatal environment, targeted maternal and child interventions for children born during high-risk periods may improve these outcomes.

Trial registration: NCT03268902 (NCT03268902).