Nitrate in drinking water and preterm birth: a national retrospective cohort study in New Zealand

Preterm birth (PTB) is a leading cause of mortality in children aged <5 years, and survivors experience increased chronic morbidity. Emerging evidence suggests that nitrate in drinking water may increase the risk of PTB.

We aimed to assess whether prenatal exposure to nitrate in drinking water is associated with PTB.

We conducted a national cohort study using linked administrative health data for all live singleton births in New Zealand between January 1, 2008, and December 31, 2021 (n = 735,831). Prenatal nitrate exposure from public supplies and domestic self-supplied drinking water was characterised as a continuous measure (mg/L nitrate-nitrogen) and by quartiles. PTB was classified by gestational age as extremely early preterm (EPT, 20-27 weeks), very early preterm (VPT, 28-31 weeks), moderate preterm (MPT, 32-36 weeks), and all PTB (20-36 weeks). Associations were estimated using generalized estimating equations, adjusting for maternal age, parity, ethnicity, body mass index, smoking status, urban-rural classification, water supply type, and season of conception.

Overall, 5.7% of singleton births were preterm (MPT 4.8%, VPT 0.6%, EPT 0.3%). Increasing nitrate concentration was associated with higher odds of all PTB (aOR 1.01; 95% CI 1.01-1.02) and each PTB category, with stronger associations for more severe outcomes. Compared with the lowest quartile (<0.04 mg/L), exposure to the highest quartile (>0.66 mg/L) was associated with increased odds of all PTB (aOR 1.08; 95% CI 1.05-1.12) and EPT (aOR 1.17; 95% CI 1.03-1.32). Under a causal assumption, population attributable fraction analysis suggested approximately 120 (95% CI 25-207) or 4% PTB per year were attributable to nitrate exposure.

Higher nitrate concentrations in drinking water were associated with increased odds of PTB. While residual confounding and exposure misclassification cannot be excluded, our findings are consistent with prior large cohort studies and suggest that low-level nitrate exposure may contribute to PTB risk.