Title: Nitrates in Drinking Water and Congenital Anomalies in Aotearoa New Zealand

Background:
Nitrate contamination of drinking water is an increasing public health concern, particularly in agricultural regions. Emerging evidence suggests prenatal nitrate exposure may be associated with adverse birth outcomes, including congenital anomalies. In Aotearoa New Zealand, groundwater nitrate levels vary widely, yet population-level evidence on adverse birth outcomes is limited. This study investigates the association between maternal nitrate exposure during pregnancy and congenital anomalies.

Methods:
We conducted a retrospective cohort study using linked administrative health data from the Integrated Data Infrastructure. The cohort included all live births in Aotearoa New Zealand from 2008–2021. Congenital anomalies were identified using ICD-10-AM Chapter Q codes from hospital discharge data within 24 months of life and classified using International Clearinghouse (ICBDSR) criteria for major anomalies. Maternal nitrate exposure was estimated using residential address at delivery linked to drinking water monitoring data. Associations were analysed using generalised estimating equations to account for clustering within mothers, adjusting for maternal age, parity, ethnicity, BMI, smoking status, urban–rural classification, water supply type, and season of conception.

Results:
A total of 854,472 births met eligibility criteria, with 68,075 children identified with congenital anomalies. Of these, 13,495 were classified as major anomalies. Preliminary results from the generalised estimating equations analyses will be presented.

Conclusion:
This study provides the first national-level assessment of nitrate exposure and congenital anomalies in Aotearoa New Zealand. Findings will inform drinking water standards and public health policy, particularly in high-exposure regions.