High-risk fertility behaviours, particularly early maternal age, advanced maternal age, short birth intervals, and high birth order, remain prevalent in Nigeria and are associated with adverse child survival outcomes. While previous studies have documented individual risk effects, limited recent evidence quantifies the combined impact of clustered fertility risks and their population-level contribution to neonatal and infant mortality using updated national data. This study analyzed nationally representative data from the 2024 Nigeria Demographic and Health Survey (NDHS). Live births occurring within five years preceding the survey were included. High-risk fertility behaviours were classified according to the standard Demographic and Health Survey definitions and categorized into single- and multiple-risk exposures. Multivariable logistic regression models, adjusted for socioeconomic and healthcare-related covariates, were used to estimate associations with neonatal and infant mortality. Population Attributable Fractions (PAFs) were calculated to estimate the proportion of deaths attributable to high-risk fertility behaviours. Neonatal and infant mortality rates were substantially higher among births exposed to high-risk fertility behaviours compared to those with no risk exposure. After full adjustment, early maternal age (