Exclusive breastfeeding for the first six months of life (EBF) is essential for infant survival and development, yet it remains challenging in low-income urban settings where mothers face substantial time and workload constraints. This study examines whether the degree of marital union formalization at time of pregnancy is associated with EBF among young mothers in Nairobi’s informal settlements, and whether pregnancy-period support from partners and kin helps explain this association.

We use retrospective survey data from 1,203 mothers in Nairobi, Kenya. EBF is reconstructed from the reported timing of first introduction of any non-breastmilk liquids and solid/semi-solid foods. UF is measured on a 0–5 scale based on the timing of key union steps reached during pregnancy (e.g., co-residence, kin introductions, bridewealth-related steps, and formal ceremony). We estimate staged logistic regression models and mediation models in a potential-outcomes framework, interpreting mediation as an associative decomposition given the observational design.

Mothers in more formalized unions during pregnancy had higher odds of EBF for six months. UF was also associated with higher odds of receiving partner support during pregnancy, particularly help with household chores. Mediation analyses suggest a small indirect association through partner housework support, while most of the UF–EBF association remains unexplained by the measured support indicators. These findings highlight the relevance of pregnancy-period relationship contexts and partner engagement for understanding breastfeeding patterns in poor urban settings.