Objective: Analyse the parental behaviours that are recognized as influencing the health of very young children based on family structure (parents separated or not). Design: Cross-sectional study, examining 79,701 infants aged 7 to 11 months as part of a free preventive medicine consultation. The data came from an assessment conducted 7 to 11 months after birth during which information was collected, namely about the parents' use of tobacco, the infant's type of nutrition, and adherence to vaccination schedules. Results: The percentage of infants whose parents were separated was 6.6%. After adjusting for the cultural and socioeconomic environment as well as for other potential confounders, in the event of separation as compared with non-separated parents, the adjusted ORs (95% CI) were as follows: 1.5 (1.3–1.7) for the infant's exposure to tobacco; 1.3 (1.2–1.4) for total lack of exclusive breastfeeding; 1.3 (1.1–1.4) and 1.2 (1.1–1.2) for breastfeeding for a duration of less than 3 and 6 months, respectively; 1.2 (1.1–1.4) for noncompliance with the vaccination schedule against rotavirus. The duration of exclusive breastfeeding was shorter when parents were separated (p<0.001; median: 10 vs. 13 weeks). Conclusions: This study reinforces the possibility that parental separation is independently associated with certain parental at-risk behaviours regarding the children’s health. This observation should be verified because this could result in major consequences for the work of family doctors, in particular in terms of parent information and targeted prevention.