According to Canadian public health recommendations, newborns should be fed exclusively with breast milk during their first 4 to 6 months of life. The aim of this paper is to identify the main social determinants of initiation, duration and exclusivity of breastfeeding from birth to 4 months, at the population level. The results will help the development of public health interventions aimed at improving the prevalence of exclusive breastfeeding for at least the first 4 to 6 months of babies' life. The analyses were performed with the data of the Longitudinal Study of Child Development in Quebec (ELDEQ 1998-2002). The study follows a representative sample (n = 2,223) of the children born in 1998 in the province of Quebec, in Canada. The relations between breastfeeding (all and exclusive) and mothers' education level, annual family income, family type, parents' working situation and mothers' age group have been evaluated by crude and adjusted odds ratios. Multivariate analyses were used to identify the most influential factors. In 1998, nearly three quarters (72%) of the Quebec newborns were breastfed at birth. When they reached 4 months, only 6% of the Quebec children were exclusively breastfed, thus following public health recommendations. Mothers' education level remains the strongest factor of influence on breastfeeding from birth to 3 months, and its impact increases with baby's age. At 4 months, it is also the most important factor for all breastfeeding, but mothers' age is more important for exclusive breastfeeding. When all the studied factors are considered equal for all children, being breastfed in accordance with public health recommendations, which refers to exclusive breasfeeding for at least 4 months, is mainly influenced by mothers' age, followed by mothers' education level. Family income, family type and parents' working situation do not influence exclusive breastfeeding at 4 months. It should be noted that when considering all breastfed children, if mothers' age and education level are equal, annual family income shows a negative relationship with breastfeeding at any of the studied ages. This negative relationship is also observed at 2 months for exclusive breastfeeding (dollar 40,000-dollar 59,999). From a public health perspective, it is important that newborns be breastfed at birth and exclusively breastfed during their first 4 to 6 months of life. The analysis indicates that once the decision to breastfeed is made, different social factors influence the duration of breastfeeding and its exclusivity. Public policy targets are necessary to evaluate whether the situation is improving from one year to another and whether disparities are increasing or decreasing for newborns accumulating the impact of material and social deprivation early in life.