Socio-demographic characteristics of mothers have been associated with exclusive breastfeeding duration, but little is known about the association with maternal full- and part-time employment and return to work in European countries. To study the associations between breastfeeding, any and almost exclusive (infants receiving breast milk as their only milk) breastfeeding, at 4 months of infant's age and the socio-demographic and occupational characteristics of mothers. We used the EDEN mother-child cohort, a prospective study of 2002 singleton pregnant women in two French university hospitals. We selected all mothers (n = 1,339) who were breastfeeding at discharge from the maternity unit. Data on feeding practices were collected at the maternity unit and by postal questionnaires at 4, 8 and 12 months after the birth. Among infants breastfed at discharge, 93% were still receiving any breastfeeding (83% almost exclusive breastfeeding) at the 3rd completed week of life, 78% (63%) at the 1st completed month, and 42% (20%) at the 4th completed month. Time of return to work was a major predictor for stopping breastfeeding: the sooner the mothers returned to work, the less they breastfed their babies at 4 months of infant's age, independently of full-time or part-time employment. The association was stronger for almost exclusive breastfeeding mothers than for any breastfeeding ones. In a society where breastfeeding is not the norm, women may have difficulties combining work and breastfeeding. Specific actions need to be developed and assessed among mothers who return to work and among employers.