Postpartum weight retention (PPWR) is associated with an increased risk for maternal obesity and is discussed to be influenced by breastfeeding. The objective was to evaluate the effect of a lifestyle intervention delivered three times during pregnancy and once in the postpartum period on PPWR and on maternal breastfeeding behavior. In total, 1998 participants of the cluster-randomized “healthy living in pregnancy” (GeliS) trial were followed up until the 12th month postpartum (T2pp). Data were collected using maternity records and questionnaires. Data on breastfeeding behavior were collected at T2pp. At T2pp, mean PPWR was lower in women receiving counseling (IV) compared to the control group (C) (−0.2 ± 4.8 kg vs. 0.6 ± 5.2 kg), but there was no significant evidence of between-group differences (adjusted p = 0.123). In the IV, women lost more weight from delivery until T2pp compared to the C (adjusted p = 0.008) and showed a slightly higher rate of exclusive breastfeeding (IV: 87.4%; C: 84.4%; adjusted p < 0.001). In conclusion, we found evidence for slight improvements of maternal postpartum weight characteristics and the rate of exclusive breastfeeding in women receiving a lifestyle intervention embedded in routine care, although the clinical meaning of these findings is unclear.