Background: To analyze (Aim 1) if there is an association between maintaining breastfeeding for 4 months after delivery and postpartum depression (n = 160) and (Aim 2) what sociodemographic, obstetric and psychosocial factors are associated to the presence of postpartum depression among the women who continue to breastfeed for 4 months after delivery (n = 81). Methods: A total of 160 women who initiated exclusive breastfeeding within an hour of childbirth participated for Aim 1. For Aim 2, a total of 81 of these same participants, specifically those who continued breastfeeding 4 months after giving birth, constituted the final sample. For Aim 2, a prospective design was used, which included four stages. During the first trimester of pregnancy, pregnancy worries, extroversion and neuroticism, external locus of control and psychiatric symptoms were assessed. In the third trimester of pregnancy, psychiatric symptoms were assessed again. Immediately after childbirth, the initiation of exclusive breastfeeding was recorded, and four months after childbirth, both postpartum depression scores and the continuation of breastfeeding were assessed. Likewise, different sociodemographic and obstetric variables were assessed, including age, type of delivery, pain during and after delivery, among others. Results: In relation to Aim 1, no statistically significant associations were found between postpartum depression and the continuation of breastfeeding four months after childbirth. In relation to Aim 2, the presence of postpartum depression among the women who continued to breastfeed was associated to higher scores on neuroticism, pregnancy worries, external locus of control and anxiety and depression during both trimesters, as well as with lower extraversion scores. The results of binary logistic regression showed that, of the above variables, anxiety during the third trimester was the strongest predictor. Conclusions: It seems as though it would be necessary to assess psychosocial risks to be able to prevent postpartum depression and therefore improve mothers’ wellbeing during breastfeeding.