Objective Low mood is common during the perinatal period, which may negatively impact breastfeeding practices. Exploring predictors of successful breastfeeding is a health priority area. This study investigated if maternal well-being during pregnancy is associated with breastfeeding practices. Study Design This is a secondary analysis of a randomized control trial of a low glycemic index diet in pregnancy. A total of 610 secundigravida women were recruited in the National Maternity Hospital, Dublin, Ireland. Data on maternal education attainment, early pregnancy body mass index (BMI), and age were collected from hospital records. Well-being was self-reported by mothers between 10 and 28 weeks' gestation using the World Health Organization 5-Item well-being index. Scores were transformed to give percentage well-being. Mothers recorded breastfeeding practices at hospital discharge and at the study follow-up appointments. Chi-squares and independent t-tests determined initial differences in breastfeeding practices. Multiple and logistic regression analyses were used to adjust for confounders. Results Average maternal age was 32.7 years; average BMI was 26.6 kg/m2, and 56% had achieved third-level education. The average well-being score was 58.2%. In unadjusted analysis, high well-being scores were associated with exclusive breastfeeding (56.2% breastfed vs. 46%, breastfed p < 0.03). After adjusted analysis, these associations were no longer significant (odds ratio: 1.00, 95% confidence interval: 0.99–1.01). No other associations were found. Conclusion Our findings indicated 25% of pregnant women in the first trimester reported low well-being scores. Associations between maternal well-being and breastfeeding patterns were explained by maternal age and education level, suggesting low mood may not be a barrier to breastfeeding initiation or duration. This trial is registered at: https://www.isrctn.com/ ISRCTN54392969. Key Points