Background: Breastfeeding provides vital nutrients and fosters maternal–infant bonding, benefiting both mother and child. However, breastfeeding duration is influenced by various factors, especially in women of high-risk pregnancy. This study aims to identify predictors of breastfeeding duration among women of high-risk and low-risk pregnancy, focusing on exclusive and mixed feeding practices. Methods: Conducted at a public hospital in Attica, Greece, this 20-month prospective cohort study (May 2020–January 2022) included 380 pregnant women, divided into high-risk and low-risk groups. The final sample of 157 women, after excluding non-breastfeeding participants, was assessed across five phases from prenatal to six months postpartum using interviews, calls, and surveys. Multiple linear regression identified key predictors, with statistical methods applied. Results: Results revealed birth weight as a consistent significant predictor of breastfeeding duration. For women with high-risk pregnancy, additional factors like infant gender, jaundice, and early introduction of solid foods influenced breastfeeding duration. The mixed breastfeeding model for women with high-risk pregnancy explained 72.9% of the variance. Exclusive and mixed breastfeeding models for women with low-risk pregnancy also highlighted birth weight’s influence. Conclusion: The findings highlight the important role of birth weight and other specific factors in determining breastfeeding duration among women of high-risk and low-risk pregnancy. Further research is needed to validate these findings across diverse populations.