Extracting milk during breastfeeding is not only caused by intra-oral vacuum pressure by the infant suckling but also is the periodic motion of the infant’s jaw, which is the focus of this study. A Fluid-structure interaction simulation provides a better understanding of the milk flow behavior in the human breast ductal system as the breast interacts with the infant’s oral cavity and jaws. Simulations were performed from the instance of latching and continued for two cycles of periodic tongue motion. The negative vacuum pressure profile was measured from the clinical data and applied in the simulation. The nipple dimensions were obtained using ultrasound images and used as boundary conditions in the simulation. The effect of vacuum pressure and the peripheral pressure on the milk flow behavior in breast ductal system were studied individually. It was observed that the deformation of the ductal system has a critical impact on milk flow behavior and the amount of expressed milk.