The interaction between breast and infant's mouth, during breastfeeding, is a complex dynamic mechanism. It remains unclear which factor plays the key role in removing milk from the breast: the intra-cavity vacuum pressure or the compressive pressure of the tongue. While there are extensive clinical data on collection of vacuum pressure, limited data exist for the positive pressure values on the breast areola exerted by infant's mouth. The goal of this study is to use a methodology to capture these positive pressure values exerted by maxilla and mandible on the breast areola during breastfeeding. In this study, the positive and negative (vacuum) pressure values are obtained simultaneously on seven lactating mothers successfully. Parallel to the pressure data measurements, ultrasound images are captured and processed to reveal the nipple deformations and the displacements of infants' tongues and jaw movements during breastfeeding. At the end, motivated by the significant differences in composition between the tissue of the breast and the nipple-areola complex, the Poisson's ratio values of the lactating nipples are obtained using these deformation measurements.