Pain below the breasts is common in pregnancy as the body undergoes various physiological changes at this time to prepare for the growth of the foetus. These pains can also be caused by an enlarged liver, gall bladder problems, the position of the foetus or constant changes in the structure of the breasts. Heartburn and acid reflux are also common painful symptoms during pregnancy. As the baby grows, the diaphragm may also move upwards, increasing the abdominal space and the pressure in the chest cavity, leading to pain under the breasts. When resting on your side, a change in the position of the organs in the abdominal cavity may cause pain under the breast.