Battering often starts, or increases, when a woman is pregnant. If you're pregnant, you're three times more likely be battered. Moreover, while most women are beaten on the face and chest, pregnant women are beaten on the abdomen. One can speculate that the batterer feels threatened by the potential loss of your attention, by increased economic pressures resulting from having a baby, or by thoughts of his own mother and the unsatisfactory support he received. It's difficult for people who haven't experienced domestic violence to understand why someone stays in a battering relationship.
Many victims do leave, but the reasons why many others stay are complex.
If I leave, he'll beat me worse when he finds me. A crazy father is still better than no father at all. Religious and family pressures to keep the family together. If he had a broken leg or cancer, I would stay with him. This is no different.