I do not know why is it important for periods to stop when a women gets pregnant, but, I can tell you what causes them to stop. When a women starts getting pregnant, the egg enters the uterus, where it co-insides with the sperm. When this happens, our body starts making progesterone. As a result, the female stops producing eggs for the next nine months. During this time the cervix remain closed, thus, women do not get their periods. Ninety-eight per cent women do not experience their periods due to this reason. The other two per cent who think they are experiencing their periods but they are actually experiencing spotting or implantation bleeding. They could also be having a miscarriage without her knowing about it yet.