I had a Google for you and found this.
Due to the extra hormones in your body during pregnancy, muscles start to become softer and more flexible. In some women, the two parallel ‘rectus abdominis’ muscles (the muscles that run down the front of the abdomen on either side of your navel) that give you your six-pack abs (or not!) may part to allow room for your expanding uterus.
Benni.
P.S All I can find on it, is pregnancys mate so not much help.