Diastasis, aka abdominal separation, occurs mostly in pregnant women and is generally tested for postpartum. The condition occurs where the rectus abdominis, top layer of abdominal muscles, (the 6 pack muscle) spreads apart at the body’s mid-line, the linea alba, creating a gap in the abdominal wall. The force of the ever expanding uterus on the abdominal wall, combined with pregnancy hormones that soften connective tissues create the separation. This separation generally occur during the second half of pregnancy, but it creates more problems postpartum by weakening the abdominal wall.
For more information, see: What's the Deal With: Diastatsis.