system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens to a fit, pregnant mother's abdominal muscles during and after pregnancy?
gpt
They get stretched out, and can separate, causing diastasis recti.  Not everyone experiences this condition, and many times the muscles just go back to normal.  Your ligaments and joints get loose as well, all caused by certain pregnancy hormones that enable a women's hips to shift during birth.  All of this usually goes back to normal in the months after birth.  Binding your belly after birth can help your muscles come back together as well, at least anecdotally.  Your skin will almost always stay a little looser than before.  

FYI, there are many exercises you can do to strengthen your ab muscles after birth (wait a couple months minimum though, to heal) and these are usually not your typical "ab strengthening" exercises/movements.  Crunches will not help, and can often make diastasis recti worse.  A quick google search will help you find something.