Since birth #2 or 3, I have had a gap in the middle of my abdominal muscles that has grown a little with each pregnancy. (It's called abdominal separation, or more technically, diastasis recti.) Now I can easily put two fingertips into the gap. If you want to check your own abdominal muscles, you can do this:
Lie on your back on the floor with your knees bent and feet flat. Place the fingers of one hand across your middle just above your navel, and the other hand under your head for support. Slowly curl your head and chest toward your navel, using stomach muscles and not neck muscles to do it. As you do, feel across your abs with your fingertips. Is there a dip in the middle? Is it two or more fingertips wide? Do you have a small mound protruding in the middle? If so, that is abdominal separation. You have plenty of company! It occurs pretty frequently for women at the end of pregnancies because of the expanding uterus within and because the connective tissue in the middle was softened by pregnancy hormones.
When I first started seeking information about this, the first thing I learned was that everything I had tried before was all wrong! Crunches and bends do nothing to close the gap, and can actually cause harm. I almost hurt myself doing these exercises last time, and they didn't seem to help shape me up at all.
Now I know that exercises like crunches and sit-ups only work the outer layer of abdominal muscles (the "six-pack" layer). What needs to happen is a slower process of strengthening and shortening the inner, foundational layers before working the outer ones. If you are in the postpartum stage, and even months or years postpartum, it might be helpful to seek out one of those workout methods that involve "core" strengthening. These tend to include exercises that have slow, controlled motions that work the inner muscle layers.
As for me, I am approaching the 12th week of pregnancy, and will not be starting any new exercise regimen. Instead, I'm concentrating on just tightening those core abdominal muscles, holding them in as I go about daily activities. Shoulders centered, erect posture, and a tight abdomen just while I'm working or walking about seem to be helping already.