During pregnancy, the abdominal muscles separate to accommodate your growing baby. Or at least, this happens to the vast majority of expectant mothers. It's easy to understand that your baby won't fit neatly behind a wall of strong abdominal muscles! Once you give birth, your joints are looser than before (useful during labor and delivery!) and your abs are still separated. Doing ab crunches will strengthen your abdominal muscles, but it will not cause your abs to move together again.
That is a gradual natural process that may require a lot of time. Doing ab crunches or similar abdominal exercises might in fact cause permanent diastasis recti; the condition during which your abdominal muscles are separated. In addition, your loose joints may cause you to strain your body too much.
Sports injuries are easier to acquire in the postpartum period. After pregnancy, walking, light yoga, or swimming are better exercises. In other words, whole-body exercises that don't focus straight on your abs are a safer bet for a while. You don't want abdominals that are permanently detached, with an empty "river" of flesh in the middle. General exercises that target your overall fitness will also help with weight loss after pregnancy, if you need it.
You should wait at least six weeks before doing ab crunches, and you should check whether your abs have moved back together before trying any. If in doubt, your doctor can be a useful source of information and you can also ask about it at your gym, if they have fitness experts who know anything about exercising after pregnancy available to you.