Before performing any abdominal exercises, all moms should check for abdominal separation (diastasis recti). Abdominal weakness, hormonal changes, pregnancy, c-sections can all cause this separation. If you do traditional abdominal exercises (such as crunches) with a separation, those exercises will only make the diastasis worse.
To do a self-check, lie on your back with your knees bent. With your fingers pointing down toward your feet, hold two fingers flat on your bellybutton. Press your fingers down as you slowly lift your head (keep your shoulders on the ground). Do you feel a gully between the two muscles? Measure how many fingers wide it is. If you can fit more than two fingers inside, you should not do crunches or sit ups. If this is you, I would recommend visiting the post below about diastasis exercises.