During pregnancy and childbirth, the abdominal and pelvic floor muscles undergo a great amount of stretching. In addition, you may have gained additional fat during your pregnancy due to a lack of ability to exercise or overeating. To gain back your core strength and flatten your stomach after giving birth, you should first wait 6 weeks before exercising and then gradually build up the intensity of your strength training and cardiovascular workout.
Let Nature Do Its Job
When it is not growing a baby, the average uterus is approximately the size of a thumb. Over the course of your pregnancy, it stretches to the size of your baby. After childbirth, it is the size of a grapefruit, and 6 weeks after childbirth, it will have shrunk to the size of a pear. For the stomach to truly flatten after pregnancy, the uterus has to revert back to its normal size. This occurs more quickly if you are breastfeeding.
While this can be frustrating for you if you've been used to having a flat stomach, having expectations of an immediate return to normal will cause you undue stress. The situation is out of your control and will resolve itself to a great degree on its own.
Seek Medical Clearance
In addition to the fact that it takes at least 6 weeks for your body to naturally reshape itself after childbirth, if you've had a normal vaginal delivery you should wait until after your 6-week checkup to begin exercising. This will allow your doctor to ensure that your organs have recovered. At that time, the doctor can check that any splitting of the rectus abdominus muscle (diastasis recti) that may have occurred during pregnancy no longer exists. You should not engage in any kind of intense exercise without a doctor's clearance, nor should you lift anything heavier than your baby.
If you've had a Cesarean section, you must wait, on average, 8 weeks and should never begin exercising without a doctor's clearance.
Once you receive clearance, you can begin a wide variety of exercises to re-firm the abdominals. These include the basic crunch, bicycle crunch, supine bridge and reverse crunch. As you begin your abdominal rehabilitation, you should exercise until fatigued or until you complete eight repetitions of each exercise—stopping at whichever comes first. After a week, you can work up to 12 repetitions and then eventually up to 20 repetitions once your rehabilitation is complete. Exercises should focus on the rectus abdominus, internal obliques, external obliques and pelvic floor.
Cardiovascular exercise will allow you to lose any fat that may have accumulated around your middle due to weight gain. It also has great psychological benefits for new mothers. Physical activity after pregnancy has been shown to decrease postpartum depression. All early cardiovascular exercise should be enjoyable and at a lower intensity than you would have engaged in before your pregnancy. Your endurance must be built up gradually to allow your body to fully recover.