A group of muscles that often gets neglected in stomach exercise routines are the transverse abdominals, the core muscles that lie below the rectus abdominus. Most abdominal exercises target the rectus abdominus and the vertical abdominals, ignoring the transverse abdominals. Even crunches, the staple of most abdominal workouts, do nothing for the transverse abdominals. These muscles are actually the most important to target, however, as they connect to both the lower back muscles and the rectus abdominus and for a girdle for the entire abdomen. Any routine aimed at flattening the stomach should include the transverse abdominals as a focus. Using the following exercises, you can work out your transverse abdominals and really make progress on that flat tummy. As with any workout routine, be sure to consult a professional before beginning and always warm up properly to avoid injury.

Pelvic Tilts
This stomach exercise requires lying on your back on a flat surface, such as the floor or a bench. Use a mat or towel to cushion your spine. Bend your knees so that your feet are flat on the floor. Raise your pelvis (and only your pelvis) off the floor, hold momentarily, and then lower it back down. Repeat for an entire set. Maintaining a controlled movement is crucial to this exercise. This will allow you to use your abdominal muscles, rather than your body's momentum, to do the work on the exercise. Also, be sure to keep your upper body on the floor throughout.

Crunchless Crunch
This first exercise is fairly simple but can also be fairly difficult. Essentially, it involves trying to pull the belly button in towards the spine. This can be tricky, as it involves using muscles which you may not be used to activating. To start, either lie or on your stomach or kneel. You might want to try both ways and see which helps you feel the exercise better. Relax your body as much as possible, then try to use only the lower abdominals to move your belly button toward your spine. Hold for ten seconds. If holding for ten seconds feels easy, hold for a longer period. The goal is to hold the contraction until you either cannot feel it, or you feel other muscles working harder than the transverse abdominus. When you feel this, let the contraction out.
Scissor Kicks
This stomach exercise also requires lying on the floor. Position your hands under your butt, keeping your back pressed against the floor. Slowly raise one leg to a height of about ten inches, then slowly lower it back to the floor. As your lower one leg, raise the other. Repeat this motion for an entire set. Maintaining control throughout is important, not allowing momentum to get the better of you. Your upper body should remain on the floor through the entire move.

There are plenty of other exercises targeting the transverse abdominals, but these three ought to be enough to get you started. Stomach exercises like these are key to any tummy-flattening plan, and they are especially good for pregnant and post-partum women.