I once watched an actress frame her belly button with her fingers during a local improvisational show. Presto! She had transformed her tummy fat into a doughy bagel.

I thought it was pretty funny until I found that I also could make an authentic-looking bagel with my stomach. After two pregnancies, it was time to do sit-ups.

But sit-ups, I later learned after performing them religiously with no visible effect, are the last thing women should do after they've had a baby. Instead of firming and tightening the stomach, they actually can lead to a bulging and protruding abdomen.

During pregnancy, as the abdominal muscles stretch to accommodate the growing fetus, the most superficial abs — the rectus abdominis, or six-pack muscles — can split down the middle like a pants seam.

This is a good thing because it allows room for the growing baby, said Cynthia Neville, director of women's health rehabilitation at the Rehabilitation Institute of Chicago. But when the split is too wide, exceeding 2 to 3 centimeters, it creates instability in the abdomen, trunk and pelvis.

It's thought that the split should spontaneously reduce to less than 2 centimeters within a few weeks after delivery and that any urine leakage should cease within a few weeks after giving birth.

But if the core muscles, which include the pelvic floor muscles and the deep abdominals, are not working as a team, then "women may continue to have flabby, bulging, weak abs, or they may leak urine, or both," Neville said. (Women who undergo Caesarean sections have a different challenge because of the way surgery affects the abs.)

What women need to do is to retrain the abdominal and core muscles to be strong and coordinated so they work at the right time.

The core should be the first muscles to kick into gear; if they're not and you're doing ab exercises that focus on the six-pack muscle, your stomach may bulge, Neville said.

A physical therapist or certified pilates instructor can help with retraining the deep transverse muscles. It starts with learning to maintain a neutral (not too arched or flattened) position of the spine and pelvis.

Then you must learn how to contract the pelvic floor and the birthing muscle — the flat sheath of muscle that wraps around and attaches to the ribs, abdominals and pelvis — while moving the arms and the legs.

"The key is to 'remind' the core to be the first to contract to prepare for the extremity movements until it becomes automatic," Neville said — and not just during exercise but during everyday movement, such as lifting the baby or pushing a stroller.

Once these basic exercises are easy, move on to leg lifts, curl-ups, bridges and planks. These exercises are demonstrated in Erin O'Brien Denton's DVD "Postnatal Rescue." Denton, a personal trainer, recommends lying on your back and doing "bridges" (articulating the raising of the pelvis off the ground) for the first three months after delivery "to teach the abs to lie sleekly against the abdominal wall."

She also suggests a modified pilates 100s exercise, which is simply lifting the head and shoulders while pulling in the abdominals.

After you have mastered those motions, Denton suggests "the old-fashioned bicycle abdominal exercise, a variety of crunches (initiating the movement from both the upper and lower abdominal wall) and various plank exercises to start toning the abs."