No matter where you are on the pre- or postnatal continuum, having a strong core (a.k.a. trunk, or abdomen/stomach/back area) is key. From the early stages through the third trimester, a solid core can nip back pain in the bud and right increasingly wobbly posture. In the delivery room, it's invaluable on the pushing front. And once the baby arrives, you'll bounce back into shape more quickly if you've consistently minded your middle.

"You need core strength to get through pregnancy, and working with a stability ball is very helpful because it strengthens your lower back," says fitness guru David Kirsch, author of The Ultimate New York Body Plan and the man who whips model Heidi Klum into shape. "Pregnancy puts a lot of strain on the body," he says. "Between the baby and the water gain, that's 20 pounds. It's a lot of weight, and it's all right in the middle."

All the more reason to keep moving—depending on your daily energy levels and a green light from your ob/gyn, of course. "Gone are the days of the 'pickles and ice cream' pregnancy," says Kirsch. "A lot of my clients are older now. When you're 25 years old, it's one thing. But if you're 35 or 40 and you're having your first child, your body isn't going to snap back as quickly as it would if you were younger. You need to reframe your mind for that."

By supporting your body weight and alleviating pressure from your back and legs, a stability ball at the center of your prenatal exercise routing lets you stay in the fitness game that much longer. Still, proper form is a must—especially as your ever-expanding belly throws your balance off and the pregnancy hormone relaxin naturally loosens your uterus, joints, and tendons in preparation for labor. To boost your safety level, wear sneakers at all times. And if you're feeling especially off-kilter, push the ball against a wall or a large, stationary object like a weight bench for extra support.

Next Page: A time-efficient workout plan for managing your middle during pregnancy