Childbirth education will mentally prepare you for labor, and all those great breathing tricks do keep you busy while labor pains are pounding your body. But you don't have to be a rocket scientist to guess that being in physical shape helps, too. Women who work out regularly have insisted on this for years, and now there's research to prove it: A study of 557 pregnant women published in the American Journal of Public Health found that women who engaged in heavy exercise (expending more than 1,000 calories a week) had faster deliveries than those who engaged in only moderate exercise or none at all. (Another plus: Heavier exercise appeared to reduce the risk of premature birth.) Other studies have shown that women who exercised while pregnant gained less weight, lost pounds more quickly after delivery, and required fewer interventions such as the use of forceps or the need for a cesarean section during delivery.

Still, you shouldn't just step outside and start jogging. Exercise during pregnancy must be carefully monitored for your own safety and the health of your baby (talk to your health care provider before beginning this or any other exercise plan). And while overall fitness is always a plus, there are very specific muscle groups that are instrumental in childbirth, and these are the ones on which you'll need to focus. Think of it as squeezing toothpaste out of a tube. The tube is the uterus and your hand represents the abdominal muscles. When squeezing out the toothpaste (the baby), your hand pushes against the tube, and the toothpaste comes out the bottom. In order for the baby to come out during labor, the abdominal muscles must be tight and the pelvic floor muscles must be open and relaxed. Many women in labor make the mistake of simultaneously tightening both sets of muscles. This is like keeping the cap on the toothpaste!

The workout on these pages has been designed to help you isolate and strengthen these key muscle groups. It's ideal if you begin the exercise program in your first trimester  -- you'll be in better shape by delivery and are likely to have a more comfortable, energetic pregnancy. But starting even as late as your eighth month  -- assuming you have no complications to worry about  -- will improve your odds of having an efficient delivery. Just clear it with your doctor, take it slow, and follow the cardinal rule of exercise: If it doesn't feel right, don't do it.

Julie Tupler, R.N., is a certified fitness instructor and childbirth educator. She is the author of Maternal Fitness (Fireside) and the creator of the Maternal Fitness Prenatal Video series. Stephanie Wood is a mother of two who writes frequently about parenting issues.

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