Exercising during the first trimester doesn't have to be any different from exercising when you are not pregnant, unless you feel too tired to get up. Once you develop a baby bump, ab crunches are out. I don't believe any woman with a bump would actually attempt any, but if you do, I think you will find it physically impossible. Exercises that raise your temperature for long periods of time could potentially be harmful for your fetus. And of course, extreme sports like bungy jumping or white water rafting are not a good idea. If you can reasonably die while doing it, just don't. And when your body tells you to stop, do. Apart from that, anything goes. Exercises that build your muscles, and don't put too much strain on your changing body include swimming, walking, cycling, prenatal yoga and pregnancy aerobics. Being active will help your body adjust to its new size, and your muscles will thank you. It is possible that women who are physically active have easier labors and births. Exercising for as long as you feel great will also help your body get back to its pre-pregnancy shape more easily after your baby arrives.