A nonstress test is a totally noninvasive procedure that's done in the third trimester to check on your baby's heartbeat and make sure he's doing okay in there. Most women don't need this test; it's mainly performed on moms-to-be with high-risk conditions like high blood pressure or diabetes, women who are expecting twins or multiples, and women who are at least a week past their due date.
To take the test (which is painless), you'll have to rest quietly for about 20 minutes on your doctor's examining table or in a reclining chair. Two belts will be wrapped around your belly and attached to an external fetal heart monitor. One belt records your contractions (if you're having any) and the other tracks your baby's heart rate and movements. The doctor is looking to see that your baby's heart rate increases slightly after he moves (just the way yours does when you go for a walk, for example). If your baby's not very active at that moment (he may be asleep) he may be nudged into action by a gentle push on your belly or with a buzzer, or by having you eat or drink something.
Results are available immediately and considered normal (or "reactive") if your baby's heart rate speeds up after he moves, and "nonreactive" if he doesn't. If that's the case, you might need follow-up tests, such as a biophysical profile (which is like a nonstress test, but also includes an ultrasound to look at your baby) or a contraction stress test (which induces mild contractions to see how your baby's heart rate responds). But there's no need to worry -- totally healthy babies sometimes have nonreactive tests. Your doctor will likely continue monitoring your baby, and may ultimately decide to induce labor or perform a c-section if he thinks your baby's ready to be born.