Good news: The nonstress test lives up to its name, both for you and baby. It's totally noninvasive, and poses no risk to baby. Be prepared, though — it can easily take up to 40 minutes (make a bathroom stop first!), and might be a little noninteresting for you.
The nonstress test evaluates baby's wellbeing by measuring how her heart rate responds to movement. It's a fetal monitoring method, meaning the purpose is simply to check on baby's well-being — not cure a problem or ensure that baby's born healthy. It's normally performed after 28 weeks, most often in post term and high-risk pregnancies.
For the test, a Doppler device will be strapped around your belly with a belt. The device is attached to ultrasound transducers which measure baby's heart rate. At the same time, baby's movements will be recorded, either with another device placed around your abdomen or a button that you press. If baby isn't doing much moving, he or she's probably snoozin'. Your doctor may use a (completely safe!) buzzer-like device to create sound and vibration to wake her, or ask you to eat or drink something to encourage a stir.
What's your doctor looking for? Ideally, baby's heart rate will quicken with movement. If this isn't the case, there could (meaning no need to freak out!) be a problem with baby's health, and your OB may recommend repeating the test or doing another procedure such as a contraction stress test. If baby doesn't respond after an hour and a half or so, you also may be asked to repeat the test.
American College of Obstetrics and Gynecologists. Your pregnancy and birth. 4th ed. Washington, DC: ACOG; 2005.