Non-stress testing is a measure of fetal well being--they measure the fetal heart rate and uterine activity. You click a buzzer if the baby moves; this is marked on the test screen/strip (so they can map it against the FHR changes). They're looking for a certain baseline FHR and (IIRC) 2 accelerations of a certain number of BPM above baseline within the test time. A nonreactive NST may be a sign of placental problems, among other things, but it is not definitive--if the test is nonreactive (the nurse performing the test may try a few simple tricks to perk the baby up, such as juice, cold water, a cookie), the usual next step is a biophysical profile.
(Apparently, a few hospitals are moving to the BPP as routine--this requires ultrasound and my OB had MFM do it.)
Aliza - 01/05/2007 - Severe preeclampsia, emergency CS 37 weeks
Isaac - 09/26/2011 - controlled on 150mg Toprol, NO PE, 39 weeks!