A fetal non-stress test (called an NST)âso named because it is non-invasive and puts no stress on the developing fetusâis designed to determine fetal âwell-beingâ by detecting changes in the fetal heartbeat, and it is administered if the doctor is concerned that the fetus may not be getting enough oxygen. fetal non-stress test (NST) is typically conducted in a doctorâs office. You may be asked to eat something or maybe drink some juice before the test, since some babies are more active after their mothers have eaten.