Fetal Surveillance

A nonstress test reads "nonreactive" — does that mean the fetus is in distress, or just asleep? Knowing how to interpret fetal surveillance results changes your next move entirely.

Fetal surveillance refers to antepartum testing methods used to evaluate fetal well-being, primarily the nonstress test (NST), contraction stress test (CST), biophysical profile (BPP), and modified BPP. The NST is the most common first-line test: you're looking for at least 2 accelerations of ≥15 bpm above baseline lasting ≥15 seconds within a 20-minute window — that's reactive, indicating adequate oxygenation (these criteria apply at ≥32 weeks; before 32 weeks, the threshold is 10 bpm × 10 seconds). If nonreactive after 40 minutes (including vibroacoustic stimulation to wake the fetus), further testing is warranted. The BPP scores five parameters — fetal breathing movements, gross body movement, fetal tone, amniotic fluid volume, and NST — each scored 0 or 2, for a total of 0–10. A score of 8–10 is reassuring; 6 is equivocal and requires retesting or delivery depending on gestational age; ≤4 warrants urgent intervention. The CST evaluates fetal response to uterine contractions: a negative CST (no late decelerations) is reassuring, while a positive CST (late decelerations with the majority of contractions) signals uteroplacental insufficiency. CST is contraindicated in placenta previa, preterm labor risk, and prior classical cesarean incision. The modified BPP combines NST with amniotic fluid index (AFI), offering a quicker screen — AFI <5 cm is oligohydramnios and signals concern.

Key Distinctions

Reactive NST = good (the fetus responds); positive CST = bad (late decelerations present). Students reverse these because "positive" sounds good — remember that the CST is testing for a negative outcome. Don't confuse a nonreactive NST with fetal distress; it may simply indicate a fetal sleep cycle. BPP and NST are noninvasive surveillance — amniocentesis for fetal lung maturity belongs in the invasive diagnostics sibling atom.

Clinical Pearl

NST: reactive is reassuring. CST: negative is normal. The tests use opposite language — when the test name has "stress," flip your instinct about positive and negative.