Clinical meaning
Antepartum fetal surveillance assesses fetal well-being before labor by evaluating autonomic nervous system integrity and placental reserve. The non-stress test (NST) relies on the coupling of fetal movement with FHR accelerations mediated by an intact sympathetic nervous system — this coupling matures by 28-32 weeks gestation. The biophysical profile (BPP) evaluates five parameters controlled by different CNS centers, with the earliest-developing centers (fetal tone, movement) being the most resistant to hypoxia and the latest-developing centers (FHR reactivity, breathing movements) being most sensitive. The contraction stress test (CST) assesses placental respiratory reserve by evaluating the FHR response to uterine contractions — contractions transiently reduce intervillous blood flow, and a placenta with adequate reserve maintains fetal oxygenation, while an insufficient placenta produces late decelerations. The modified BPP combines the NST (short-term marker) with amniotic fluid index (long-term marker of placental function) as a more efficient screening approach.
Diagnosis & workup
Diagnostics & workup: - Non-stress test (NST): reactive = 2 or more accelerations of ≥15 bpm lasting ≥15 seconds within 20 minutes (≥10 bpm x 10 sec if <32 weeks); nonreactive requires further testing (extend to 40 min, vibroacoustic stimulation, or BPP) - Biophysical profile (BPP): 5 components scored 0 or 2 each — FHR reactivity (NST), fetal breathing movements (≥1 episode of ≥30 sec in 30 min), fetal movement (≥3 body/limb movements in 30 min), fetal tone (≥1 flexion/extension episode), amniotic fluid volume (deepest pocket ≥2 cm); total 8-10 = normal, 6 = equivocal (repeat in 24h), ≤4 = abnormal (consider delivery) - Modified BPP: combines NST + amniotic fluid index (AFI); normal = reactive NST + AFI >5 cm; abnormal in either component warrants full BPP or further evaluation - Contraction stress test (CST): negative (no late decelerations) = reassuring; positive (late decelerations with ≥50% of contractions) = uteroplacental insufficiency; equivocal/suspicious = intermittent lates; unsatisfactory = inadequate contractions - Umbilical artery Doppler velocimetry: assess for absent or reversed end-diastolic flow (indicates severe placental insufficiency in IUGR); used serially to guide delivery timing - Amniotic fluid index (AFI): normal 5-25 cm; oligohydramnios <5 cm (associated with cord compression, renal anomalies); polyhydramnios >25 cm (associated with GDM, GI atresia, neural tube defects)