Pathophysiology
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.