The goal of antepartum fetal assessment is to both identify fetuses at risk of intrauterine death or other complications of intrauterine asphyxia and intervene to prevent these adverse outcomes, if possible. The main techniques for fetal assessment are the nonstress test, biophysical profile, modified biophysical profile, contraction stress test, and fetal movement count. Assessment of amniotic fluid volume and Doppler velocimetry provide additional information about fetal status. Despite widespread use of these techniques, there is limited evidence to guide their optimal use or to demonstrate their effectiveness for improving perinatal outcomes.
This topic will provide an overview of antepartum fetal assessment. Detailed discussions of the various techniques and their use in specific clinical settings are available separately (see individual topic reviews).
PHYSIOLOGICAL BASIS FOR FETAL TESTING
Antepartum testing is based on the premise that the fetus responds to hypoxemia with a detectable sequence of biophysical changes, beginning with signs of physiological adaptation and potentially ending with signs of physiological decompensation (figure 1) [1,2]. Studies in chronic animal models support this premise by demonstrating that fetal biophysical activities (eg, heart rate, movement) are sensitive to fetal oxygenation and pH levels, and changes in fetal biophysical activities occur in response to hypoxemia and acidemia . However, fetal biophysical parameters can be affected by factors unrelated to hypoxemia, such as gestational age, maternal medication/smoking, fetal sleep-wake cycles, and fetal disease/anomalies.
Periodic fetal antepartum testing rarely identifies fetuses at risk of death from a sudden insult, such as complete placental abruption, and thus does not provide an opportunity for intervention to prevent these deaths.
Antepartum fetal surveillance has had an established role in obstetrical practice since the 1970s , although its ability to improve pregnancy outcome has not been evaluated by large, well-designed randomized trials . Efficacy is based primarily on two lines of evidence: (1) observational studies that reported lower rates of fetal death in pregnancies that underwent fetal testing than among historic controls with the same indication for testing but no fetal testing and (2) the same or lower rates of fetal death in tested pregnancies (primarily high risk) than in a contemporary untested general obstetrical population (primarily low risk) [6-10].