Surfactant, a combination of surface-active phospholipids and proteins, is produced by the maturing fetal lung and eventually is secreted into the amniotic fluid. The amount of surfactant in amniotic fluid is a direct reflection of surface-active material in the fetal lung and can be used to predict the presence or absence of pulmonary maturity. Because phosphatidylcholine, or lecithin, is a principal component of surfactant, the determination of lecithin in amniotic fluid is used to predict a mature fetus. Lecithin concentration increases with increasing gestational age, beginning at 32 to 34 weeks.
Methods used to assess fetal well-being before the onset of labor are focused on identifying a fetus at risk for asphyxia or a fetus already compromised by uteroplacental insufficiency. The oxytocin challenge test simulates uterine contractions through an infusion of oxytocin sufficient to produce three contractions in a 10-minute period. The development of periodic fetal bradycardia out of phase with uterine contractions (late deceleration) is a positive test result and predicts an at-risk fetus.
The nonstress test examines the heart rate response to fetal body movements. Heart rate increases of more than 15 beats/min lasting 15 seconds, are reassuring. If two such episodes occur in 30 minutes, the test result is considered reactive (versus nonreactive), and the fetus is not at risk. Additional signs of fetal well-being are fetal breathing movements, gross body movements, fetal tone, and the presence of amniotic fluid pockets more than 2 cm in size, detected by ultrasound. The biophysical profile combines the nonstress test with these four parameters and offers the most accurate fetal assessment.
Doppler examination of the fetal aorta or umbilical arteries permits identification of decreased or reversed diastolic blood flow associated with increased peripheral vascular resistance, fetal hypoxia with acidosis, and placental insufficiency. Cordocentesis (percutaneous umbilical blood sampling) can provide fetal blood for Po2, pH, lactate, and hemoglobin measurements to identify a hypoxic, acidotic, or anemic fetus who is at risk for intrauterine fetal demise or birth asphyxia. Cordocentesis also can be used to determine fetal blood type, platelet count, microbial testing, antibody titer, and rapid karyotype.
In a high-risk pregnancy, the fetal heart rate should be monitored continuously during labor, as should uterine contractions.Fetal heart rate abnormalities may indicate baseline tachycardia (>160 beats/min as a result of anemia, β-sympathomimeticdrugs, maternal fever, hyperthyroidism, arrhythmia, or fetal distress), baseline bradycardia (<120 beats/min as a result of fetaldistress, complete heart block, or local anesthetics), or reducedbeat-to-beat variability (flattened tracing resulting from fetalsleep, tachycardia, atropine, sedatives, prematurity, or fetaldistress). Periodic changes of the heart rate relative to uterinepressure help determine the presence of hypoxia and acidosiscaused by uteroplacental insufficiency or maternal hypotension(late or type II decelerations) or by umbilical cord compression(variable decelerations). In the presence of severe decelerations (late or repeated prolonged variable), a fetal scalp bloodgas level should be obtained to assess fetal acidosis. A scalppH of less than 7.20 indicates fetal hypoxic compromise. A pHbetween 7.20 and 7.25 is in a borderline zone and warrants repeat testing.