Evaluation of fetal status. This necessitates obtaining a history of fetal activity. Electronic monitoring of fetal heart rate (FHR) and uterine activity also is recommended after 24 weeks’ gestation. In certain situations (such as in the event of a nonreactive nonstress test or concern about possible premature rupture of membranes), a biophysical profile and/or evaluation of adequate placental function (for example, by documenting appropriate amniotic fluid volume) are required.11
During the third trimester, the fetus makes gross body movements about 10% of the time, with 30 such movements every hour.
The method used for assessment and documentation depends on the gestational age. For example, starting at 20 weeks’ gestation, the uterine fundal height should be measured. From that point on, the fundal height in centimeters usually correlates with gestational age in weeks. During the third trimester, the fetus makes gross body movements about 10% of the time, with 30 such movements every hour.7 The mother is able to perceive 70% to 80% of these movements.
- Evaluation of a previable pregnancy. Prior to 24 weeks’ gestation, fetal status is usually assessed by auscultation of FHR using a doppler-scope. Documentation of FHR ranging from 120 to 160 beats per minute usually is a sufficient assessment. In certain situations (such as first-trimester bleeding or suspected premature rupture of membranes), portable ultrasound is helpful to document fetal viability, assess amniotic fluid, and reassure the mother.
- Assessment between 24 and 28 weeks’ gestation. More thorough evaluation is warranted at this time. Because of the limited predictive value of clinical risk factors, the interpretation of the FHR pattern has become the primary method of assessing fetal well-being. A monitor strip documenting FHR and uterine activity should be obtained and classified as reactive or nonreactive (TABLE 3). A reactive FHR pattern usually requires at least 2 accelerations of 15 beats above the baseline. However, when the FHR is measured prior to 30 weeks’ gestation, 10-beat accelerations are sufficient for reassurance. Although mild variable FHR decelerations may be present, they are not associated with adverse fetal outcomes as long as they are of short duration (less than 30 seconds) and nonrepetitive (less than 3 in 20 minutes).8 When the FHR tracing is nonreassuring, a biophysical profile may be performed.
- Assessment between 28 and 37 weeks’ gestation. As gestational age advances, the FHR baseline lowers, variability increases, and the cycles of fetal activity and rest become better defined. The amplitude of FHR accelerations is inversely proportional to the baseline rate.8 In uncompromised pregnancies, most nonstress tests are reactive (50% of nonstress tests are reactive between 24 and 27 weeks, and 85% are reactive at 28 to 32 weeks).9 However, some of the tests are non-reactive even when the fetus is not compromised (false positive); additional tests are necessary to assess the fetus’ well-being and avoid unnecessary interventions such as induction of labor or cesarean delivery.
- Assessment at term. During a triage visit between 37 and 42 weeks’ gestation, the maternal and fetal conditions should be carefully assessed. When active labor is ruled out, the potential risks of continuing the pregnancy and of labor induction are weighed against the potential benefits of delivering the fetus.10 Conditions that should prompt the caregiver to consider induction of labor are listed in TABLE 4. Prior to labor induction, dating of the pregnancy should be carefully evaluated. Evaluation of fetal lung maturity should be considered when induction of labor is deemed indicated but dating of the pregnancy is insufficient to indicate whether it is at term.
At the conclusion of this initial evaluation, usually performed by a nurse, the patient’s physician is consulted by phone. He or she then decides the patient’s disposition. The woman may be discharged home, kept for further evaluation or observation, or reassessed by the physician.
TABLE 3
Classification of fetal heart rate pattern8,9
| REACTIVE (REASSURING) A tracing with a baseline of 120 to 160 beats per minute, good variability (5 beats above and below the baseline), and at least 2 accelerations more than 15 seconds long of 15 beats above the baseline during a 20-minute period coinciding with fetal movements. |
| NONREACTIVE Suspicious Mild fetal tachycardia (more than 160 beats per minute) or bradycardia (less than 110 beats per minute), decreased variability, no fetal heart rate accelerations or decelerations. This pattern may be associated with fetal hypoxia, maternal fever, chorioamnionitis, or use of certain drugs. A normal pattern may be restored by treating the underlying cause. |
| Threatening Minimal variability, stable baseline, variable or late decelerations. This pattern suggests fetal response to impaired umbilical blood flow (variable decelerations) or uterine blood flow (late decelerations). The cause of variable decelerations is not always clear. Possible conditions that need to be ruled out include oligohydramnios; premature rupture of membranes; placental abruption; umbilical cord compression, prolapse, or entanglement; imminent delivery; and maternal hypotension after regional anesthesia. Late decelerations with normal variability may be caused by hypotension (for example, due to regional anesthesia) or uterine hyperstimulation (such as use of oxytocin or a cervical ripening agent). Intrauterine resuscitation maneuvers that may be used include changing the patient position, administering oxygen or IV fluid, and discontinuing oxytocin. |
| Ominous Absent variability, unstable baseline, late decelerations, significant fetal tachycardia or bradycardia. These patterns suggest severe, acute fetal distress. |
| Chronic condition Absent variability. This pattern suggests an earlier neurological insult; intervention is likely to be of little benefit. |