SPECIFIC TECHNIQUES OF FETAL ASSESSMENT
Continued evaluation of the fetus includes techniques for assess-ment of fetal (1) growth, (2) well-being, and (3) maturity. These tests must be interpreted in light of the clinical con-text and provide a basis for management decisions.
Fetal growth can be assessed by fundal height measurement, as the initial measure, and ultrasonography. The increase in fundal height through pregnancy is predictable. If the fundal height measurement is significantly greater than expected (i.e., large for gestational age [LGA]), possible considerations include incorrect assessment of gestational age, multiple pregnancy, macrosomia (large fetus), hyda-tidiform mole, or excess accumulation of amniotic fluid (polyhydramnios). A fundal height measurement less than expected, or small for gestational age (SGA), suggests the possibility of incorrect assessment of gestational age, hydatidiform mole, fetal growth restriction, inadequate amniotic fluid accumulation (oligohydramnios), or even intrauterine fetal demise. Deviation in fundal height mea-surement should be closely evaluated.
Ultrasound is the most valuable tool in assessing fetal growth. Ultrasound has many potential uses for both fetal datingand identifying any fetal anomalies. In early pregnancy, deter-mination of the gestational-sac diameter and the crown-to-rump length correlates closely with gestational age. Later in pregnancy, measurement of the biparietal diameter of the skull, the abdominal circumference, the femur length, and the cerebellar diameter can be used to assess gestational age and, using various formulas, to estimate fetal weight.
Assessment of fetal well-being includes subjective mater-nal perception of fetal activity and several objective tests using electronic fetal monitoring and ultrasonography. Tests of fetal well-being have a wide range of use, includ-ing the assessment of fetal status at a particular time and prediction of future well-being for varying time intervals, depending on the test and the clinical situation.
Evaluation of fetal activity is a common indirect measure of fetal well-being. A variety of methods can be used to quan-tify fetal activity, including the time necessary to achieve a certain number of movements each day, or counting the number of movements (“kick counts”) in a given hour. This type of testing is easily performed and involves the patient in her own care. If the mother notices less move-ment, further evaluation may be needed.
Fetal monitoring tests can provide more objective information about fetal well-being. These tests include the nonstress test (NST), contraction stress test (CST) (called the oxytocin challenge test [OCT] if oxytocin is used), biophysical profile (BPP), and ultrasonography of umbilical artery blood flow velocity. Although there is no optimal time to initiate fetal testing, there are several maternal and pregnancy-related indications (Box 6.1).
The nonstress test measures the fetal heart rate, which is mon-itored with an external transducer for at least 20 minutes. Thepatient is asked to note fetal movement, usually accom-plished by pressing a button on the fetal monitor, which causes a notation on the monitor strip. The tracing is observed for fetal heart rate accelerations (Fig. 6.2). The results are considered reactive (or reassuring) if two or more fetal heart rate accelerations occur in a 20-minute period, with or without fetal movement discernible by the mother. A nonreactive (nonreassuring) tracing is one without suffi-cient heart rate accelerations in a 40-minute period. A non-reactive NST should be followed with further fetal assessment.
Whereas the NST evaluates the fetal heart rate response to fetal activity, the contraction stress test measures the response of the fetal heart rate to the stress of a uterine contraction. During auterine contraction, uteroplacental blood flow is temporarily reduced by the contracting myometrium.
A healthy fetus is able to compensate for this intermittent decreased blood flow, whereas a fetus that is compromised may be unable to do so. To perform a CST, a tocodynamometer is placed on the maternal abdomen along with a fetal heart rate trans-ducer for a baseline tracing for 10 to 20 minutes. If there are no contractions, they are induced by nipple self-stimulation or oxytocin (this test is called an OCT). A neg-ative (normal) result is indicated if there is no change from the baseline fetal heart rate and no fetal heart rate deceler-ations. If decelerations occur, the results can be considered posi-tive, equivocal, or unsatisfactory, depending on the pattern, frequency, and strength of the deceleration.
These tests of fetal well-being have a significant inci-dence of false-positive results (i.e., results suggesting that the fetus is in jeopardy, though the fetus is actually healthy). Forthis reason, the results of these tests must be interpreted collectively, and the tests themselves repeated to verify the results. When mul-tiple test results are reassuring, they tend to rule out a prob-lem. When all results are nonreassuring, they tend to signify the presence of a problem.
If an OCT is positive, evidence to support fetal well-being, such as that provided by a biophysical profile (BPP), is sought. The BPP is a series of five assessments of fetal well-being, each of which is given a score of 0 (absent) or 2 (pres-ent) [Table 6.3]. The parameters include a reactive NST, the presence of fetal breathing movements, the presence of fetal movement of the body or limbs, the finding of fetal tone (flexed extremities as opposed to a flaccid posture), and an adequate amount of amniotic fluid volume. A total score of 8 to 10 is considered reassuring. A total score of 6 is equivocal and generally should lead to delivery if the patient is at term. If the patient is preterm, retesting within 12 to 24 hours may be appropriate. A score of 4 or less is nonreassuring and warrants further evaluation and consideration of delivery. Irrespective of the score, more fre-quent BPP testing or consideration of delivery may be war-ranted when oligohydramnios is present.1(p116) Management based on the BPP depends not only on the score itself, but also on the gestational age of the fetus.
Modified BPP combines use of an NST and assessment of an amniotic fluid index (AFI). The AFI is a semiquantita-tive, four-quadrant assessment of amniotic fluid depth. Theimportance of adequate amniotic fluid volume is well-established. Diminished amniotic fluid is thought to represent decreased fetal urinary output caused by chronic stress and shunting of blood flow away from the kidneys. The decreased amniotic fluid provides less sup-port for the umbilical cord, which may be more com-pressed, reducing blood flow. The modified BPP is lesscumbersome than the BPP and appears to be as predictive of fetal well-being.
Umbilical Artery Doppler flow ultrasonography is anoninvasive technique to assess resistance to blood flow in the placenta. It can be used in conjunction with other biophysical tests in high-risk pregnancies associated with suspected intrauterine growth restriction. Umbilical cord Doppler flow velocimetry is based on the characteristics of the systolic blood flow and the diastolic blood flow. The most commonly used index to quantify the flow velocity waveform is the systolic/diastolic ratio. As peripheral resis-tance increases, diastolic flow decreases and may become absent or reserve, and the systolic/diastolic ratio increases. Reversed end-systolic flow can be seen with severe cases of intrauterine growth restriction secondary to uteroplacen-tal insufficiency and may suggest impending fetal demise.
Fetal maturity should always be taken into consideration when delivering a fetus electively or preterm in high-risk pregnancies. Several tests are available to assess fetal matu-rity (Box 6.2). Because the respiratory system is the last fetal system to mature functionally, many of the tests avail-able to assess fetal maturity focus on this organ system. Several phospholipids, collectively known as surfactant, enter the amniotic fluid where they can be obtained by amniocentesis and measured. Surfactant is necessary for normal lung function, as it maintains the patency of the alveolar sacs. The ratio of two phospholipids, lecithin andsphingomyelin, called the L/S ratio, has been used todetermine fetal lung maturity, but other tests are rapidly replacing use of this ratio. Another important phospholipid contained in the surfactant complex is phosphatidylglyc-erol (PG), a marker of complete pulmonary maturationthat is present after 35 weeks of gestation.
Neonates delivered before their lungs have matured are at risk of respiratory distress syndrome (RDS), a serious and life-threatening condition caused by lack of surfactant. RDS in newborns is manifest by signs of res-piratory failure—grunting, chest retractions, nasal flar-ing, and hypoxia—possibly leading to acidosis and death. Management consists of skillful support of ventilation and correction of associated metabolic disturbances until the neonate can ventilate without assistance. Adminis-tration of synthetic or semi-synthetic surfactant to the neonate has resulted in improved outcomes for infants with RDS.
Results of pulmonary function tests that indicate immaturity do not have a high predictive value for RDS. Because no test indicating maturity can completely elimi-nate the risk of RDS or other neonatal complications, the risk of adverse fetal outcome following delivery must be weighed against the potential risk of allowing the pregnancy to continue