Assessment of Fetal Well-Being

Key Points

Fetal assessment demands a view into the somewhat inaccessible intrauterine environment; the primary tools are ultrasound and fetal heart rate monitoring.
It is important to make the distinction between intrapartum (during labor) and antepartum (before labor) fetal assessment; the latter is the focus of this chapter.
Current debate centers on who should undergo sonographic examination and what type of evaluation these patients should have.
Fetal assessment in the third trimester is centered on prediction and/or detection of fetal hypoxemia and academia.
Antepartum fetal testing in high-risk pregnancies includes the nonstress test, amniotic fluid volume measurement, biophysical profile, and Doppler velocimetry.
The primary difficulty with antepartum fetal testing is the as-yet unproven utility to improve outcome.

A primary objective of obstetric care is the assessment and prevention of adverse fetal and neonatal outcomes. Maternal care is an integral step toward this goal. Optimization of the maternal state, through monitoring and treatment of chronic conditions such as diabetes or hypertension or acute states like preeclampsia or preterm labor, is one important facet of care to achieve desirable perinatal outcomes. Monitoring and management of the fetus, although a more obvious step toward this goal, is somewhat less straightforward. Fetal assessment demands a view into the somewhat inaccessible intrauterine environment. The ability to gain access to this space to gauge the needs and health of the fetus improved dramatically with developments in technology and increased understanding of fetal physiology over the past several decades. As a result, perinatal morbidity and mortality decreased considerably over that time ( Fig. 12.1 ).

In general, antepartum fetal assessment utilizes various techniques to assess fetal health and well-being in pregnancies that are at increased risk of fetal death due to preexisting maternal conditions (chronic hypertension) or pregnancy-related complications (fetal growth restriction [FGR]). Selecting appropriate patients at risk for adverse perinatal events can enhance the prediction of these events, although some tests may be appropriate even for a low-risk population. The assessment may allow for certain therapeutic options—often, timely delivery—to prevent fetal harm. The overall goal of these efforts is to reduce perinatal mortality, although the reduction of morbidities such as cerebral palsy or preventable birth injury is intertwined with this objective. In antenatal assessment in the third trimester, the prediction and detection of fetal acidemia and hypoxemia form a central principle underlying these efforts.

It is important to make the distinction between antepartum and intrapartum fetal assessment: the latter is specifically related to monitoring the fetus during labor. The nature of labor affords certain advantages (e.g., dilation allows blood samples from the fetus) and restrictions (the lack of fluid after rupture of membranes creates difficulties for ultrasound examination) that do not occur in the antenatal period. As a result, this chapter focuses only on events and assessments preceding labor.

General Principles

Principles of Testing

Many of the tests used for antepartum fetal assessment are screening tests that will lead to further testing allowing for diagnosis and decision making; therefore it is important to note the principles guiding such tests. The outcome, principally perinatal morbidity and mortality, is a significant burden to both the individual and the overall healthcare system. The primary tools for assessment, ultrasound examination and fetal heart rate (FHR) monitoring, are generally easy, safe, and acceptable to patients. Screening has the potential to allow important and timely interventions, such as antenatal steroid administration or delivery. The predominant difficulty with fetal testing comes in the unproven utility of testing to improve outcomes. Furthermore, some tests, such as the nonstress test (NST), have high false-positive rates; therefore when used as a diagnostic test (e.g., to decide on delivery), they can lead to the overuse of interventions. The specificity and sensitivity of the tests vary, and the critical step to enhancing test performance is patient selection. The utility of fetal surveillance involves the judicious application of the tests in patients with specific risk profiles.

Fetal Physiology and Behavior

The first trimester (≤14 weeks’ gestation) is mainly a time of system development and organogenesis. The hyperplastic enlargement during the first 11 weeks produces standard rates of growth, with deviation being rare. At the completion of the first trimester, the major organ systems have developed, allowing the opportunity during the second trimester to assess for anomalies in development. The second and third trimesters involve the maturation of these systems. Fetal assessment, primarily performed in the third trimester, is concerned with the prediction or detection of fetal hypoxemia and acidemia. Integration of the fetal neurologic and cardiovascular systems as they relate to acid-base status is the cornerstone of this assessment. By the beginning of the third trimester, there is usually adequate maturation present in the neurologic and cardiovascular systems to enable meaningful fetal assessment. We are thus able to monitor the manifestations of hypoxemia and acidemia as shown by neurologic and cardiovascular changes. In pregnancies at extremely high risk for adverse perinatal outcome or stillbirth, a fetal assessment may be performed in the second trimester, as early as 23 to 24 weeks’ gestation.

Technology

The technologic underpinning of fetal assessment is ultrasound. FHR monitoring during the antepartum period depends on a Doppler cardiogram; movements of the fetal heart, in particular the sounds of the valves, are detected by this monitor. The time between the beats is translated into a heart rate, which is then graphically represented on a chart over time. This process produces the FHR monitoring strip that becomes the NST or contraction stress test (CST).

Contemporary ultrasound imaging technology involves a wide array of features, including B-mode (basic imaging), M-mode (mapping the movement of structures over time), pulsed Doppler (demonstrating flow velocity in a particular area, such as a vessel), color Doppler (showing intensity and direction of flow through shades of red and blue), and power Doppler (a more sensitive form of colorized Doppler). Magnetic resonance imaging is often used to supplement ultrasound imaging, especially for imaging of the fetal brain.

Indications and Timing

Most fetal testing protocols involve a stepwise approach, and the first step is the selection of the appropriate patient. Suggested assessments for low-risk pregnancies include one ultrasound examination for dating and one for the basic anatomic survey. Prenatal risk assessments for chromosomal disorders, such as cell-free fetal DNA analysis, first-trimester risk assessment with maternal serum analysis and fetal nuchal translucency assessment, or the second-trimester maternal quadruple serum screen, are additional options (see Chapter 26 , Prenatal Diagnosis and Counseling, for additional discussion of these tests). Whereas up to 30% of perinatal morbidity may occur in low-risk patients, routine fetal testing beyond that described previously in a low-risk pregnancy is an ineffective use of resources.

High-risk pregnancies are those at greater peril for perinatal morbidity and mortality. These pregnancies often have more justification for targeted or detailed anatomic ultrasound examinations and for regular assessment of fetal growth or FHR. Common conditions requiring increased fetal surveillance are shown in Box 12.1 .

BOX 12.1

Common Indications for Antepartum Fetal Assessment: High-Risk Pregnancies

Fetal

Abnormal fetal testing, fetal distress
Decreased fetal movement
Fetal growth restriction
Monochorionic multiple gestation
Oligohydramnios

Maternal–Fetal

Placental abruption (abruptio placenta)
Alloimmunization
Late term or postterm pregnancy
Gestational hypertension or preeclampsia
Gestational diabetes
History of fetal death

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