The assessment of fetal well-being is an imprecise science and is carried out in a number of ways:

• fetal growth assessment
  
• amniotic fluid volume assessment
  
• nonstress tests (NST), contraction stress tests (CST) (both also known as cardiotocography (CTG))
  
• biophysical profile
  
• fetal Doppler velocimetry
  
• maternal Doppler velocimetry.

Each of these modalities is briefly summarized below. While all of these modalities are commonly used in clinical practice, the evidence for their impact upon pregnancy outcome remains unclear.

Fetal growth

Maternal medical disorders such as hypertension, hypoxia, and chronic renal disease can affect placental function and lead to intrauterine growth restriction (IUGR). The greater and earlier the onset of the IUGR, the greater the risk of perinatal morbidity and mortality [1]. Fundal height (the height of the uterus as measured from the pubic bone) is a screening test for appropriate fetal growth but has limited sensitivity and specificity. Ultrasound is currently the most useful method of determining fetal growth. Clinical determination of fetal growth such as fundal height can be misleading and should always be backed up by ultrasound. In cases where there is high risk of IUGR, serial ultrasound examinations to assess fetal growth should be carried out every 2–4 weeks depending upon the severity of the problem. The routine growth parameters used are biparietal diameter, head circumference, abdominal circumference and femur length. Due to wide normal ranges for these measurements, particularly at later gestations, and the errors of measurement, growth is best assessed by ultrasound examinations carried out 2–4 weeks apart. The classic picture of IUGR related to maternal medical disease is asymmetric IUGR where the abdominal measurements are more decreased than the fetal head and femur measurements. In severe early-onset IUGR, all parameters are often decreased.

Amniotic fluid volume

It is well documented that amniotic fluid volume correlates well with perinatal outcome and is very helpful in identifying those babies that are small due to uteroplacental insufficiency from those that are constitutionally small. It has been demonstrated in animal models that fetuses subject to chronic hypoxia redirect blood flow from less vital organs (such as the kidneys) to more critical organs (such as the brain). This diversion of blood flow from the kidneys leads to reduced fetal urine production and thus to a decreased amniotic fluid volume or oligohydramnios.

Amniotic fluid volume is usually measured in one of two ways. The first is the maximum vertical pocket of amniotic fluid as measured by ultrasound. Normal values fall between 2 cm and 8 cm, with oligohydramnios being present when the value is less than 2 cm. The Amniotic Fluid Index (AFI) is the second ultrasound method of assessing amniotic fluid volume. With this method, the maximum vertical pocket in each of the four quadrants is measured and then the values are summed. In general, the normal range is between 5 and 24, but gestational age-related ranges have been developed [2].

Nonstress tests and contraction stress test (also known as cardiotocography)

The measurement of fetal heart rate and its relationship to fetal activity (NST) or uterine contractions (CST) are the most commonly used methods in assessing fetal well-being [3], despite lack of convincing data on the overall benefits of such testing. Although used extensively antenatally, it is the mainstay of intrapartum fetal monitoring. It is beyond the scope of this chapter to cover all aspects of CTG interpretation. Many authorities have published guidelines for the use and interpretation of CTG, including the American College of Obstetricians and Gynecologists and the Royal College of Obstetricians and Gynaecologists [1,4].

Fetal heart rate is measured either externally by a Doppler ultrasound transducer placed on the maternal abdomen or internally by an electrode (fetal scalp electrode) affixed to the fetus. Maternal contractions are measured by an external tocodynamometer attached to the maternal abdomen or an intrauterine pressure catheter. A number of fetal heart rate parameters including baseline rate, heart rate variability, and accelerations and decelerations are measured when assessing a CTG record. In general, a reactive NST as assessed by these parameters is very predictive of a nonhypoxic, noncompromised fetus. On the other hand, a nonreactive NST pattern is not highly predictive of a compromised fetus and should be interpreted in association with the clinical situation (such as pre-eclampsia, antepartum hemorrhage) and should be used as a screening test for further investigation. Either an unsatisfactory or a nonreactive NST provides no reassurance of fetal well-being and usually warrants further investigation by a CST (administering oxytocin and assessing the fetal heart rate response to uterine contractions) or ultrasound (a biophysical profile –see below). Similarly, a nonreassuring CTG in labor requires further evaluation, including the possibility of the collection of a fetal scalp blood sample for assessment of pH. If, for example, the scalp pH is less than 7.20, delivery should be expedited. There are also very sinister CTG patterns (such as prolonged decelerations) that require immediate delivery without other testing.

Biophysical profile

The biophysical profile (BPP) combines an assessment of fetal NST and ultrasound assessments of amniotic fluid volume, fetal tone, fetal breathing and spontaneous fetal movement in an attempt to improve upon the specificity and sensitivity of NST and amniotic fluid levels alone. The BPP is often used as an alternative test to monitor fetal well-being in pregnancies at increased risk of fetal loss or as a follow-up test for patients who have an abnormal NST [5].

Fetal Doppler velocimetry