The at-risk fetus

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Chapter 20 The at-risk fetus

The birthweight of an infant depends on its genetic growth potential, which may be restricted or enhanced by the growth support provided by the mother, the functional integrity of the placenta and the ability of the fetus to use the nutrients provided.

Most fetuses grow normally throughout pregnancy (see Fig. 6.13, p. 46). Some fetuses are genetically programmed to have a low growth potential. They are healthy, but small at birth. Some have a genetic defect which reduces their growth potential and causes slow intra-uterine growth, which may not become apparent until some time in the second half of pregnancy. Some fetuses grow normally initially, but in the last trimester of pregnancy their growth is restricted by alterations in uteroplacental function. This has led to the use of the descriptive term placental dysfunction or placental insufficiency.

Within genetically set limits, the actual fetal growth depends on:

Of all these factors, an adequate blood supply reaching the placenta so that exchange of nutrients across the placenta can take place seems the most important.

With this background the identified causes of fetal growth restriction can be defined (Table 20.1). The degree to which these causes affect fetal growth depends on the amount of placental functioning reserve, and so not all women having these complications will give birth to a growth-restricted fetus.

Table 20.1 Aetiological factors in fetal growth restriction (placental dysfunction)

  Percentage
Maternal causes  
Fetal causes 10
 
Unknown 20

A growth-restricted fetus is more likely to develop metabolic disturbances, such as acidosis, hypoglycaemia and erythroblastosis. If the disturbances are severe the fetus may die in utero. Less severe disturbances tend to become worse during labour, causing clinical or monitor-detected fetal distress, or the baby may be born with signs of severe hypoxia. Thus the fetus in an affected pregnancy is at greater risk of dying in utero or, if born alive, of needing resuscitation and possibly of being brain damaged (cerebral palsy). The pregnancy is high risk. Not all high-risk pregnancies have a growth-restricted fetus, but many do.

TESTS FOR FETAL WELLBEING IN HIGH-RISK PREGNANCIES

Having identified that the pregnancy is high risk for the fetus, tests to determine fetal wellbeing should be started after the 30th week. In the past 10 years, biophysical tests to determine fetal wellbeing have superseded biochemical tests. These tests are:

None of these has a high positive predictive value, but each has a high negative predictive value.

A note of caution should be sounded. The authors of Effective Care in Pregnancy and Childbirth point out that, although the tests may provide ‘a minimum level of care and attention in settings where these are adequate’, in other settings their use may result in ‘a variety of unwarranted interventions’. It should be added that unwarranted interventions may lead to an increase in obstetric interventions and possible medicolegal problems. The doctor may be ‘damned if he does and damned if he does not’. It is essential that the doctor explains to the patient what is intended and why, including the limitations of the tests.

Cardiotocography – the non-stress test

Cardiotocography (CTG) depends on the assumption that a healthy fetus will normally be more active than an at-risk fetus, and that its heart will respond to a uterine contraction by accelerating. An external cardiotocograph is applied to the woman’s abdomen as she sits in a reclining position (not on her back, as this may result in erroneous findings). The heartbeat variations in relation to the contraction are recorded. If the fetus is lethargic, it may be stimulated to move by tapping the uterus gently. CTG is usually done in hospital but can be done at home using domiciliary fetal heart monitoring and a telephone link. The term fetus sleep/wake cycle is approximately 20 minutes so if the tracing is initially non-reactive, the CTG should be continued for at least 30 minutes.

The Royal College of Obstetricians and Gynaecologists’ Cardiotograph Classification is shown in Table 20.2. There are three grades:

A normal pattern indicates that the fetus is not at risk of dying in the next 7–10 days (Fig. 20.1). Such a fetus is termed reactive. If a suboptimal pattern is found the fetus is at slightly increased risk and the test should be repeated in 3–4 days. The clinical management reaction to a suspicious or pathological CTG tracing will depend on the overall clinical picture and the gestational age of the fetus. A suspicious tracing merits closer surveillance and/or delivery if the fetus is at term, a pathological tracing intensive surveillance and, if persistent, consideration of delivery (frequently by caesarean section) if the fetus has reached viability (Fig. 20.2).

A problem with CTG is that normal patterns predict that the fetus is at very low risk, but an abnormal pattern does not give an accurate prediction of fetal danger, i.e. the false positive rate is high, the positive predictive value is low, but the false negative rate is also low.

Serial ultrasound examinations

The growth of the fetus, as a measure of its health, may be monitored by examining it using real-time ultrasound at 3-weekly intervals. The fetal abdominal circumference, head circumference, femur length and estimated fetal weight are calculated and related to the centile chart for the sex of the fetus. In addition, the longest column of amniotic fluid is measured and the amniotic fluid index (AFI) calculated (see Ch. 19, p. 150–151). In some centres customized growth or centile charts are used which make adjustment for the mother’s parity, height, weight and ethnic origin. This approach has been shown to improve the accuracy of identification of growth-restricted fetuses and helps differentiate those fetuses from ones that are genetically small but developing normally.

MONITORING THE AT-RISK FETUS IN LABOUR

When caesarean section is not chosen to perform an immediate delivery, the health of the mother and fetus should be monitored during the labour. The care of the woman in labour is described in Chapter 8. In this section the care of the fetus will be described.

An at-risk fetus, particularly if growth restricted, has fewer reserves on which it can draw for energy should placental gas and glucose transfer be reduced further during labour. Normally, the fetus converts glucose (released from glycogen stores) by anaerobic pathways into ketopyruvic acid, and then, in the presence of oxygen, into carbon dioxide and water via the Krebs’ cycle. The process releases ‘30 high-energy bonds’ per molecule of glucose. When the supply of oxygen is limited, the conversion cycle stops before the Krebs’ cycle is initiated and only the anaerobic cycle operates. Anaerobic glycolysis releases far less energy and leads to the accumulation of lactic acid in the fetal blood, causing acidaemia. In addition, if the fetus is growth restricted and has limited glycogen and fat stored, it is at great risk of developing acidaemia and may die.

Fetuses which are at particular risk of developing hypoxia and acidaemia are those whose mother has had a pregnancy complication, or those who have been compromised either before labour or during the birth process (Box 20.1).

During labour the wellbeing of the fetus may be monitored by either external Doppler cardiotocography or by direct electronic monitoring by placement of a fetal scalp electrode once the cervix is sufficiently dilated, i.e. 2–3 cm or more.

The physiological basis of fetal heart monitoring is that when fetal hypoxia occurs the altered composition of the blood causes a rise in sympathetic and vagal tone, which differ in character and effect. In mild hypoxia the sympathetic response predominates, with resulting tachycardia; the onset of the tachycardia is delayed and it persists for 10–30 minutes after the cause of the hypoxia has ceased. In moderate or severe hypoxia the vagal response predominates. Bradycardia is rapid in onset and lasts as long as the hypoxic episode, and then disappears rapidly.

The fetal heart monitor records these changes in the fetal heart rate in relation to uterine contractions. The abnormal patterns are shown in Figure 20.5:

A further piece of information can be obtained from the heart monitor tracing, i.e. beat-to-beat variation (baseline variability). Normally the beat-to-beat heart rate shows variations of more than 5 bpm, leading to a wavy line on the trace. The diminution, or absence, of beat-to-beat variation indicates some degree of fetal hazard.

If, in addition to abnormal fetal heart tracings, the fetus passes thick, green meconium, the probability of severe fetal hypoxia is increased.