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:

• An adequate supply of nutrients (especially glucose) and oxygen from the mother

• An adequate placental transfer of nutrients, which depends on a good blood supply reaching the placenta

• A good fetal circulation

• Functioning fetal pancreatic β cells, as insulin is one of the main regulators of fetal growth, provided that nutrients are available.

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
Hypertensive disorders Prolonged pregnancy Maternal diseases (especially eating disorders, renal disease and severe anaemia)	60 5 5
Fetal causes	10
Malformations, transplacental infections Multiple pregnancy
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.

HIGH-RISK PREGNANCY

Women who have a high-risk pregnancy should be identified early if possible, and given more intensive antenatal care, particularly after the 30th week. As mentioned, in some high-risk pregnancies the fetal growth rate is normal or, in the case of pregnancy in a diabetic woman, the baby may be large (macrosomic). In other words, a large number of normal-size-for-dates babies occur in high-risk pregnancies, and their morbidity and mortality is less than if the fetus is growth restricted.

A second method of detecting fetal growth restriction is to measure the symphysiofundal height with a tape measure and refer to a chart (see Fig. 6.12, p. 46).

A third test is to use ultrasound to determine the size of the fetus, the amniotic fluid volume and Doppler umbilical blood flow velocity measurements.

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:

• Fetal movement (fetal kick) counts

• Cardiotocography

• Serial ultrasound examinations

• Doppler flow velocity wave forms.

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.

Fetal movements

In the last half of pregnancy the fetal movements become more regular and at term the average fetus moves 40–50 times per day with a range of 10–130. The woman is asked to either report if she perceives that the fetus is moving less or to formally count the number of times the fetus kicks until she has recorded 10 kicks. Usually this number occurs in a 3-hour period, but as fetuses have different patterns of activity the woman may have to check the number of kicks during another 3-hour period that day. In one study, of those fetuses that had less than 10 movements per day, 36% of babies were subsequently stillborn, compared with none in those whose movements were more than 10 per day. If the fetus fails to kick 10 times in a day, then the woman should be instructed to contact her care giver and cardiotocographic assessment should be performed.

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:

• Normal

• Suspicious

• Pathological.

Table 20.2 RCOG cardiotocographic (CTG) classification

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).

Fig. 20.1 Fetal heart acceleration during a uterine contraction (reactive).

Fig. 20.2 Preterminal fetal heart rate pattern. Note marked reduction in short-term variability and repeated deceleration with Braxton Hicks contractions.

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.

Fetal blood flow velocity

Uteroplacental and fetoplacental circulations are low-resistance systems in which the flow of blood towards the placenta continues throughout the cardiac cycle, causing waveforms of different types. If fetoplacental vascular resistance increases because of acute uteroplacental atherosclerosis, fetal disease or unknown causes, the waveforms are altered. In the umbilical artery the difference between the peak systolic flow (S) and end-diastolic flow (D) can be measured (Fig. 20.3). The greater the S : D ratio the more likely it is that the fetus will be compromised, particularly if absent or reversed end-diastolic flow velocity (AEDFV) or extreme S : D ratio is found.

Fig. 20.3 Doppler umbilical blood patterns.

Fetal blood flow velocity is a useful method of determining whether the placental function is insufficient, and hence whether the wellbeing of the fetus is affected.

Biophysical profile

Some obstetric units, especially in North America, believe that the at-risk status of a fetus can be detected with greater accuracy if a biophysical profile is obtained. This consists of monitoring fetal movements, breathing, tone and reactivity and amniotic fluid volume by real-time ultrasound. The procedure is costly and time-consuming, and at present there is no evidence that it is superior to non-stress testing.

MANAGEMENT OF A HIGH-RISK PREGNANCY

Although fetal growth may be restricted at all stages of pregnancy (commonly in malformed fetuses), it is unusual for problems to arise before the 30th week. The effects of placental dysfunction are observable between the 30th and 35th weeks, and become more marked after that time. In three-quarters of cases the expectant mother has a clinical disorder.

A multicentre trial has demonstrated that a daily low dose of aspirin (75 mg) will reduce the risk of the development of severe pre-eclampsia and of fetal growth restriction in women who have previously had a compromised pregnancy.

All that can be done is to monitor the maternal disease if it is present, to detect deterioration and to check fetal wellbeing (Fig. 20.4). If the maternal condition worsens, for example pre-eclampsia deteriorates or if the fetal monitoring becomes grossly abnormal, e.g. ominous CTG, persistent reverse end-diastolic blood flow, the pregnancy should be terminated either by inducing labour or by caesarean section. If the gestation is less than 34 weeks then if feasible the mother should be given corticosteroids before delivery to enhance fetal lung maturity.