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

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