Chapter 10 Specialised Fetal Investigations

Advances in technology mean that assessment of the fetus during pregnancy has become increasingly sophisticated and more widespread. Biochemical tests on maternal serum are commonly performed in order to identify which pregnancies carry a high risk of Down syndrome (trisomy 21) and ultrasound scanning is continually being refined.

Psychological aspects of prenatal testing

Prenatal testing is a two-edged sword. It enables midwives and doctors to give people choices. However, they may increase the amount of anxiety and psychological trauma experienced in pregnancy.

Anxiety caused by consideration of possible fetal abnormality may be accompanied by moral or religious dilemmas. For instance, tests that can diagnose chromosomal or genetic abnormalities also carry a risk of procedure-induced miscarriage. Many parents agonise about whether to subject a potentially normal fetus to this risk in order to obtain this information. Parents may then need to consider whether they wish to terminate or continue with an affected pregnancy. Informed consent must be given (Box 10.1).

Box 10.1 Information required for obtaining informed consent

• Purpose of the procedure

• All risks and benefits to be reasonably expected

• Details of all possible future treatments that could arise as a consequence of testing

• Disclosure of all appropriate techniques that may be advantageous

• The option of refusing any tests

• The offer to answer any queries

Tests for fetal abnormality

Broadly speaking, there are two types of test for fetal anomaly:

screening tests

diagnostic tests.

Screening for fetal abnormality

Screening tests aim to identify a proportion of individuals (often around 5% of a population) who have the highest chances of a named disorder. This makes it possible to target further investigations towards those with the best indication. Mothers who undergo screening tests will be classified as above or below an action limit, whereby they are recalled and offered follow-up procedures.

The performance of a screening test is defined in a number of ways (Box 10.2).

Box 10.2 Performance criteria for screening tests

Detection rate/sensitivity

• The proportion of affected pregnancies that would be identified as high risk

False-positive rate

• The proportion of unaffected pregnancies with a high-risk classification

• The higher the specificity, the fewer the false positives

False-negative rate

• The proportion of affected pregnancies that would not be identified as high risk

• The higher the sensitivity, the fewer the false negatives

Diagnosis of fetal abnormality

Diagnostic tests are performed in order to confirm or disprove the presence of a particular abnormality.

The use of ultrasound in obstetrics

Most mothers undergo at least one ultrasound scan during pregnancy. This procedure can enable assessment and monitoring of many aspects of the pregnancy and is often presented as ‘routine’. It can be used in order to screen for and to diagnose fetal abnormalities. Ultrasound works by transmitting sound at a very high pitch, via a probe, in a narrow beam. When the sound waves enter the body and encounter a structure, some of that sound is reflected back. The amount of sound reflected varies according to the type of tissue encountered. Generally, pictures are transmitted in ‘real time’, which enables fetal movements to be seen.

First trimester pregnancy scans

Many areas offer mothers a scan in early pregnancy. The purpose of this is to establish:

that the pregnancy is viable and intrauterine

gestational age

fetal number (and chorionicity or amnionicity in multiple pregnancies)

detection of gross fetal abnormalities, such as anencephaly.

There is evidence to suggest that at least one scan is beneficial, mainly in reducing the need to induce labour for postmaturity. A gestation sac can usually be visualised from 5 weeks’ gestation and a small embryo from 6 weeks. Until 13 weeks, gestational age can be accurately assessed by crown–rump length (CRL) measurement. Mothers are asked to attend with a full bladder, since this aids visualisation of the uterus at an early gestation.

Measurement of nuchal translucency at 10–14 weeks as a screen for Down syndrome

Information about the fetus can be gained by observation of the nuchal translucency (NT) at 10–14 weeks’ gestation. This involves measuring the thickness of the subcutaneous collection of fluid at the back of the neck. Increased NT is used as a basis upon which to screen for Down syndrome. The main advantage of this test is that it offers an early way of assessing the mother’s risk for Down syndrome. Increased NT is also associated with other structural (mainly cardiac) and genetic syndromes, and this enables increased pregnancy surveillance to be arranged. However, a disadvantage of this knowledge is that parents may suffer considerable anxiety until later scans offer some degree of reassurance.

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1. The Placenta
2. Trauma During Birth, Haemorrhage and Convulsions
3. Obstetric Emergencies
4. Problems of Pregnancy

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