1. GESTATIONAL AGE
The traditional method of determining gestational age is from the date of the last menstrual period and clinical examination (see Chapter 5).
These methods may not be reliable:
1. LMP uncertain or forgotten.
2. Calculation depends on a ‘normal’ 28 day cycle.
3. The widespread use of hormonal contraception, oral and parenteral, makes ovulation unpredictable.
4. Uterine size may be difficult to determine, particularly in the patient who is obese or tense.
ULTRASOUND
Ultrasound examination, pioneered by Donald in Glasgow in the late 1950s and early 1960s, has become the cornerstone of fetal assessment. Increased resolution and portability have led to the widespread use of ultrasound equipment from the earliest stages of pregnancy through to care during labour. Further, ultrasound is the only reliable technique which can be used in early pregnancy to identify both fetal viability and multiple pregnancy.
The contribution of ultrasound to obstetric practice is so great that it is useful to summarise the uses to which ultrasound may be applied:
| Routine Use: | Confirmation of ongoing intra-uterine pregnancy.Assessment of gestational age (measurement of crown–rump length, biparietal diameter, femur length).Identification of multiple pregnancy.Recognition of major anomalies. |
| Specific Uses: | Threatened miscarriage (to confirm fetus is alive).Antepartum haemorrhage (placental localisation).Fetal growth studies (head–trunk ratio, estimated fetal weight, liquor volume, placental grade).Assessment of high-risk cases (maternal disease, elevated serum alphafetoprotein, history of anomaly).Postpartum (retained products). |
Pelvic masses.
| Adjunct to Interventional Procedures: | Chorion villus samplingAmniocentesis.Fetal blood sampling (cordocentesis).Intra-uterine therapy, e.g. intra-uterine transfusion.Fetoscopy |
FETAL ABNORMALITY
2. IDENTIFICATION OF FETAL ABNORMALITY
The identification of fetal abnormality is an important component of antenatal care. It is now possible to identify a large number of abnormalities. Many of these are sufficiently severe to be detectable at early gestations, allowing the woman to elect for termination of the pregnancy if she wishes.
In other cases, termination may not be indicated but knowledge of the presence of the condition permits preparation of the woman, her partner and the paediatric and obstetric teams for treatment following delivery. A number of conditions may be amenable to fetal surgery but this expertise remains largely within a small number of highly specialised centres.
The term ‘prenatal diagnosis’ has become established to describe these diagnostic techniques.
SCREENING TESTS AND DIAGNOSTIC TESTS
It is important to distinguish between tests which are applied as screening tests and those which are diagnostic.
Screening tests are offered to a population in which there are no specific risk factors that would indicate the need for diagnostic testing. Thus a woman who had previously had a pregnancy complicated by fetal trisomy may elect to opt for diagnostic testing from the outset. The more sensitive and specific a screening test the closer it approximates a diagnostic test.
A. Screening at 11 to 14 weeks
Screening for congenital abnormality may be undertaken using biochemical assays, ultrasound or both.
Ultrasound can be used at this stage to identify structural abnormalities. Anencephaly and anterior abdominal wall defects may be seen but more subtle signs are now used, the foremost of which is measurement of nuchal translucency. In normal first trimester pregnancy a fluid-filled area on the posterior surface of the neck may be seen and measured. An association between increased size of nuchal translucency (NT) and chromosomal and heart abnormalities is now well recognised. At any given maternal age the measurement of NT can be used to modify the underlying age-related risk of a fetal trisomy.
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The measurement of NT can be analysed in combination with biochemical markers such as Pregnancy Associated Proteins and oestriol and HCG to give a specific risk of the fetus being affected. In many centres a risk of 1:250 would be used to identify women at sufficiently high risk to merit diagnostic testing such as chorionic villus sampling or amniocentesis.
B. Screening at 15 to 21 weeks
Detailed ultrasound assessment of fetal anatomy is commonly undertaken around 18 to 20 weeks gestation but at this stage ultrasound is not of value in identifying Down’s syndrome since nuchal translucency has disappeared at this stage (in the absence of associated abnormalities such as anterior abdominal wall defects such as exomphalos). Biochemical measurement of maternal serum alphafetoprotein (AFP), HCG and oestriol can be used, in conjunction with maternal age, to calculate a risk for Down’s syndrome, so-called triple testing. Again a high risk indicates the need for amniocentesis to obtain a fetal karyotype.
Measurement of AFP alone can be used as a screening test for a number of defects if detailed anatomy screening by ultrasound is not available. AFP is elevated in a number of conditions and, if elevated, indicates the need for further ultrasound assessment.
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