Introduction

The field of antenatal investigations has grown greatly in the past few years. Tests are being offered today – and decisions from women are becoming necessary – that would have been unthinkable previously. Although NICE (2008) guidelines for antenatal care recommend a schedule of antenatal tests, there is still a wide variation in what tests are considered ‘routine’ in various parts of the UK. The increase in use of information technology has meant that women and their partners often have accessed much specialized information themselves, and this may shape their questions.

Therefore, midwives need to have a better-than-ever knowledge of what tests are being offered, in order that they can ensure women are making their choices based on up-to-date and comprehensive information. They also need to be effective counsellors as it is acknowledged that the skills and attitudes of midwives influence the uptake of screening tests (Heyman & Henriksen 2001, van den Berg et al 2007). The midwife should also appreciate that the complete clinical antenatal examination is one of the most effective and efficient screening and diagnostic tools, if undertaken systematically and skilfully.

Screening and diagnosis

Although the meanings of screening and diagnosis are very different, they are often confused, and the midwife must ensure that the woman fully understands the difference.

Screening can be defined as determining the risk or likelihood of a condition, whereas a diagnostic test will give a definite answer. Sometimes, action will be taken following the results of a screening test. For example, a low haemoglobin (Hb) result in pregnancy may be assumed to be caused by pregnancy-induced anaemia and iron tablets will probably be given without further investigation, although in a few rare cases the anaemia may be caused by uncommon conditions, such as chronic renal infection, which would need further investigations to obtain a diagnosis. However, it would not be cost-effective to do an infinite range of investigations for every woman who presented with a positive screening test where the usual cause can be easily treated.

Some screening tests will produce results which mean an invasive test will be necessary to obtain a diagnosis. This needs to be made clear to the woman by the midwife providing counselling – should a woman be undertaking a serum screening for Down syndrome test if she would not undergo amniocentesis in the case of a ‘high risk’ result? Some tests, such as ultrasound, can be both screening and diagnostic (see Table 33.1) – for instance, a scan can diagnose a missing limb or neural tube defect, but can also discover anomalies (for example, ‘soft markers’) which would need further investigations to determine a diagnosis. (See the NHS screening website for the timeline for antenatal tests.)

Table 33.1 Common procedures used for fetal assessment

Test		Time
Nuchal translucency (screening)	Chromosomal abnormality	10–14 weeks
Chorionic villus sampling (diagnostic)	Chromosomal abnormality Genetic disease Metabolic disorders Haemoglobinopathies Infection	>10 weeks
Amniocentesis (diagnostic)	Chromosomal abnormality Genetic disease Metabolic disorders Haemoglobinopathies Infection	10–14 weeks (early) 15–18 weeks
Ultrasound (screening and diagnostic)	Assess fetus (dates/growth/viability/number) Diagnosis of some abnormalities (e.g. structural) Screening for abnormalities (e.g. soft markers) Assessment of placental site Liquor volume	All gestations
Cordocentesis (diagnostic)	Obtain fetal blood sample	2nd/3rd trimester
Doppler (screening)	Assess fetal/placental/uterine blood flow	2nd/3rd trimester

It is obviously not enough just to have the tests explained by the midwife – the implications of both positive and negative results also need to be explored before a woman can be said to be making an informed choice. As tests become more varied and complex and midwives’ time more limited, ensuring properly informed choice is becoming a greater challenge for midwives.

Blood tests

Blood is taken from a woman during pregnancy to detect conditions which may influence her wellbeing and that of the developing fetus.

Blood tests for assessment of maternal wellbeing

See Table 33.2 for normal blood laboratory values.

Table 33.2 Normal blood levels and specific diagnostic tests

	Non-pregnant	Pregnant
General screening assays
Haemoglobin	12–16 g/dL	11–13 g/dL
Packed cell volume (PCV)	37–45%	33–39%
Red blood cell count (RBC)	4.2–5.4 million/mm³	3.8–4.4 million/mm³
Mean corpuscular volume (MCV)	80–100 fL	70–90 fL
Mean corpuscular haemoglobin (MCH)	27–34 fL	23–31 fL
Mean corpuscular haemoglobin concentration (MCHC)	32–35 fL	32–35 fL
Reticulocyte count	0.5–1%	1–2%
White blood cells (WBC)	4–11 × 10⁹/L	6–16 × 10⁹/L
Platelets	150–400 × 10⁹/L	150–400 × 10⁹/L
C-reactive protein (CRP)	0–7 g/L	0–7 g/L
Specific diagnostic tests
Serum iron	50–100 mg/dL	30–100 mg/dL
Unsaturated iron binding capacity	250–300 mg/dL	280–400 mg/dL
Transferrin saturation	25–35%	15–30%
Iron stores (bone marrow)	Adequate ferritin	Unchanged
Serum folate	6–16 mg/mL	4–10 mg/mL
Serum vitamin B₁₂	70–85 ng/dL	70–500 ng/dL
Serum ferritin	15–300 pg/L	Unchanged

Serum ferritin levels and total iron-binding capacity (TIBC) may be assessed (McGhee 2000) and causes of insidious blood loss, such as from chronic renal infection or parasitic infestation, may be investigated.

In the past, iron supplements were routinely given to pregnant women, but this is no longer considered appropriate (NICE 2008). Measurement of serum ferritin at booking may predict those who will develop anaemia during pregnancy and therefore treatment could be commenced before the Hb becomes low (Letsky 2002).

Mean corpuscular volume (MCV)

The earliest effect of iron deficiency is a reduced MCV. MCV is also reduced with alpha and beta thalassaemia minor. A raised MCV is associated with folate deficiency (high alcohol intake can reduce absorption of folic acid) or B₁₂ deficiency.

Platelets

Platelets usually stay within the normal range for non-pregnant women, although levels may fall during pregnancy within this range. An abnormal fall could indicate various medical conditions and would need further investigation.

White cell count

The total number of white cells rises in pregnancy, mainly due to the increase of neutrophils. However, an abnormal rise could indicate an infection, and this cause needs additional exploration.

Haemoglobinopathies

Haemoglobinopathies are a diverse group of inherited single-gene disorders involving abnormal haemoglobin patterns which constitute two major conditions: thalassaemia (minor or major) and sickle cell disorders: sickle cell trait (SCT or HbAS); sickle cell haemoglobin C disease (HbSC); and sickle cell disease/anaemia (HbSS).

Both sickle cell disease and thalassaemia are recessive conditions (see Ch. 26), therefore only those inheriting an affected gene from each parent will have the disease. If a woman is found to be carrying either the HbS gene or the thalassaemia trait (thalassaemia minor), it is necessary to test her partner before a prediction about the baby’s condition can be made. If both parents carry the gene, prenatal diagnosis can be made by chorionic villus sampling (CVS), amniocentesis or, more rarely, cordocentesis.

Currently, in high-prevalence areas, booking bloods for all women are automatically screened by hospital laboratories. In areas considered low prevalence, the Family Origin Questionnaire (DH 2007) should be used by midwives to identify who to test (see website).