Chapter 7. Blood tests in pregnancy
Introduction
Pregnancy is a time when women are faced with many choices and decisions. There is a plethora of information about why various tests are being made, but it does not always reach those women who need it most. An important aspect of the student midwife’s progression from novice to competent practitioner is the development of an ability to present information to women in a meaningful manner, in a way that makes sense to them. As well as developing the necessary communications skills and self-awareness to approach women with confidence and competence, the student needs to feel that s/he has a firm underpinning knowledge of the facts. Being a student is challenging. Just as you thought you were getting to grips with a subject, a new dimension emerges. As you become familiar with one policy another is produced to take its place. Change is a constant factor in professional life, but an understanding of key principles provides a stable base from which new approaches can be explored.
This chapter considers the routine blood tests offered in pregnancy (Table 7.1). Although women may be advised to undergo such blood tests, they should not be pressured or coerced. It must also be noted that some maternity units offer different tests at different times, depending on local policy and the needs of the local community. Antenatal screening for fetal abnormality, including inherited disorders, will be considered separately in Chapter 8.
| Blood test | Who | When |
|---|---|---|
| Full blood count (FBC) | All | Booking and third trimester |
| Hepatitis B virus (HBV) | All | Booking |
| Blood group and rhesus factor | All | Booking |
| Rhesus antibody screening | Rhesus negative women | 28 weeks |
| Red cell alloantibodies | All | Booking and 28 weeks |
| Rubella antibodies | All | Preconceptually or booking |
| Syphilis (VDRL) | All | Booking |
| Human immunodeficiency virus (HIV) | All | Booking |
| Maternal serum screening for Down’s syndrome (see Chapter 8) | All | Combined test at 11 + 0 days–13 weeks + 6 days. Triple/Quad test 15–20 weeks |
| Sickle cell anaemia | Women (or those with partners or relatives) from Africa, Mediterranean, Far/Middle East, Asia and Caribbean | Booking |
| Thalassaemia (see Chapter 8) | Booking |
Midwives involved in taking blood from pregnant women need to consider the following:
▪ Occasionally some women do not want to have what are considered by professionals to be ‘routine’ tests. They have a right to decline. Although you have a responsibility to inform them of the consequences of their decision and to document that you have done so, take a non-judgmental approach. As a student, you must always refer any controversy or uncertainty to a registered professional.
▪ Just because there is a lot of information to absorb during pregnancy, does not mean that women do not want to receive it.
▪ Some women have needle phobia and need to be treated with sensitivity.
▪ Some women faint when they have blood taken – always ask a woman if this is the case!
▪ See Midwifery Essentials: Basics, Chapter 9: Venepuncture, for the procedure for taking blood.
The booking history was explored in Chapter 3. It was seen that a detailed account of the woman’s current health, previous medical, social and obstetric history is taken in order to identify potential factors that might impact on the woman’s or her baby’s wellbeing. Taking maternal blood is another procedure that enables further risk factors to be identified or potential pathology avoided.
The sooner that these important blood tests are taken, the sooner results can be acted upon. NICE guidance (NICE 2008) recommends that the booking appointment takes place by week 10 of the pregnancy. However, when blood is taken from women depends on the model of care in operation and on the local services available. Some community midwives who undertake the booking history in the community also take the booking bloods at the same time. It is important that there is a system to ensure safe transportation of these blood samples to the local laboratory, which is usually situated in the hospital Trust. The midwife will often make her booking appointments in the morning so that the samples can meet the pick-up service that serves the general practitioner’s surgery or health centre that she links with. Where such services do not exist, women attend the hospital for booking bloods, usually at the same time as the first scan. In some areas, the practice nurse takes blood, after appropriate counselling by the midwife, and there is a local arrangement for samples to be taken at a time that coincides with other clinics.
Blood tests in pregnancy
For a summary of the blood tests routinely offered in pregnancy, see Table 7.1.
Full blood count
The full blood count is taken to identify maternal anaemia with the aim of treating the condition, if necessary. However, it also provides a useful picture of the woman’s current health status in many more respects (see Table 7.2). For example, a fall in platelets may be seen in women who develop pre-eclampsia, and a raised white cell count may be indicative of underlying infection. In some trusts, serum ferritin levels are also measured as they reflect the woman’s iron reserves and fall before haemoglobin levels.
| Test | What | Normal (women) | Pregnancy |
|---|---|---|---|
| Haemoglobin (Hb) | Amount of haemoglobin in the blood measured in grams per decilitre: g/dL or g/100ml | 12.5–15.5 | Falls 2g until about 30 weeks (steepest drop by 20 weeks) then rises slightly to term |
| Red cell count (RCC) | Number of red cells per litre of blood: ×1012 per litre | 4.2–4.5 | Falls by about 1 until 30–34 weeks |
| Haematotcrit (Hct) of packed cell volume (PCV) | Percentage of blood cells to total blood volume | 35–45 | Falls by about 6% until 30 weeks then rises slightly to term |
| Mean corpuscular (cell) volume (MCV) | An estimated volume of indivdual red blood cells (fL) | 80–100 | Increase in macrocytic anaemia Decrease in microcytic anaemia |
| Mean cell haemoglobin (MCH) | Haemoglobin content within erythrocyte of average size (pg) | 27–32 | Increase in macrocytic anaemia Decrease in microcytic anaemia |
| Red cell mass | Total volume of red blood cells in the circulation | 1400 | Up to 1650 by term |
| White cell count (WCC) | Total number of white blood cells in the circulation | 4–11 | May be raised up to 15 |
| Platelets | Number of platelets per litre of blood (×103) per mm3 | 150–400 | Slight decrease |
It is normal for a woman’s haemoglobin to fall in pregnancy and this reflects an expansion in plasma volume which exceeds the increase in red cell mass. A failure of the haemoglobin concentration to fall has been linked with an increase in the incidence of preterm birth and low birthweight (Steer et al 1995). Stephansson et al (2000), in a matched case-control study, found an association between high first haemoglobin and increased risk of stillbirth. The National Institute for Health and Clinical Excellence (NICE 2008) recommend that a haemoglobin level of less than 11g per decilitre (g/100ml) at booking and below 10.5g/100ml at 28 weeks should be investigated and iron supplementation considered. Barrett et al (1994) argue that during normal pregnancy, the woman increases her absorption of iron from her diet and this adaptation is sufficient to meet the requirements of pregnancy if her diet is adequate. There are many issues regarding correct diagnosis of iron deficiency anaemia (Coggins 2001). According to Enkin et al (2000:43):
an individual’s haemoglobin concentration depends much more on the complex relation between red-cell mass and plasma volume than on deficiencies of iron or folate.
It is suggested that mean corpuscular volume (MCV) is a better indicator of iron deficiency (Stables & Rankin 2005): it falls during iron deficiency anaemia and is higher in folic acid deficiency anaemia (Bewley 2004). However, further research is required to account for potential confounding by the macrocytic effect of folate deficiency (Steer et al 1995). Reveiz et al (2007), in a systematic review of the evidence regarding the treatment of iron deficiency anaemia in pregnancy, did not find any conclusive evidence as to when or how it should be treated.
Blood group and rhesus factor
The batch of blood samples taken at the booking appointment include blood group and rhesus factor. There are four blood groups and each may be either rhesus positive or negative, giving eight different types in total (see Table 7.3).
| Group | Rhesus + | Rhesus − |
|---|---|---|
| A | A + | A − |
| B | B + | B − |
| AB | AB + | AB − |
| O | O + | O − |
It is important to know the woman’s blood group in the event that she might require an emergency blood transfusion. However, when it is anticipated that a woman might need a blood transfusion, during a caesarean section or following a post-partum haemorrhage, for example, a blood sample will be taken so that her blood and donor blood can be ‘cross-matched’ in the laboratory, to ensure compatibility of the transfused blood with the woman’s blood.
Identifying a woman’s rhesus factor is important during pregnancy for the 15% of women who are rhesus negative. If blood from a rhesus negative person mixes with blood from a rhesus positive person, rhesus antibodies will be formed. If this happens again, the body is already armed to fight the invasion of a foreign factor, and haemolysis occurs. Consider that a rhesus negative woman is carrying a rhesus positive baby. If some of the baby’s blood transfers across the placenta into her bloodstream, after approximately 72 hours she will develop rhesus antibodies. On this occasion there is little consequence; however, if the woman were to become pregnant again with a rhesus positive baby, antibodies would already be present and able to cross the placenta causing haemolysis of the fetal red blood cells (erythrocytes). The potential for harm depends on the degree of haemolysis. The developing baby may become anaemic through the development of this condition known as ‘haemolytic disease of the newborn’ (HDN). This is a potentially fatal condition: the fetus may develop heart failure ‘hydrops fetalis’ or if born alive, severe jaundice. Rhesus negative women should have their rhesus antibody status checked in early pregnancy and again at 28 weeks’ gestation. For a summary of how the development of rhesus antibodies can be prevented, see Box 7.1.
Situations that increase the likelihood of blood from the fetus entering the maternal bloodstream are known as ‘sensitizing events’. Identify five such events.
Box 7.1
Prevention of rhesus antibodies
Development of rhesus antibodies by the mother can be prevented if:
▪ the sensitizing event is known
▪ the significance of the event is realized
▪ the event is reported
▪ the practitioner acts on the knowledge
▪ Anti-D immunoglobulin is administered within 72 hours of the event
▪ prophylactic anti-D is administered during pregnancy.
Anti-D
If a sensitizing event is known, reported and confirmed within 72 hours an intramuscular injection of anti-D immunoglobulin can be given to the woman to prevent her from making antibodies in response to the feto-maternal transfusion (fmt). Such an event can be confirmed and quantified by the Kleihauer test or the more accurate flow cytometry (RCOG 2002).
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