Oxford Handbook of Midwifery (27 page)

Rh-negative mother to test for maternal antibodies, and fetal cells (Kleihauer’s).

It is the midwife’s responsibility to carry out tests during pregnancy to identify women who require anti-D prophylaxis. A full explanation should be given to the woman and her consent obtained for any tests or administration of anti-D Ig.

1
National Institute for Health and Clinical Excellence (2008). Routine antenatal anti-D prophy- laxis for women who are rhesus D negative. Technical appraisal 156. London: NICE. Available at: M
www.nice.org.uk/ta156.

CHAPTER 4
Antenatal care

64‌‌

Screening for Down’s syndrome risk

The incidence of Down’s syndrome is approximately 1:600–1:700 across the age range of the childbearing population. There are variations in inci- dence according to maternal age:

• At 18 years of age the incidence is 1:2300
  
• At 35 years it is 1:200–350
  
• At 40 years it is 1:100
  
• At 45 years it is 1:45.
  

  Taking age as the only risk factor would mean that <30% of affected fetuses would be detected by diagnostic testing, as it would not be appropriate to offer amniocentesis to all women.
  

  Down’s risk screening was developed in the 1980s to enable all pregnant women to be given an estimate of individual risk if they choose to be screened. The risk is calculated by examining a combination of the following factors:
  

• Maternal age
  
• Gestational age in completed weeks
  

  •
  Maternal body weight
  

• Serum screening.
  

  Maternal serum screening
  

• This enables examination of a combination of hormones and proteins present in the maternal bloodstream during early pregnancy.
  
• Levels vary according to fetal gestational age.
  
• Abnormally low or high levels are linked to genetic, chromosomal, and structural abnormalities of the fetus.
  
• High levels of A-fetoprotein (AFP) are associated with neural tube defects, and low levels with Down’s syndrome.
  
• Neural tube defects can be confirmed by ultrasound scan and also amniocentesis.
  

  Recommended screening: aims
  

• Screening for Down’s syndrome should be performed by the end of the first trimester (13 weeks 6 days), but provision should be made to allow later screening (which could be as late as 20 weeks 0 days) for women booking later in pregnancy.
  
• The ‘combined test’ (nuchal translucency, B-hCG, pregnancy- associated plasma protein-A) should be offered to screen for Down’s syndrome between 11 weeks 0 days and 13 weeks 6 days.
  
• For women who book later in pregnancy the most clinically and cost- effective serum screening test (triple or quadruple test) should be offered between 15 weeks 0 days and 20 weeks 0 days.
  
• When it is not possible to measure nuchal translucency, owing to fetal position or high BMI, women should be offered serum screening (triple or quadruple test) between 15 weeks 0 days and 20 weeks 0 days.
  

SCREENING FOR DOWN’S SYNDROME RISK

65

Sensitivity

Results and consequences of screening

Further reading

National Institute for Health and Clinical Excellence (2008). Antenatal care: Routine care for the healthy pregnant mother. Clinical guideline 62. London: NICE. Available at: M www.nice.org. uk/cg62.

CHAPTER 4
Antenatal care

66‌‌

Group B haemolytic streptococcus

Group B haemolytic streptococcus (GBS) is one of a number of common bacteria found in the gut of 30% of men and women. It is estimated that 25% of women carry this organism in the vaginal tract with no ill effect. Its significance is that it can be transmitted to the baby during delivery and is the most common cause of fatal bacterial infection in the early neonatal period.

Incidence

In the UK, approximately 1:2000 babies annually acquire GBS infection, pre- senting with septicaemia, pneumonia, or meningitis. A UK survey in 2001 identified 376 cases of early-onset GBS disease, 39 of which were fatal.

Presentation

There are two ways in which the infection will present:

• 90% of the infections are early onset, and 70% of babies are symptomatic at birth
  

  •
  10% are late onset, occurring after 48h and up to 3 months after birth.
  

  Screening
  

  There is little organized antenatal screening for GBS carriage in the UK at present and most maternity units rely on risk factor estimation to iden- tify carriers and situations where the infection may be transmitted to the neonate.
  

  Risk factors
  

• Previous baby affected by GBS.
  
• GBS bacteriuria detected in the present pregnancy.
  

• Pre-term labour.
  
• Prolonged rupture of the membranes.
  
• Pyrexia during labour.
  

  Intrapartum antibiotic prophylaxis (IAP) is offered to women who have any of these risk factors. This approach differs from that in the USA, where all pregnant women are offered bacteriological screening at 35–37 weeks’ gestation. This involves taking vaginal and rectal swabs, and all women who carry GBS are offered IAP. This results in 27% of all pregnant women being offered IAP during labour and a reduction in early onset GBS disease of 86%.
  

  Royal College of Obstetrics and Gynaecology recommendations
  

  The Royal College of Obstetrics and Gynaecology (RCOG)
  ¹
  has made the following recommendations in the absence of clinical trials and recent data on the prevalence of GBS carriage in the UK.
  

• Routine antenatal screening is not recommended.
  
• Antenatal treatment with penicillin is not recommended if GBS is detected incidentally.
  
• IAP should be considered if GBS is detected incidentally.
  
• There is no good evidence to support IAP in women who carried GBS in a previous pregnancy.
  
• IAP should be offered to women who had a previous baby with GBS disease.
  

  GROUP B HAEMOLYTIC STREPTOCOCCUS
  

  67
  

1
Royal College of Obstetrics and Gynaecology (2007).
Preventing Group B Streptococcus Infection in Newborn Babies
. London: RCOG.

CHAPTER 4
Antenatal care

68‌‌

Sickle cell anaemia

Haemoglobin is a complex molecule with the ability to absorb oxygen easily and reversibly. The molecule is composed of iron and protein. The protein structure is inherited and is the part affected in haemoglobino- pathies, being either abnormal or partly missing.

A normal red blood cell (RBC) in an adult is filled with adult haemoglobin. Everyone inherits their haemoglobin type from their parents, half the responsible gene copies from each, and the usual type is HbAA.

Sickle cell trait

• Sickle haemoglobin (abbreviated HbS) has an abnormality of the protein part of the molecule.
  
• An individual inheriting HbS from one parent and HbA from the other will have haemoglobin type HbSA.
  
• This is known as the sickle-cell trait.
  
• The RBCs of such individuals will function normally and they will have few, if any, symptoms.
  

  •
  However, these individuals have a 50% chance of passing this type of haemoglobin on to their children.
  

• This condition confers immunity from the malaria parasite, which explains the prevalence of the condition in areas where malaria infection is endemic.
  
• Due to population movement, individuals can inherit this type of haemoglobin even if their ancestry is from malaria-free areas.
  

  Sickle cell anaemia
  

• In this case, the individual inherits HbS from both parents, so the haemoglobin type is HbSS.
  
• RBCs containing only HbSS react to hypoxia, acidosis, or dehydration by changing shape, from the usual bi-concave disc to a crescent or sickle shape.
  
• These RBCs are more fragile, easily damaged, and will clump together, blocking capillaries.
  
• Painful crises are provoked by the blockage of small blood vessels.
  
• The overall effect is that the RBCs are haemolysed, rapidly causing chronic haemolytic anaemia.
  

  Effects on childbearing
  

• Subfertility
  
• Impaired placental function
  
• Increased risk of pregnancy-induced hypertension
  
• Pulmonary and renal problems
  
• Phlebitis
  
• Women with this type of anaemia need to be well hydrated during labour and need careful monitoring to avoid hypoxia should an anaesthetic be required.
  

SICKLE CELL ANAEMIA

69

Screening

Further reading

National Institute for Health and Clinical Excellence (2008). Antenatal care: Routine care for the healthy pregnant mother. Clinical guideline 62. London: NICE. Available at: M www.nice.org. uk/cg62.

CHAPTER 4
Antenatal care

70‌‌

Thalassaemia

In this recessively inherited condition part of the haemoglobin protein is missing. The protein is made from structures called A and B chains. As several genes are responsible for the structure of these chains, it is pos- sible to have varying degrees of the condition.

B
-thalassaemia minor

The individual inherits one normal gene from one parent and one affected gene from the other parent. This is a carrier state, and has little effect on health other than mild anaemia. Affected individuals can pass on the defective gene to their children.

B
-thalassaemia major

The individual inherits defective genes from both parents and can make no, or very few, B chains, so does not produce sufficient haemoglobin. This results in severe anaemia requiring regular blood transfusions and therapy to remove excess iron from the blood.

A
-thalassaemia

People with normal haemoglobin carry four A globin genes, two from each parent. A-thalassaemia results from the deletion of one or more of these genes. Table 4.2 shows the result of deletions of one or more of the genes and the effect on the type of haemoglobin produced.

Table 4.2
Effect of gene deletions

Screening for thalassaemia

• As with sickle cell anaemia, antenatal screening should be offered to all pregnant women as recommended.
  ¹
  
• A routine FBC will identify women with hypochromic, microcytic anaemia. Haemoglobin electrophoresis will then identify the underlying haemoglobinopathy.
  
• Knowing the carrier state of each parent allows counselling about the risks to the fetus of being a carrier or having the disease.
  
• Haemoglobinopathy screening is carried out on newborns as part of the neonatal blood spot test.
  

  THALASSAEMIA
  

  71
  

  Impact of maternal thalassaemia major on pregnancy