Free Oxford Handbook of Midwifery by Janet Medforth, Sue Battersby, Maggie Evans, Beverley Marsh, Angela Walker Page A

sensitizing event is suspected, Kleihauer’s test (on maternal venous blood) estimates the amount of fetal red cells in the maternal circulation, and a measured dose of anti-D can be administered. A
500IU dose is enough to deal with an 8mL transplacental transfusion of fetal blood. Prior to 20 weeks’ gestation it is usual to give 250IU as a prophylactic dose.
Blood samples are taken within one hour of delivery from the
Rh-negative mother to test for maternal antibodies, and fetal cells (Kleihauer’s).
Also from the neonate, to discover its blood group and Rh factor. If the neonate is Rh-negative, then the mother requires no further anti-D Ig.
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
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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
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Sensitivity
The sensitivity of the test is a measurement of how many affected fetuses are detected. This means that around 5% of women having the test will be recalled for further investigation.
The false-positive rate is between 2.6% and 5%. This percent of women will be carrying a normal baby despite a high-risk screening result. About 60 women will be recalled for every affected baby diagnosed.
The false-negative rate is 20%. This percent of women will be carrying an affected baby despite a low-risk screening result.
Results and consequences of screening
Offer diagnostic testing if a woman’s