Case 54: Antenatal screening

 

Case 54: Antenatal screening

CASE 54: ANTENATAL SCREENING

History

A woman aged 23 years is referred by her general practitioner to the antenatal clinic at 12

weeks in her first pregnancy. She has booked late having only just discovered she is pregnant.

She separated from her partner of 2 years a few weeks ago but is supported by her family and

friends. She has no significant medical history and is one of four siblings. On direct questioning her mother apparently had a stillbirth attributed to some form of congenital abnormality

28 years ago. Otherwise the pregnancy is assessed to be low risk.

She is offered a screening test for Down’s syndrome, which she agrees to. This is performed

at 12 weeks 2 days’ gestation.

INVESTIGATIONS

Combined test for Down’s syndrome:

• Pregnancy-associated plasma protein-A (PAPP-A): 0.4 multiples of the mean (MoM)

• Free beta human chorionic gonadotropin (hCG): 1.7 multiples of the mean (MoM)

• Nuchal translucency: 2.9 mm

Overall Down’s syndrome risk calculated as 1 in 118

Questions

• What tests are available for screening for Down’s syndrome and how accurate are they?

• How would you counsel this woman about her options now?100 Cases in Obstetrics and Gynaecology

144

ANSWER 54

Screening for Down’s syndrome

PAPP-A, free beta hCG and nuchal translucency (by ultrasound) are used in combination in

the ‘combined’ test, one of the screening tests available to detect Down’s syndrome. Down’s

syndrome is associated with a decreased level of PAPP-A and increased level of NT and free

beta hCG.

Serum marker levels are however affected by other variables. For example PAPP-A is decreased

in heavier women, about 60 per cent higher in Afro-Caribbean and about 20 per cent lower in

women who smoke, though these factors are taken account of in the risk assessment.

Other screening tests include the ‘triple test’, ‘quadruple test’ and ‘integrated test’. All such

tests aim for a detection rate of at least 90 per cent of affected fetuses and a screen positive rate

of less than 2 per cent of the unaffected fetuses.

The assessment of ‘high’ or ‘low’ risk is dependent on the viewpoint of the individual, but risk

higher than 1 in 150–250 is generally considered ‘high risk’. Such women should be offered

a diagnostic test.

Counselling

The woman should be counselled through the following options.

Expectant management

• The woman may choose not to have any further testing and accept the chance of a

baby with Down’s syndrome.

• Detailed ultrasound scan: at 20 weeks, features of Down’s syndrome (skull abnormalities, ventriculomegaly, atrial septal defect, duodenal atresia, echogenic bowel,

hydronephrosis and short limbs) may be apparent on detailed anomaly scan. If none

of these ‘soft markers’ are found then the woman might choose still to avoid further

diagnostic tests.

Diagnostic tests

• Chorionic villous sampling (CVS):

• performed 11 to 14 weeks’ gestation

• ultrasound guidance

• sample of placental tissue collected generally using needle inserted through the

abdominal wall

• small risk of miscarriage (about 1 per cent) associated with the procedure

• inconclusive result in 1 per cent of cases (so amniocentesis then required).

• Amniocentesis:

• performed any time from 15 to 16 weeks’ gestation

• ultrasound guidance

• sample of amniotic fluid collected using needle inserted through the abdominal wall

• small risk of miscarriage (about 1 per cent).

• Cell-free fetal DNA: This recently developed non-invasive diagnostic test detects

cell-free fetal DNA in a sample of maternal blood from 10 weeks’ gestation. It is used

to identify the common trisomies (21, 18, 13) and fetal gender.Case 54: Antenatal screening

145

Women’s decisions depend on many factors. Some will want to know the diagnosis in order

to consider termination of pregnancy, whereas other couples may not opt for termination but

wish to be prepared for a baby with Down’s syndrome (and any medical needs it may have,

such as cardiac abnormalities) without the ongoing uncertainty throughout the duration of

the pregnancy. Time for discussion and sensitivity to the woman’s own situation are imperative in counselling.

KEY POINTS

• The possibility of screening for Down’s syndrome should be considered with all

women regardless of age.

• Women who choose not to undergo screening or diagnostic tests should have this

choice respected.

• Screening tests produce a risk for an individual pregnancy being affected by a

chromosomal abnormality, following which a woman may choose to undergo a

diagnostic test.

• Diagnostic tests are chorionic villus sampling, amniocentesis and testing for cellfree fetal DNA in maternal blood.