Case 58: Antenatal screening

 

Case 58: Antenatal screening

CASE 58: ANTENATAL SCREENING

History

A woman aged 34 years is 9 weeks’ gestation in her third pregnancy. Her first pregnancy

ended in a first-trimester suction termination at 18 years of age and she had a miscarriage

8 months ago requiring surgical management. She is generally well except for mild asthma.

She has no family history of congenital abnormalities. She is a non-smoker and currently drinks

approximately 3 units of alcohol per week. Her only medication is folic acid 400 mcg daily.

Her partner is 31 years old and was adopted. He has no known medical problems.

The routine booking blood and urine tests are normal. The couple opt for Down’s syndrome

screening and a first-trimester ultrasound appointment is booked for 12 weeks.

INVESTIGATIONS

The first-trimester ultrasound findings are shown in Fig. 58.1.

Ultrasound report:

Single fetus. Fetal heart action normal

Crown–rump length: 62.4 mm (corresponds to 12 weeks 3 days’ gestation)

Nuchal translucency (NT): 3.2 mm

Risk of trisomy by maternal age (34 years): 1:276

Adjusted risk of trisomy after NT: 1:30

Questions

• How would you explain the report to the couple?

• The couple chose to have a further test and the results are normal (46 XY normal

karyotype). What further diagnoses should be considered?

Figure 58.1 First-trimester transabdominal ultrasound scan.100 Cases in Obstetrics and Gynaecology

158

ANSWER 58

Explanation of the report

Down’s syndrome screening can be difficult to explain, and any discussion should start with

checking that the couple understands what Down’s syndrome is:

• a chromosomal ‘genetic’ problem that usually occurs sporadically (‘by chance’)

• associated with physical abnormalities which may be relatively minor, such as short

stature, abnormal facial appearance, or major, such as severe cardiac abnormality

• always associated with learning disability, though the extent is variable

• generally associated with life expectancy up to 40 or 50 years.

The ‘nuchal translucency’ test is a screening test and does not give a definite answer as to

whether the pregnancy is affected or not. The risk in this case based on the mother’s age alone

is 1 in 276, but the high NT measurement combined with the maternal age suggests that the

risk for this particular baby is 1 in 30. Most women even with a high risk result actually have

a normal fetus (a false-positive result).

Further investigation

Fetuses with a high-risk NT but normal karyotype have an increased likelihood of other

structural defects such as congenital heart disease, exomphalos, diaphragmatic hernia and

skeletal defects. This couple should therefore have a detailed anomaly scan and fetal cardiac

echo at around 20 weeks’ gestation.

KEY POINTS

• Nuchal translucency (NT) is a screening test not a diagnostic test.

• The NT result may increase their risk but most women with increased NT have a

normal fetus (false positive). However women who have a high-risk NT and normal

karyotype are at risk of other structural defects, so a detailed anomaly scan should

be performed at around 20 weeks.