Chromosomal abnormalities

Question 1

Screening tests available?

Answer 1

Combined (10-14 weeks)
Triple
Quadruple (14-20 weeks)

Question 2

Risk factor common to trisomy 13, 18 and 21?

Answer 2

Maternal age

Question 3

What is performed during the combined screening test

Answer 3

Scan (nuchal translucency) + bloods (PAPP-A and HCG)

Question 4

Nuchal scan

Answer 4

The nuchal translucency is measured - the larger it is (>5mm) the higher the risk of structural defects

Question 5

PAPP-A and HcG

Answer 5

PAPP-A (Pregnancy associated plasma protein-A):
Produced by placenta
Low levels associated with chromosomal abnormality

HcG:
Initially produced by embryo, then placenta
Raised level of HCG associated with down’s
Low levels associated with Edward’s and Patau’s

Question 6

How is the risk of trisomy 21 calculated?

Answer 6

Maternal age x risk ratio from combined screening test

Positive if >1 in 150 chance - offer diagnostic test

Question 7

Other than chromosomal abnormalities what can the combined test screen for?

Answer 7

Structural defects - cardiac, renal, spina bifida

Anencephaly

Question 8

Sensitivity and specificity of combined test?

Answer 8

85% sensitive
2.2% specific (false +ve rate)

1/9 who have +ve screening result have a downs baby

Question 9

Quadruple test

Answer 9

15% women present after 1st trimester:
Offered serum testing for DOWNS ONLY - less effective than combined (80% sensitive, 4% false +ve)
AFP
HCG
Estriol
Inhibin-A

Question 10

Quadruple test indicators for downs

Answer 10

AFP and estriol = decreased

HCG and inhibin- A = increased

Question 11

Non-invasive pre-natal testing

Answer 11

99% detection rate
Available in private sector (becoming more common in NHS)

Placental DNA extracted from maternal blood sample

Question 12

Anomoly scan

Answer 12

20 weeks (18-21) - offered to all women

USS scan for anencephaly, gastroschesis, Trisomy 13 etc
If abnormality is detected they are referred\

Counsel - many defects are hard to diagnose and maybe missed - esp. cardiac defects

Question 13

Prenatal diagnostic tests

Answer 13

Performed if there is suspicion of disease:
FH
Past obstetric Hx
+ve screening or anomaly scan

Question 14

What can be detected using USS

Answer 14

Neural tube defect
Twin to twin transfusion syndrome
gastroschisis

Question 15

What is detected using invasive procedures (CVS or amniocentesis)

Answer 15

Trisomy 21
CF
Thalassaemia

Question 16

Counselling on invasive testing

Answer 16

1. Why it is being offered
2. What the test involves
3. Risks
4. Limitations (not all conditions are detected)
5. Management options:
   A. Continue - help plan delivery and emotionally
   B. TOP
6. Complete a formal consent form

Question 17

Chorion villus sampling (CVS)

Answer 17

10-13 weeks (usually offered after combined)

Aspiration of trophoblastic tissue –> karyotype, FISH, PCR
Guided by USS

Results within 48 hrs
Miscarriage rates = 1-2%

Question 18

Benefits/disadvantages of CVS

Answer 18

Benefit - can TOP in 1st trimester

Disadvantage - miscarriage + risk of vertical transmission of HIV and Hep B

Question 19

Amniocentesis

Answer 19

15+ weeks

Transabdominal aspiration of amniotic fluid

results in 3 weeks

Miscarriage rates = 0.5%

Question 20

benefit and disadvantages of amniocentesis

Answer 20

Benefits - reduced rate of miscarriage and vertical transmission

Disadvantages - 2nd trimester and full karyotyping takes 3 weeks