Prenatal diagnosis of genetic diseases

Question 1

List the indications for referral to Genetics Services regarding prenatal testing

Answer 1

Indications

1. Following abnormal findings at nuchal scan or mid-trimester scan
2. Following results of combined test which give an increased risk of Down Syndrome
3. If previous pregnancy affected with a condition e.g. DS, CF
4. If parent(s) carrier of chromosome rearrangement or genetic condition, e.g. t(13;14), DMD, HD.

Combined test for Down’s syndrome

1. Combined test =levels of the hormone free beta-hCG +protein PAPP-A
2. Down’s babies HIGH hCG and LOW levels of PAPP-A.

Question 2

Explain the reproductive options available, summarise the psychosocial aspects associated with reproductive decision making

Answer 2

1. Planning prenatal testing
2. Facilitating decision making
3. Seeing patients in clinic following diagnosis in utero
4. Arrange termination if necessary
5. Discuss recurrence risks and plans for future pregnancies
6. Taking into account: previous experiences, family situation, personal beliefs, psychosocial situation, miscarriage risk with genetic risk

Question 3

Explain the use of non-invasive tests (maternal serum screening, ultrasound, NIPT) and the use of invasive tests (amniocentesis, chorionic villus sampling)

Answer 3

Invasive;

1. Amniocentesis
   
   1. From 16 weeks
   2. Sample of amniotic fluid which contains fetal cells
   3. 1% miscarriage risk, infection, Rh sensitisation
2. Chorionic villus sampling
   1. 11-14 weeks, 1-2% miscarriage risk
   2. Transabdominal or transvaginal
   3. Sample of chorionic villi-part of developing placenta- same DNA as fetus
   4. Earlier result than amnio

Non -invasive;

1. Maternal serum screening
   
   • Serum markers in blood can detect increased risk of trisomy 21, 18 and/or neural tube defects
2. Ultrasound
3. cffDNA (cell free fetal DNA)

• Analyses DNA fragments in maternal plasma during pregnancy – 10-20% comes from placenta and is representative of unborn baby (maternal blood test => accurate at 9weeks)