5- Prenatal Testing

Question 1

What prenatal testing is done in normal pregnancy?

Answer 1

Positive pregnancy test – no longer confirmed at GP
•Book into antenatal care – see midwife
•Nuchal scan – 10-14 weeks gestation. Different tests dependent upon NHS Trust e.g. nuchal translucency, combined test etc.
•Mid-trimester anomaly scan
•Ultrasound examination is the main method for prenatal diagnosis of fetal abnormalities. All pregnant women should be offered routine ultrasound scans at 11-14 weeks and again at 20-22 weeks.

Question 2

What can the nuchal translucency test at 12 weeks detect

Answer 2

Thickness of fluid at back of fetal neck
•Increased > 3mm can indicate:
–Chromosome abnormalities
   (e.g. Downs, Edwards, Patau, Turners)
•NT + maternal age detects up to 75% of Down syndrome with 5% false positive rate
–Birth defects: -
•Cardiac anomalies
•Pulmonary defects (diaphragmatic hernia)
•Renal defects
•Abdominal wall defects
–Skeletal dysplasias

Question 3

What are the 3 types of prenatal test

Answer 3

1) Non invasive
•Ultrasound / MRI
2) Minimally invasive
•Maternal blood test
•Cell-free fetal DNA
3) Invasive
•Chorionic villus sampling (CVS)
•Amniocentesis

Question 4

What maternal serum screening tests are carried out?

Answer 4

Tests maternal serum markers in the blood to detect increased risk of fetal trisomy 21, trisomy 18 and/or neural tube defects
–1st trimester maternal serum screening (with nuchal translucency measurement): 11-14 weeks [hCG, PAPP A]
–2nd trimester maternal serum screening (triple screen): 16-20 weeks [AFP, uE3, hCG]
–Nuchal translucency measurement: 11-14 weeks

Question 5

What is a cell-free fetal DNA test used to test for?

Answer 5

Maternal blood test at around 9 weeks of pregnancy
➢Achondroplasia
➢Thanatophoric dysplasia
➢Apert syndrome

Question 6

What are limitations of NIPD and NIPT

Answer 6

Multiple pregnancies - It is not possible to tell which fetus the DNA is from when carrying twins/triplets etc.
•The relative proportion of cell-free fetal DNA is reduced in women with a high BMI as they have more of their own cell-free DNA.

Question 7

What is chronic villus sampling?

Answer 7

11-14 weeks
–1-2% risk of miscarriage
–Transabdominal or transvaginal
–Takes sample of chorionic villi – part of developing placenta – same DNA as fetus
–Allows patient to have an earlier result than amnio

Question 8

What is Amniocentesis?

Answer 8

From 16 weeks
–Takes sample of amniotic fluid which contains fetal cells

•Risks
–Up to 1% risk
   of miscarriage
–Infection
–Rh sensitisation

Question 9

What tests are done with DNA sample?

Answer 9

Test for the genetic disorder in question
–Timing for results dependent upon condition

•Karyotype if chromosomal abnormality in family
–Results 2 weeks (dependent upon the cells growing)

•QF-PCR

Question 10

What does low levels of alpha feto protein indicate

Answer 10

Trisomy 21

Question 11

What is an agent which interferes with normal embryonic development

Answer 11

Teratogen

Question 12

What are some other autosomal dominant diseases

Answer 12

Osteogenesis imperfecta, familial hypercholesterolaemia and Achondroplasia.