Genetics 5 (Prenatal Diagnosis)

Question 1

What are the scans offered for a normal pregnancy?

Answer 1

• Nuchal scan: 12 weeks
• Mid-trimester anomaly scan
• ultrasound scans: 20-22 weeks

Question 2

When is a nuchal scan offered and what is it used to determine?

Answer 2

• 12 weeks
• Dates pregnancy
• Multiple pregnancies
• Major foetal abnormalities
• Early miscarriage
• Risk of chromosomal abnormalities

Question 3

What are the features of a Nuchal Translucency Test?

Answer 3

• 10-14 weeks
• measures fluid at back of baby’s neck
• <3mm normal
• can indicate: Chromosomal abnormalities, Birth defects & Skeletal dysplasia

Question 4

What are the three types of prenatal testing and which test fall under each category?

Answer 4

• Non-invasive (ultrasound & MRI)
• Minimally invasive (maternal blood test & cell free foetal DNA)
• Invasive (chorionic villus sampling & amniocentesis)

Question 5

What can ultrasound tests be used for?

Answer 5

• Nuchal translucency
• Nasal bone
• Dating

Question 6

When is maternal serum screening done and what does it look for?

Answer 6

• 11-14 weeks: hCG and PAPP A

- 16-20 weeks: hCG, PAPP A, AFP and uE3

Question 7

When is cell-free foetal DNA testing offered and what does it determine?

Answer 7

• accurate around 9 weeks
• if baby has a chance of having an X-linked condition
• sex of baby (looks for presence of SRY gene)

Question 8

What are the limitations of NIPD and NIPT?

Answer 8

• Multiple pregnancies: can’t tell which foetus DNA is from
• High BMI: relative proportions of cffDNA reduced in obese women
• Ethical issues

Question 9

What are the benefits of NIPD and NIPT?

Answer 9

• No risk of miscarriage

- Reduces need for more invasive testing

Question 10

What is the problem with invasive prenatal testing?

Answer 10

Small risk of miscarriage

Question 11

What is Chorionic Villus Sampling (CVS) and when is it done?

Answer 11

• transabdominal/transvaginal
• sample taken from from chorionic villus as has same genetic material as foetus
• 11-14 weeks

Question 12

What is amniocentesis and when is it done?

Answer 12

• sample of amniotic fluid with foetal cells taken

- 16+ weeks

Question 13

What further tests are done to the DNA obtained via CVS or amniocentesis?

Answer 13

Karyotype QF-PCR (test for trisomies)

Question 14

Describe pre-implantation genetic diagnosis.

Answer 14

• IVF used to produce zygote
• cell sampled at 8-cell stage and tested to identify any genetic defects
• Only cells with no genetic defects will be implanted

Question 15

What are the eligibility criteria for PGD?

Answer 15

• Female partner under the age of 40
• BMI above 19 and less than 30
• No living unaffected children from relationship
• Both partners are non-smokers

Question 16

What are the conditions for arranging a prenatal test?

Answer 16

• abnormal findings at nuchal scan
• results of combined test
• previous pregnancy affected with condition
• parent is carrier
• family history of genetic condition

Question 17

What is a teratogen?

Answer 17

agent that interferes with normal embryonic or foetal development

Question 18

What does the combined test for Down’s Syndrome measure?

Answer 18

• Levels of hormone free beta-hCG and protein PAPP A

- Down’s: high hCG and low PAPP A

Question 19

When is a foetal MRI undertaken and what does it show?

Answer 19

• 20+ weeks
• shows foetal organs, shows if further tests needed
• foetal cardiac scans show cardiac problems