Genetic Testing

Question 1

Why might a Normal Nuchal scan (ultrasound) be taken and when are they taken

Answer 1

To read thickness of fluid at the back of the neck at 12 weeks

Question 2

Aim of 12 week scan

Answer 2

To date the pregnancy accurately

To diagnose multiple pregnancies

To diagnose major foetal abnormalities

To diagnose early miscarriage

To assess the risk of Down Syndrome and other chromosomal abnormalities

Question 3

Nuchal translucency

Answer 3

• Done at 10-14 weeks
• It is thickness of fluid at back of foetal neck
• Is a screening test and NOT diagnostic
• Can indicate a range of genetic syndromes
  Increased >3mm indicated chromosome abnormalities eg diwns syndrome,Edward’s,turners,patau

Question 4

When is prenatal testing arranged

Answer 4

• If previous pregnancy was affected with a condition e.g. DS, CF
• If parent(s) is a carrier of chromosome rearrangement or genetic condition e.g. t(13;14), DMD, Huntingtons
• Family history of genetic condition
• Following abnormal findings at nuchal scan or mid-trimester scan
• Following results of combined test which given an increased risk of Down syndrome

Question 5

Non invasive prenatal diagnosis

Answer 5

Fetal dna found in mothers plasma at around 9 weeks

Question 6

NIPD for gender

Answer 6

SRY gene in Y chromosome

Question 7

When would there be an increase in the amount of cfDNA for chromosome 21?

Answer 7

In trisomy 21

Question 8

• For Cystic Fibrosis, what test is used to detect both maternal and paternal mutation in the fetal DNA?

-

Answer 8

NIPD offered privately via nhs

Question 9

Aneuploidy testing

Answer 9

Offered privately or via research studies
Non invasive prenatal test is used which isn’t diagnostic

Question 10

Limitations of NIPD and NIPT

Answer 10

not possible in multiple pregnancies

Question 11

Benefits of NIPD and NIPT

Answer 11

• The number of invasive tests carried out is likely to reduce as a result
• There is no increased risk of miscarriage- invasive do
• Less expertise is required to perform a blood test than an invasive test
• In many cases we can offer NIPD/NIPT earlier than traditional invasive testing, thereby getting a result much earlier

Question 12

Invasive testing

Answer 12

Only given if known risk

Question 13

CVS

Answer 13

A sample of chorionic villi which is a part of the placenta which has the same dna as the fetus is taken trans abdominal or transvaginally
Carried out at 11-14 weeks

Question 14

Amniocentesis

Answer 14

At 16 weeks
Sample of amniotic fluid which contains fetal cells is taken
Risk of miscarriage infection and Rh sensitisation

Question 15

CGH array

Answer 15

If there are concerns on 20 week scan, the gold standard is to offer CGH array

Look for small/large imbalances in chromosomes (microdeletions and duplications)

Question 16

What happens if something is found on CGH array

Answer 16

Standard test parents to see if either is a carrier

Question 17

What are the 4 steps in carrying out a CGH array?

Answer 17

Extract and label dna
Hybridize and wash
Scan
Analyze data

Question 18

Trio exome

Answer 18

Allows us to see exome of baby to find anomalies
Exome is the coding region

Question 19

Why are there fewer egg and sperm donors

Answer 19

As donations are no longer anonymous

Question 20

PGD

Answer 20

uses IVF to genetically test embryo before implantation

1. Stimulation of ovaries
2. Egg collection
3. Insemination - intracytoplasmic sperm injection (ICSI) of only a single sperm to reduce contamination from more sperm cells
4. Fertilisatiton
5. Embryo biopsy
6. Embryo testing - at blastocyst stage
7. Embryo transfer - of those that did not inherit the genetic condition into the womb
8. Pregnancy test

Question 21

What disorders do PGD usually test for

Answer 21

Translocation carriers
HD
DMD
CF

Question 22

Timings of scans

Answer 22

Nuchal translucency (12 weeks)
NIPD cell free foetal dna (9weeks)
CVS (12 weeks)
Amniocentesis (15 weeks)
Mid trimester scan (20 weeks)
CGH array
Prenatal exome

Question 23

Prenatal exome

Answer 23

Done if there are major fetal abnormalities
Exome is the coding region take dna from parents and children