Prenatal Testing Flashcards
Prenatal screening
focuses on finding problems among a large population with affordable and noninvasive methods.
Example are – Down syndrome, Blood Borne Virus, USS
Prenatal diagnosis
focuses on pursuing additional detailed information once a particular problem has been found, and can sometimes be more invasive.
Example are - amniocentesis and chorionic villus sampling.
purpose of prenatal testing
To enable timely medical or surgical treatment of a condition before or after birth
To give the parents the chance to terminate the pregnancy with the diagnosed condition
To give parents the chance to prepare psychologically, socially, financially, and medically for a baby with a health problem or disability, or for the likelihood of a stillbirth.
To organise appropriate fetal surveillance
screening in the UK
- Offeredtoallpregnantwomen
- To assess either woman or baby have particular health issue or disability
- Simple tests – Blood test, USS or questionnaire
- Not diagnostic
- Follow on test for diagnosis
What kind od blood tests can be performed
- Full blood count
- Blood group and Rhesus state
- Haemoglobinopathies – Thalasemmia and Sickle cell disorder
- Infectious diseases
- Chromosomal disorders – Trisomies ( 21, 18 and 13)
Screening for Haemoglobinopathies
- high risk groups
- If woman is carrier then partner testing will be offered - If partner is positive - genetic counselling
- Amniocentesis or CVS – to see if fetus is affected
- Newborn – blood spot screening test
Screening for Trisomies
Assess the chance of the baby being born with Down’s syndrome or Edwards’ syndrome or Patau’s syndrome.
screening for trisomies, what is the test sensitive to
The test is sensitive for singleton and twin pregnancies , not for any other higher multiple pregnancy.
factors that can effect screening for trisomies
Smoking, ethnicity, BMI, age Assisted pregnancy( donor egg or frozen embryo)
Down’s syndrome (Trisomy 21 )
Commonest cause of identifiable learning disability – Usually due to non- disjunction at chromosome 21 at meiosis.
50 % will have a congenital abnormality
80% profound or severe intellectual impairment
trisomy 18
Edwards’ syndrome
trisomy 13
Patau’s syndrome
First trimester combined test
optical time
Optimal time - 11+2 weeks to 14+1 weeks of gestation
Detection Rate fro down syndrome in first trimeter
Down’s syndrome detection Rate (DR) is 84%
• Edwards’ syndrome detection Rate (DR) is in first trimester
about 85% and False Positive Rate is 0.2%
Second Trimester Screening
• Optimal time is 14+2 -20 weeks
detection for don syndrome 2nd trimester
Detection rate is 80% and FPR is 3.5%. for Down’s syndrome..
detection for Edwards 2nd trimester
• Detection Rate (DR) is about 73% and FPR 0.1% for Edward’s syndrome
• If screening for all trisomies, then, woman will receive 2 results:
1 chance of having a baby with Down’s syndrome,
2 combined chance of having a baby with Edwards’ syndrome or Patau’s syndrome.
• Women who receive a higher-chance result 1:150 from first or second trimester screening tests, further options are
• no further tests
• a new, more accurate blood screening test called non-invasive
prenatal testing (NIPT)
• diagnostic tests: chorionic villus sampling (CVS) or amniocentesis
Non Invasive Prenatal Testing (NIPT) - this is screening and not a diagnostic test
- can be performed from 10 weeks of pregnancy
• The results are given as low or high chance result. If the results are low chance
then no further test is offered.
What is NIPT also known as
cell free DNA screening test ( cfDNA) , as cell-free DNA material is
extracted and analysed from maternal blood test.
• NIPT is better at finding babies who have
Down’s syndrome than finding babies with Edwards’ syndrome or Patau’s syndrome.
Twin pregnancies
- NIPT can be as accurate in identical twin pregnancies as if you were pregnant with one baby.
- NIPT may be less accurate in non-identical twin pregnancies because there are two placentas releasing their own DNA.
Diagnostic tests
- These are offered to women who have had a ‘higher- chance’ result from screening.
- These are invasive tests like chorionic villus sampling or amniocentesis.
- Alternatively- a detailed ultrasound scan.
Chorionic Villi Sampling CVS
- Fine needle inserted through abdomen and into uterus; or through cervix, & small piece of developing placenta removed
- Done from 11 weeks on wards
what does Chorionic Villi Sampling CVS test for
Tests for inherited disorders (cystic fibrosis, sickle cell, thalassemias, muscular dystrophy) and chromosomal disorders
ethical issues with Chorionic Villi Sampling CVS
2% risk of miscarriage (also, delay in getting results) infection, heavy bleeding
Amniocentesis
• Needle inserted through the abdomen and into amniotic fluid For karotyping if screening tests suggest aneuiploidy
DNA analysis if parents carrier of an identifiable gene
Enzyme assays for inborn errors of metabolism
• Done after 15 weeks
Preimplantation Genetic Diagnosis (PGD)
- Offered to couples who are at risk of passing on a genetic disorder
- It involves removing 1 cell from the early embryo (4-8 cell embryo)
Huntington’s Disease
- Inherited, late onset, degenerative condition
- Manifests ~ 30-50 yrs •
- 50/50 chance of inheriting it from affected parent
Saviour Sibling
Saviour Sibling
• A saviour baby or saviour sibling is a child who is born to provide an organ or cell transplant to a sibling that is affected with a fatal disease, such as cancer or Fanconi anemia, that can best be treated by hematopoietic stem cell transplantation.
