Prenatal Testing Flashcards
Prenatal Screening
Focuses on finding problems among a large population with affordable and non-invasive methods. Examples are Down Syndrome, Bloodborne virus and Ultrasound Scan
Prenatal Diagnosis
Focuses on pursuing additional detailed information once a particular problem has been found, and can sometimes be more invasive. Examples are Amniocentesis and Chorionic Villus Sampling.
Why do we do prenatal testing?
- To enable the 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 foetal surveillance.
Screening in Pregnancy UK
- Offered to all women to assess if either woman or baby have particular health issue or disability.
- Simple tests - Ultrasound, blood test, questionnaire.
- Not diagnostic, there is a follow on diagnostic test.
Blood tests in Pregnancy Screening
- FBC
- Blood group and Rhesus state
- Haemoglobinopathies - thalasaemia and sickle cell
- Infectious diseases
- Chromosomal disorders - e.g. trisomies (21, 18, 13)
Screening for Haemoglobinopathies
- Inherited altered haemoglobin gene disorders, usually autosomal recessive.
- High risk - family origins from Africa, the Carribean, the Mediterranean, South East Asia, the Middle East and Far East. Can be found in all ethnic groups but is less common.
- If the woman is a carrier then partner testing will be offered. If partner is positive, genetic counselling will be offered.
- The screening involves an Amniocentesis or Chorionic Villus Sample to see if the foetus is affected. And Blood Spot Screening Test which will be done when the baby is born.
Screening for Trisomies
- Assess the chance of the baby being born with Down’s, Edwards’ or Patau’s Syndromes.
- The test is sensitive singleton or twin pregnancies, not for higher multiple pregnancies.
- Risk factors which affect screening tests include; smoking, ethnicity, BMI, age, assisted pregnancy (donor egg or frozen embryo).
Down’s Syndrome
Commonest cause of identifiable learning disability usually due to non-disjunction at chr.21 at meiosis. 50% will have congenital abnormality, 80% profound or severe intellectual impairment. 1:1000 live births. Approx. 40,000 people in UK have Down’s Syndrome.
Edwards’ Syndrome
Trisomy 18 - these babies rarely survive to adulthood, most die before or shortly after birth.
Patau’s Syndrome
Trisomy 13 - not many babies survive to adulthood, many die before or shortly after birth.
First Trimester Combined Test
- Optimal time for this test - 11+2 weeks to 14+1 weeks of gestation.
- Factors used for calc. - maternal age, nuchal translucency measurement, two biochemical markers (free beta hCG and PAPP-A), gestational age calculated from the crown rump length measurement.
- Result is low or high chance. 1 to 150 chance is the cut-off point.
Nuchal Translucency
Measure the amount of fluid under the skin at the back of the baby’s neck.
Detection Rates
- Down’s Syndrome detection rate is 84%. False positive rate is 2.2%.
- Edwards’ Syndrome detection rate is 85% and false positive rate is 0.2%
Second Trimester Screening
- Quadruple Test
- The optimal time for it is between 14+2 weeks and 20 weeks.
- Factors used in this calculation are; maternal age, four biochemical markers (alpha-fetoprotein, hCG (total, intact or free beta subunit), uE3 and Inhibin-A).
- No scan in 2nd trimester screening.
- Quadruple test has a lower detection rate than the combined test. Detection rates are lower; 80% for Down’s Syndrome with a FPR of 3.5%. And 73% detection rate for Edwards Syndrome and a FPR of 0.1%.
- If risk of having a term pregnancy affected with Edwards’ Syndrome is 1 in 100 or higher an ultrasound and amniocentesis are offered.
Getting Results from Screening
- Tells you if the woman has a higher or lower chance of having a baby with one of the conditions screened for.
- Woman will receive 2 results for trisomies: 1 chance of having a baby with Down’s Syndrome and a 2 combined chance of having a baby with Edwards’ or Patau’s Syndrome.
- If it shows high chance, results should be told within 3 working days of result being available. If lower chance then should be told within 2 weeks.
- Further options for women who receive high chance result are: no further tests, Non-Invasive Prenatal Testing or Diagnostic Tests.