Lecture 5- Prenatal Testing Flashcards
(33 cards)
What happens in a normal pregnancy?
1) booked into antenatal care with midwife
2) Nuchal scan- 10-14 weeks
3) mid trimester anomaly scan
4) ultrasound scans at 11-14 and 20-22 weeks
What is the purpose of the nuchal scan at 12 weeks?
- date the pregnancy
- multiple pregnancies
- major foetal abnormalities
- risk of miscarriage
- assess risk of Down syndrome and chromosomal abnormalities
How do you diagnose chromosomal abnormalities around 10-14 weeks of gestation?
Nuchal translucency: measure fluid at back of baby’s neck
Normal = <3.5mm
What can an increased NT indicate?
1) CHROMOSOMAL ABNORMALITIES
- downs, patau, Turners, Edwards
- NT + maternal age = 75% of Down’s syndrome diagnoses and 5% false positive rate
2) BIRTH DEFECTS
- cardiac anomalies, pulmonary defects, renal defects
3) SKELETAL DYSPLASIA
During the diagnosis of Downs, a combined test is done as well as NT, what does the combined test look for?
High hCG levels and low PAPP-A levels
What are the criteria for arranging prenatal testing?
- abnormal findings at Nuchal scan or mid- trimester scan
- after results of combined test
- if previous pregnancy was affected by DS or CF etc
- if parents are carriers of chromosomes (HD or DMD)
- FH of genetic condition
What is the purpose of prenatal testing?
- to inform and prepare parents for the birth of an affected baby
- to allow in utero treatment
- manage the remainder of pregnancy
- prepare for complications post birth
- allow termination of affected foetus
What are three different types of prenatal tests?
1) Non- invasive (ultrasound/ MRI)
2) Minimally invasive (maternal blood test/ cell- free foetal DNA)
3) invasive (chorionic villus sampling/ amniocentesis)
What does the high level anomaly scan detect?
- hydrocephalus
- neural tube defects
- limb deformities
- heart defects
- ‘soft markers’ Nasal bone - DS
What can maternal serum screening be used to diagnose?
Trisomy 21, trisomy 18 and neural tube defects
In maternal serum screening what is investigated in the 1st trimester?
11-14 weeks
Look for hCG (human chorionic gonadotropin) and PAPP-A (pregnancy associated plasma protein A)
In maternal screening during the second trimester what is investigated?
hCG, PAPP-A, AFP(alpha foetal protein) and uE3 (oestriol)
What is cell free fetal DNA?
- DNA fragments present in maternal plasma during pregnancy
- which comes from the placenta
- And is representative of the unborn baby
When is cffDNA detectable and when can it be used for testing?
- detectable from 4-5 weeks
- used for testing until around 9 weeks
How can you determine the sex of a baby using cffDNA?
Look for SRY gene on Y chromosome
When is cffDNA testing offered?
If there is an X-linked disease in the family and the baby is male–> prenatal testing
What are the limitations of NIPD?
- multiple pregnancies, can’t tell which foetus the DNA came from
- not suitable for women of high BMI because cfDNA is mostly theirs
- Invasive test may still be required
What are the benefits of NIPD?
- reduced number of invasive tests taken
- no increased risk of miscarriage
- less expertise required
- can get earlier result
When is CVS performs?
11-14 weeks
What is the risk of miscarriage for CVS?
1-2-% risk of miscarriage
How is CVS performed?
- transabdominal or transvaginal
- take sample of chorionic villi (developing placenta)
- contains same DNA as foetus
- earlier diagnosis than amniocentesis (important for TOP)
When is amniocentesis done?
From 16 weeks
What are the risks of amniocentesis?
- up to 1% risk of miscarriage
- infection
- Rh sensitisation
How is amniocentesis performed?
- transabdominal needle
- take sample of amniotic fluid which contains foetal cells
