Lecture 5- Prenatal Testing Flashcards
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
What is done with the DNA samples
- karyotype if chromosomal abnormality
- QF-PCR (quantitative fluorescence PCR) to test for T13, T18 and T21 and sex chromosomes
What are the different reproductive options?
- conceive naturally (no prenatal testing)
- conceive naturally (with prenatal testing)
- use egg/ sperm donors
- adoption
- choose not to have children
- PGD
Where should you go to find a egg/ sperm donor?
A UK HEFA licensed fertility centre which conforms to strict medical, ethical and legal practices
What are the two stages of adoption?
1) registration and checks (medical and criminal background)
2) assessment and home approval ( home visits, prospective adopters report, panel review)
What is PGD?
IVF with an additional test of genetically testing the embryo before implantation
Describe the process of PGD
1) stimulation of the ovaries
2) egg collection
3) insemination- by intracytoplasmic sperm injection
4) Fertilisation
5) Embryo biopsy- once embryo is 8 cells in size
6) Embryo testing
7) Embryo transfer
8) Pregnancy test
What is the age criteria and weight criteria for PGD?
Under 39
Between 19-30 BMI
What are some of the problems with PGD?
- may not be eggs to collect
- eggs may not have been fertilised
- all embryos may have been affected
- embryos might not have survived biopsy
- lengthy emotional process
- low success rates
What is the role of genetic counselling in prenatal testing?
- arrange and explain the different procedures
- facilitate decision-making
- give results
- see patients in clinic following a diagnosis in utero
- arrange termination if necessary
- discuss recurrence risks and plans for future pregnancies
