Genetics 5 Flashcards
(59 cards)
What happens during a ‘normal’ pregnancy
Positive pregnancy test- no longer confirmed at GP
Book into antenatal care – see midwife
Nuchal scan – 10-14 weeks gestation. Different tests dependent upon NHS Trust e.g. nuchal translucency, combined test etc.
Mid-trimester anomaly scan
All pregnant women should be offered routine ultrasound scans at 11-14 weeks and again at 20-22 weeks.
What is the main method for prenatal diagnosis of fetal abnormalities
Ultrasound
When does the mid-trimester scan normally take place
20-22 weeks
What are the aims of the 12 week nuchal scan
To date the pregnancy accurately.
To diagnose multiple pregnancy.
To diagnose major fetal abnormalities.
To diagnose early miscarriage- no heart beat
To assess the risks of Down Syndrome and other chromosomal abnormalities- further testing is required for this.
What does the 12 week nuchal scan take into account
Taking into account the maternal age, blood hormone levels, nuchal translucency thickness, nasal bone, blood flow through the fetal heart and fetal abnormalities.
What does a nuchal translucency > 3mm indicate
Chromosome abnormalities (e.g. Downs, Edwards, Patau, Turners) NT + maternal age detects up to 75% of Down syndrome with 5% false positive rate Birth defects: - Cardiac anomalies Pulmonary defects (diaphragmatic hernia) Renal defects Abdominal wall defects Skeletal dysplasias
How should the nuchal translucency develop
It should grow in proportion with the rest of the body. Hence the bones not being in the normal proportion may be due to skeletal dysplasia and this should be investigated during pregnancy,
What is the issue with the Nuchal scan
The baby needs to be in he correct orientation for it to be measured, hence tests may need to be rescheduled accordingly.
What is the nuchal
Fluid at the back of the neck.
What is the important thing to remember about screening tests
They are not diagnostic! Further tests are required.
What is the purpose of the mid-trimester scan
To measure the circumference of long bones- especially after seeing family history or looking at defects from previous pregnancies.
When are prenatal tests arranged
Following abnormal findings at nuchal scan or mid-trimester scan
Following results of combined test which give an increased risk of Down Syndrome
If previous pregnancy affected with a condition e.g. DS, CF
If parent(s) carrier of chromosome rearrangement or genetic condition, e.g. t(13;14), DMD, HD.
FH of genetic condition
What are the aims of prenatal testing
To inform and prepare parents for the birth of an affected baby
To allow in utero treatment
Manage the remainder of the pregnancy
To be prepared for complications at or after birth
To allow termination of an affected fetus
What are the 3 different types of prenatal tests
1) Scanning Ultrasound / MRI 2) Non-invasive Maternal blood test Cell-free fetal DNA 3) Invasive Chorionic villus sampling (CVS) Amniocentesis
Describe the different types of scans available
Ultrasound in pregnancy
Early / dating scan- confirmation- check for heartbeat, ectopic, twins
Nuchal translucency (NT) & nasal bone
High level / anomaly scan- structural anomalies
Fetal MRI- indicate CF/ show development of tissues
Usually around 20 weeks+
When is the early/dating scan normally done
8-10 weeks
When is the high level/anomaly scan normally done
18-20 weeks
Why are fetal cardiac scans necessary
Cardiac abnormalities can be treated in utero.
What are the different types of non-invasive tests available
Maternal serum screening
Cell-free fetal DNA
Describe maternal serum screening
Tests maternal serum markers in the blood to detect increased risk of fetal trisomy 21, trisomy 18 and/or neural tube defects
1st trimester maternal serum screening (with nuchal translucency measurement): 11-14 weeks [hCG, PAPP A]
2nd trimester maternal serum screening (triple screen): 16-20 weeks [AFP, uE3, hCG]
Nuchal translucency measurement: 11-14 weeks
Other variations combining 1st and 2nd trimester screening results available privately
Describe cell free fetal DNA
Non-invasive prenatal diagnosis (NIPD) works by analysing the DNA fragments present in the maternal plasma during pregnancy (cell-free DNA).
Most of this DNA comes from the mother
10%-20% of it comes from the placenta, which is representative of the unborn baby (cell-free fetal DNA).
Cell-free fetal DNA (cffDNA) is first detectable from about 4 -5 weeks’ gestation, but cannot accurately be detected on testing until around 9 weeks
What is a problem with cffDNA
It is difficult to obtain sufficient quantities of cffDNA for analysis
What can the cffDNA be used for
PCR for fetal sexing (check for Y chromosome sequences) and testing for rhesus incompatibility. Not routinely offered- do not want sex selection
NGS- can be used to test for fetal trisomies. However, it is still currently regarded as a screening test with positive results needing confirmation from amniocentesis.
What is abnormal about the cffDNA in DS
There is a higher quantity of cffDNA for chromosome 21.
