Prenatal Screening Flashcards
What types of testing is available during pregnancy?
Non-invasive:
Ultrasound scan
Blood tests at various stages of pregnancy inclu. NIPT.
Invasive:
CVS
Amniotic Fluid
Specialist centres:
PGS / PGD
What is the purpose of the Ultrasound Scan?
Check NT measurement, date the pregnancy, check growth and check for structural abnormalities.
Check the uterine environment e.g. how much fluid, placental position, umbilical blood flow.
What blood tests are offered?
1st trimester: blood test for pregnancy associated alpha protein-A and free beta-human chorionic gonadotrophin.
2nd trimester: quad test - triple hormones and inhibin A.
DNA test - cell free DNA, fetal DNA circulating in mums plasma from 10 weeks onwards. NIPT.
What is NIPT? Why is it useful?
Non-Invasive Prenatal Testing.
It utilises cell-free fetal DNA in mums plasma.
Comes from placenta and is caused by death of old/unwanted cells.
Can be tested to find out genotype of fetus.
Can be used from around 10 weeks onwards.
What are the basics of NIPT?
Mum has blood test, plasma is isolated and DNA is extracted.
DNA is amplified and then sequenced. The small fragments that are produced are aligned against the genome to work out where they come from. The ratio of sequences for each chromosome is calculated and regions that have a higher than expected number of reads indicate trisomy e.g. higher than expected number of reads for chromosome 21 = trisomy 21.
What are the drawbacks to NIPT? What could be the cause?
False positive and false negative e.g. test comes back positive and baby is normal or test comes back normal and baby is abnormal at birth.
Sample uses cells from placenta so could be due to confined placental mosaicism (CPM).
Uses cells sloughed off from placenta so could be affected by a vanishing twin or maternal mosaicism.
What would happen if a patient was given a positive NIPT?
They are offered an invasive test to confirm.
What is NT and why is it important?
Nuchal translucency.
Measurement of translucent space at back of neck of fetus (>4mm could be aneuploidy).
Fluid accumulates here when certain abnormalities are present such as +21. Can be measured between 11 and 14 weeks.
What types of abnormalities can be seen by ultrasound?
Size of NT.
Neural tube defects such as Spina Bifida and anencephaly.
Abdominal wall defects such as omphalocele and gastrochisis.
Heart defects.
Facial defects such as cleft lip/palate.
List some US markers that would be seen in trisomy 13.
Associated with midline defects due to defect in fusion of the prechordal plate during 3 week of development. Holoprosecephaly Club/rocker bottom feet Heart defect Cleft lip/palate Talipes Polydactyly Cystic kidneys Omphalocele
List some US markers that would be seen in trisomy 18.
Diaphragmatic hernia Omphalocele Rocker bottom feet Neural tube defect Ventriculomegaly Clenched fists Talipes Micrognathia Cleft lip/palate Heart defect
What US finding might you expect in a case of Turner’s?
Cystic hygroma.
List some US markers that you might see in trisomy 21?
Cystic hygroma Heart defect (Atrial septal defect, ventricular septal defect, AVSD, tetralogy) Ventriculomegaly Short femur Nasal bone absent in 60-70% of t21 fetus Duodenal atresia Hydrops (accumulation of fluid) Increased NT May also be able to see sandal gap and macroglossia.
What is a soft marker?
Minor abnormality seen in the normal population and in isolation nothing of concern.
However, occurs in higher proportion of pregnancies with aneuploidy.
Their occurrence prompts a search for other ‘abnormalities’.
Includes: increased NT, echogenic bowl, short long bones, choroid plexus cysts.
What is the cut off at which a termination can be completed on non-medical (social) grounds?
What would allow a termination to be carried out after this date?
<24 weeks of gestation.
A ‘clause E’ termination, this is a termination on medical grounds. The abnormality has to be barn door pathogenic for it to fall under this category.
