Prenatal testing

  • Created by: amh4
  • Created on: 15-02-17 16:58

Nuchal Scan

Done at 10-14 weeks. 

Aims: date the pregnancy, diagnose multiple pregnancy, diagnose major fetal abnormalities, diagnose early miscarriage, asses the risks of DS and any other chromosomal abnormalites. 

Nuchal translucency measues the thickness of fluid at the back of the fetal neck. Increases of >3mm can indicate chromosome abnormalities. Also can show birth defects and skeletal dysplasias. 

Is a screening test and is not diagnostic. 

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Cell-free fetal DNA

Non invasive prenatal diagnosis, analyses DNA in maternal plasma during pregnancy.

10-20% of cfDNA comes from the placenta. Cannot be accurately detected on testing until around 9 weeks. 

cfDNA are short DNA fragments. for example in trisomy 21 the amount of DNA for chromosome 21 is higher than in normal pregnancies. 

Uses: sex linked condition, detects SRY gene on Y chromosome so can determine if male or female fetus (if male go to prenatal test, if female no invasive test is needed). Can also be used for aneuploidy, rhesus D tyoing and autosomal dominant disorders inherited from the father or de novo.

Limitations: if multiple pregnancies its not possible to tell which fetus the DNA is fom. Women with high BMI have a higher ratio of their own cfDNA. An invasive test may still be required to confirm the result. Mosaicism in the placenta. 

Benefits : Much earlier result than invasive tests, no risk of miscarriage. 

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Chorionic Villus Sampling

11-14 weeks, 1-2% risk of miscarriage.

Transabdominal or transvaginal, take sample of chorionic villi part of the placenta 

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From 16 weeks, takes sample of amniotic fluid which contains fetal cells.

Needle is passed through the mothers abdomen into amniotic cavity, amniotic fluid is aspirated and taken to the laboratory for analysis. 

Up to 1% risk of miscarriage and risk of infection. 

Tests done - chromosome abnormality - karyotype (2 weeks) or Quantitative Fluorescence-Polymerase Chain Reaction result within 24-48 hours.

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Pre implantation genetic diagnosis

Uses IVF with an additional step to test the embryo before implantation.

Process: Hyper stimulation of ovaries, egg collection, insemination (by IVF), fertilisation, embryo biopsy, embryo testing, embryo transfer, pregnancy test. 

Eligibilty criteria to have on the NHS - female partner under 39, BMI 19-30, both partners are not smokers, stable relationship, hormone levels normal. Normally funded for 3 rounds of PGD. 

Emotional and lengthy process, 30% per cycle success rate, 40% per embryo transfer

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The study of karyotypes is made possible by staining. A suitable dye, such as Giemsa s applied after cells have been arrested during cell division by a solution of colchicine usually in metaphase  when most condensed. 

Can be done on blood, bone marrow, skin, amniotic fluid or chorionic villus. 

An Ideogram is the chromosomes grouped by size and shape (submetacentric, metacentric, acrocentric)

All bands are numbered. Number of bands - quality of the analysis. 

What can you see? Balanced and unbalanced rearrangements, reciprocal and robertsonian translocations, inversions, insertions or aneuploidies, microdeletions.

Advantages - genome wide screen in a single test, results available in 24-72 hours so allows swift patient management. Disadvantages: resolution is low, cells must be dividing, staff training needed, reporting time can be 2 weeks. 

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Fluorescence in-situ hybridisation

Targets specific areas of genome. In metaphase you need cultured material, in interphase you use uncultured material. 

Probes used: centromeric, locus specific, whole chromosome, partial chromosome, single clone probes, BAC probes. 

Technique: dependent upon hybridizing a probe with a fluorescent tag, complementary in sequence to a section of DNA on a target gene. The tag and probe are applied to a sample of interest under conditions that allow for the probe to attach itself to the complementary sequence in the specimen if it is present. After the specimen has been treated, excess fluorophore is washed away, the signal is amplified and the sample can be visualized under a fluorescent microscope. 

FISH techniques have a single major downfall: Interphase and metaphase FISH can only detect known genetic aberrations, providing the specific probe is available. FISH analysis with locus-specific probes or chromosome-specific DNA libraries is restricted to the targeted chromosome or chromosomal subregion. Additionally small deletions may not be found, probes are expensive and potential mosacism. 

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