Invasive diagnostic procedures like amniocentesis and chorionic villus sampling (CVS) are performed under ultrasound guidance to obtain amniotic fluid and chorionic villi (or placental tissue) respectively to check for certain conditions during the pregnancy.
These procedures are safe when performed by experienced operators though they are associated with procedure-related risks of miscarriage ranging from 0.1-0.3%.
CVS can be performed at 10-15 weeks onwards. This is normally performed from the abdominal (aka transabdominal) approach, and involves getting a small sample of placental tissue or chorionic villi from a needle inserted abdominally under local anaesthesia. From the transabdominal approach, the main technical difficulty is that placentas that are located behind may not be reached from the front in about 1% of cases. In such cases, an amniocentesis may be more appropriate.
Amniocentesis is performed from 16 weeks-21 onwards. It involves getting some amniotic fluid from a needle inserted abdominally under ultrasound guidance. There is no need for local anaesthesia here.
These procedures may be required when:
1. there is increased risk of chromosomal abnormalities from First Trimester Screening / OSCAR, non-invasive prenatal test (e.g. Panorama and Harmony tests), or minor and major abnormalities detected on the Second Trimester Fetal Anomaly Scan.
2. both parents are carriers of recessive genes e.g. thalassaemia, cystic fibrosis.
3. there is a history of chromosomal abnormalities in previous pregnancies.
4. there is a possibility of certain infections in the fetus.
These samples can then be sent to check for:
1. Polymerase chain reaction (PCR)
a) to look for number of specific chromosomes (chromosome number 21, 18, 13, X and Y) within 2-3 working days.
b) to look for certain infections e.g. toxoplasmosis, cytomegalovirus (CMV), chickenpox (varicella zoster virus or VZV), rubella.
2. Chromosome culture or karyotype to examine the chromosomes of the body within 12 -14 days. This test is able to detect the "large" chromosomal abnormalities up to 5MB in resolution, but will not detect smaller chromosomal abnormalities below that resolution. This allows detection of:
a) aneuploidies i.e. abnormalities in the number of chromosomes e.g. Trisomies 21, 18, 13, Turner syndrome, sex chromosomal abnormalities, other rarer types of aneuploidies
b) structural changes in the chromosomes e.g. deletions, duplications, inversions, translocations
3. Chromosomal microarray (CMA) test, which is a molecular test, which "chops" up the chromosomes into important areas up to 0.1-0.2MB in resolution (higher resolution than chromosome culture or karyotype), and then measure the number of copies of these areas. This allows detection of similar conditions like in chromosome culture or karyotype (except inversions and balanced translocations), and even microdeletion and microduplication syndromes that are beyond the resolution of chromosome culture or karyotype. This still means that smaller genetic defects smaller than this resolution cannot be detected.
4. FISH for certain genetic mutations e.g. thalassaemia.
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