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Amniocentesis and CVS (chorionic villus sampling)


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%. Antibiotics used during the procedure have not been shown to reduce the risk of miscarriage.

CVS can be performed from 10 weeks onwards. This is often done under local anaesthesia. We performed CVS from the abdominal (a.k.a 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 at the back of the uterus may not be reached from the front in about 1% of cases. Often, filling up the bladder or emptying the bladder may change the position of the uterus and allow access to the placenta from the front. In cases where the placenta cannot be reached from the front, delaying the procedure by up to 1 week later or an amniocentesis from 16 weeks onwards may be more appropriate.

Amniocentesis is performed from 16 weeks onwards. It involves getting some amniotic fluid from a needle inserted abdominally under ultrasound guidance. There is no need for local anaesthesia here as studies have shown no difference in the pain experienced by patients whether or not local anaesthesia is used for amniocentesis. Most patients described the pain of amniocentesis as similar to that of routine blood taking.

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. Quantitative Fluorescent Polymerase chain reaction (QF PCR)

QF PCR analysis includes amplification, detection and analysis of chromosome-specific DNA sequences known as genetic markers. It is used for rapid identification of the common chromosomes (i.e. chromosome number 21, 18, 13, X and Y) within 3 working days. It allows for detection of Down syndrome (Trisomy 21), Edward syndrome (Trisomy 18), Patau syndrome (Trisomy 13), sex chromosomal abnormalities including Turner syndrome, and triploidy.

2. Chromosome culture or karyotype to study the chromosomes of the human cells under the light microscope within 12 -14 days. This test is able to detect the "large" chromosomal abnormalities up to 5 MB 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. Even if QF PCR had already confirmed these, karyotype should be performed to show the arrangement. For example, 95% of Down syndrome is due to Trisomy 21 (a random event that implies a low risk of recurrence, usually about 1%, in the next pregnancy) while 2-3% may be due to unbalanced translocation Down syndrome. If one of the parents is subsequently found to be a balanced translocation (i.e. healthy but with an unusual rearrangement of the chromosomes), then the risk of recurrence becomes substantially higher, about 25-100% depending on the type of balanced translocation.

b) structural changes in the chromosomes that are at least 5 MB in size e.g. deletions, duplications, inversions, translocations.

A "normal" karyotype does not exclude all genetic conditions as many genetic conditions may be smaller than the limited resolution afforded by the karyotypic study. It also does not guarantee that the fetus will be healthy, or will not develop genetic diseases later in life.

An "abnormal" karyotype may result in some forms of discrimination in insurance and employment or others as they will form part of the medical record, and may be accessed by and / or disclosed to a third party when consent has been obtained from you or your spouse, or when such access is otherwise allowed or required by law. It may also reveal incidental findings not related to the original reason for doing this test.

3. Chromosomal microarray (CMA) test, which is a molecular test, which "chops" up the chromosomes into important segments up to 0.1-0.2MB in resolution (50-100 x higher resolution than chromosome culture or karyotype), and then measure the number of copies of these segments. 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 (less than 0.1-0.2 MB) cannot be detected. Microdeletion and microduplication syndromes cause a spectrum of conditions ranging from mild to severe, and may include variable effects on the intellectual function, multiple structural abnormalities and/or autism spectrum disorders. Possible results from a CMA test include:

a) normal - This is very reassuring but does not include functional disorders (e.g non-chromosomal causes of mental retardation / autism spectrum disorders, hearing loss, visual loss), structural abnormalities (the major ones of which ultrasound scan may be able to detect) and diseases caused by smaller genetic disorders (e.g. single gene disorders like thalassaemia and cystic fibrosis).

b) abnormal or pathogenic - Some of these conditions are known to be associated with early-onset diseases and therefore likely to be severe, while other conditions may be of variable onset and of variable severity. Whilst the diagnosis of the aneuploidy / microdeletion / microduplication may be definite, the prediction of the severity of the manifestations of the disorder may not be as clear.

c) variation of unknown significance - Some microdeletion or microduplication may not be clearly known to cause disorders. Where there are no data to suggest that diseases may be associated with such a microdeletion or microduplication, the laboratories do not report them in general. Where there are only a few cases with abnormalities associated with such a microdeletion or microduplication but such associations are not strong, the laboratories may report them. Such a report may cause undue anxiety in the parents.

d) incidental findings - Rarely incidental findings from CMA studies may reveal that:

i) the partner or husband may not be the biological father

ii) that the mother may not be the biological mother (i.e. pregnancy resulting from a donor egg)

iii) the biological father and mother of the fetus are descendents of the same ancestors (e.g. cousins)

4. Whole exome sequencing (WES) or whole genome sequencing (WGS). These are the highest resolution for gene studies. It allows the detection of single gene disorders especially if there are multiple ultrasound abnormalities detected in the fetus. A consultation with a geneticist is highly recommended to understand the implications of doing such a test.

Whole exome sequencing (WES) allows sequencing of the exons of the DNA (i.e. sequences of the DNA that are involved in producing proteins for the cells) while whole genome sequencing (WGS) allows sequencing of the introns and exons of the DNA (i.e. sequences of the DNA that are involved in producing proteins and regulation of the genes for the cells). Whole genome sequencing negates the necessity to do the chromosomal microarray test. If WES alone is sent for, some microdeletions / microduplications that can be detected on chromosomal microarray test may be missed on WES alone.

WES or WGS is increasingly being performed now for major fetal structural abnormalities especially if there are multiple abnormalities detected on ultrasound scan, when there's history of consanguinity (i.e. marriage between close relations) and for grossly increased nuchal translucency >= 4.5 mm or when there's other abnormalities detected with increased nuchal translucency.

The British Society for Genomic Medicine in 2021 recommend stated the following opinions on prenatal WES:

  • there is an additional diagnostic yield of 8.5-10.3% over and above standard chromosomal testing in fetuses with major structural abnormalities, and up to 15.4-18.9% in fetuses with abnormalities in multiple systems.

  • it is limited to assessing 85% of known disease-causing variants, which represent 1-2% of the genome

  • parents may feel more empowered to make an informed decision about their pregnancy care with the findings of prenatal whole exome sequencing.

  • prenatal WES testing of fetus and both parents (i.e. trio) is the recommended approach (as opposed to prenatal WES testing of the fetus only) as it allows more rapid analysis through exclusion of familial, most likely benign, genomic variants

  • As sequencing becomes more common, our knowledge of the pathogenicity (i.e. disease-causing potential) of variants may change.

Prenatal whole genome sequencing (WGS) may increase the diagnostic yield slightly higher (when compared to prenatal WES) though large studies are not yet available.

Similarly, the possible results of WES / WGS could be:

a) normal. This is very reassuring but does not exclude all genetic disorders as it does not include genetic syndromes with variable manifestations that are not known to be associated with the ultrasound findings detected.

b) abnormal or pathogenic. You would require a consultation with a geneticist when abnormal results are found to understand the predicted outcome of the fetus.

c) variation of unknown significance. You would require a consultation with a geneticist when abnormal results are found to understand the predicted outcome of the fetus.

5. FISH for certain genetic mutations. This may be indicated if there is a family history of a known autosomal recessive (e.g. thalassaemia, cystic fibrosis) or X-linked recessive genetic disorder (e.g. haemophilia), or when there is a strong suspicion of a particular genetic disorder based on abnormalities detected on ultrasound scan.

Risks of the CVS / amniocentesis

  1. 0.1-0.3% risk of procedure-related miscarriage (above the baseline risk of miscarriage that exists in all pregnancies) when performed by experienced operators

  2. Small sample obtained or poorly dividing cells in the sample that may take a longer time for results to be reported, or very rarely may result in a failure to obtain a result

  3. Amniocentesis only: 2% risk of transient amniotic fluid leakage

  4. CVS only: 1% risk of confined placental mosaicism (this is a rare scenario where the cells of the placenta shows an abnormality in the chromosomes while the fetus shows normal chromosomes). For this reason, CVS may not be a good test to do in some cases when the ultrasound scan of the fetus is normal. If the scan of the fetus is abnormal, CVS's main advantage is the provision of earlier diagnosis of the genetic make-up of the fetus.

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