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 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. QF-Polymerase chain reaction (PCR)
a) QF-PCR to look for number of specific common chromosomes (i.e. chromosome number 21, 18, 13, X and Y) within 2-3 working days.
b) to look for certain specific 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. 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 5MB in size e.g. deletions, duplications, inversions, translocations.
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 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 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). The current platform for whole genome sequencing that the laboratory (the one that we send to) uses negates the necessity to do the chromosomal microarray test. If WES is sent for, some microdeletions / microduplications that can be detected on chromosomal microarray test may be missed on WES alone.
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
0.1-0.3% risk of procedure-related miscarriage (above the baseline risk of miscarriage that exists in all pregnancies).
Amniocentesis only: 2% risk of transient amniotic fluid leakage
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, then CVS may provide earlier diagnosis of the genetic make up of the fetus.
Email This
BlogThis!
Share to Twitter
Share to Facebook
Share to Pinterest
Comments