Amniocentesis and CVS are diagnostic tests that are performed prenatally to detect whether a fetus has a chromosome abnormality such as Down Syndrome (Trisomy 21). Less commonly they are performed to detect inherited genetic disorders in couples known to have an increased risk of a particular condition. These procedures enable laboratory testing of fetal chromosomes from fetal cells in the amniotic fluid (amniocentesis) or placental cells (CVS).
These are diagnostic tests with very high accuracy for the detection of chromosomal anomalies in particular and in some cases of heritable genetic disorders.
Although considered quite safe, these procedures are not without risk. In general terms the risk of the procedure needs to be balanced against the risk of the condition targeted.
Patients with a specific past history of a genetic or chromosomally abnormal pregnancy may elect to have a diagnostic test without any preliminary screening procedures and of course no patient is compelled to have a diagnostic test regardless of an assessment of increased risk.
It is important to be aware of your blood group when booking either of these procedures. If your blood group is rhesus negative you will need to have an injection of anti D following the procedure to prevent the development of rhesus disease in future pregnancies.
Chorionic villi are small moss like projections that are part of the early placenta. The cells present in the chorionic villi almost always have the same chromosomes as the fetus hence abnormalities in the chromosomes of the chorionic villi are also found in the fetus.
During a CVS procedure a fine needle is passed through the mother’s abdomen and a sample of the small fragments of chorionic villi are obtained which can then be examined for chromosome anomalies.
A CVS is usually performed from 11 weeks of gestation with ultrasound guidance.
It is not without risk. The main risk is of pregnancy loss, which for CVS in most reported studies is between 1:100 and 1:200.
The preliminary results for Down syndrome and other major chromosomal defects are usually available within 3 – 4 days. The full chromosome analysis takes between 2 and 2 1/2 weeks.
Common Reasons for Amniocentesis and CVS
- High risk First Trimester Screen results
- Increased risk by maternal age, without a prior screening test.
- Previous pregnancy with chromosomal abnormalities or genetic disorder.
- Family history of chromosomal anomaly or genetic disorders. Most chromosomal anomalies are random events and do not have a familial predisposition.
- Major abnormality. Amniocentesis may be indicated if a major abnormality is found at the time of a fetal anatomy scan irrespective of a low risk First Trimester Screening result.
The patient: in conjunction with her partner after discussing the pros and cons with their medical practitioner and being satisfied with the level of information provided.
You only need to prepare yourself for the ultrasound examination. You do not need to fast. It is advisable to have your partner or support person drive you to and from your appointment. Some patients experience some cramping discomfort after the procedure.
It is best to rest for 24 hours after the procedure.