Chorionic villus sampling and Amniocentesis are prenatal diagnostic screening tests used to detect certain foetal genetic abnormalities, including Down’s syndrome and spina bifida. By Kim Bell
You want to be reassured that your unborn baby is healthy and growing strong. One of the scientific ways of doing this is through pregnancy screening tests conducted while your baby is in-utero. However, many parents are concerned that these tests can be invasive and do more harm than good.
According to cochrane.org, a research organisation that aims to provide high-quality, relevant and accessible information, it is vital that the prenatal screening and diagnostic tests are safe and accurate and can be done early enough to allow parents to make informed choices regarding their baby’s future.
Chorionic villus sampling (CVS) and amniocentesis are tests offered during pregnancy to identify any chromosomal abnormalities such as Down’s syndrome or genetic conditions such as cystic fibrosis.
According to the South African Society for Ultrasound in Obstetrics and Gynaecology, it is important to realise that no test or combination of tests is foolproof. However, they can help you make an informed decision.
Dr Oscar Shimange, specialist obstetrician and gynaecologist at Mediclinic Medforum Hospital in Pretoria explains that there are various tests available that are generally performed between 10 and 20 weeks gestation. While there is some risk of bleeding or miscarriage, they are generally considered safe and effective.
Used to detect chromosome disorders, with Down’s syndrome the most common, this diagnostic test can also help pick up spina bifida and other genetic disorders. Your doctor and the laboratory need to be informed if there is a history of genetic disorders in order to test for these.
Who can have the test?
Dr Shimange says that the final decision is yours, but an amniocentesis will be suggested if:
- You are 35 years or older at the time of conception (women over 30 years have a one in 900 risk of having a baby with Down’s syndrome, while women who are 45 years old have a one in 25 risk.
- Women who have had a blood test that has indicated an increased risk for Down’s Syndrome or a genetic condition.
- Couples who have a family history of genetic problems or a previous baby with a genetic condition.
- Women who have already had a previous pregnancy terminated or if anything unusual is noticed during ultrasound.
What does the test involve?
A sample of amniotic fluid is taken and tested. This is an outpatient test and is usually done between 14 and 16 weeks of pregnancy. An ultrasound is done to check the stage of pregnancy, position of the baby and the placenta. A fine needle is inserted through your belly into the womb, and a sample of fluid is taken. It is recommended that you go home and rest after the procedure and take things easy for a few days, avoiding any heavy lifting or strenuous exercise. You may experience a tightening feeling in your womb or feel a bit sore the next day. This is not unusual, but contact your doctor if you’re concerned.
Dr Shimange says that this is a well-established and widely available screening method. There is a minimal risk of miscarriage, with one in 150 women miscarrying as a result of the procedure within two weeks of having the test. “It is believed, in general, that amniocentesis is not harmful to you or your baby.”
The chromosome test (which is mainly for Down’s syndrome) involves “growing” the cells in lab conditions, so the results can take up to four weeks. The spina bifida results take about two days.
Chorionic villus sampling (CVS)
Used to detect Down’s syndrome and other genetic disorders since the cells found in the chorionic villi (placental tissue) almost always have the same chromosomes as the developing baby.
Who can have the test?
This is the same as for amniocentesis. The difference is that CVS can be done earlier, at around 11 to 13 weeks, so you can find out earlier if there is a concern.
What does the test involve?
There are two types of CVS: Transabdominal and transcervical. In transabdominal CVS, the procedure is done under local anesthetic. A needle is inserted through your belly to extract a sample of the placental tissue. This is observed at all times by ultrasound and the needle doesn’t enter the amniotic sac or go near the baby. In some circumstances, this is done through the cervix, using a spectrum and monitored throughout suing ultrasound.
The risk of miscarriage is around one in 50 procedures. Light bleeding can occur, but usually settles without further problems.
It takes around a week to get the results back. There is also around a 1% risk of chromosomal mosaicism, which may give you an inconclusive result. If this is the case, and you are considered a high-risk pregnancy, your doctor may recommend a follow-up amniocentesis.
Maternal serum screening test
This is a blood test to measure the levels of three substances in your blood – alpha-fetoprotein (AFP), unconjugated oestriol (uE3) and human chorionic gonadotrophin (hCG). The test is usually carried out about 16 weeks of pregnancy and the levels of the three substances are used in combination with the woman’s age to estimate the risk of Down’s syndrome. The level of AFP alone is used to determine if there is an increased risk of a neural tube defect.
Kim Bell is a wife, mother of two teenagers and a lover of research and the way words flow and meld together. She has been in the media industry for over 20 years, and yet still learns more about life from her children everyday. You can learn more about Kim Bell here.