Risks include: Miscarriage - around 1% for either test - it's a good idea to research your clinic in advance.
Around 5% of pregnant women in the UK, around 30,000 women a year, are offered the choice of an invasive prenatal diagnostic test of their baby.
The two main diagnostic tests used during straightforward pregnancies are amniocentesis and chorionic villus testing (CVS). These tests are used by obstetricians to assess whether the foetus could have developed, or be developing, some sort of abnormality or serious health condition.
Conditions tested for include Down’s syndrome, sickle cell anaemia, thalassaemia, haemophilia, cystic fibrosis, chromosome abnormalities or spina bifida. The tests also assess whether the foetus is a boy or a girl, so if you don’t want to know, tell your obstetrician beforehand.
The tests involve various risks, outlined below, so are normally offered only to women for whom the risk factor is high. High-risk groups include older women (over 35), women from families with genetic health histories, women whose scans have suggested an abnormality, and women with particular individual health histories.
There is a small (around 1%) risk of miscarriage involved in either test – slightly higher for CVS than amniocentesis. It’s a personal choice, and you need to decide whether you think the small risk is worth knowing whether your baby may have a serious abnormality.
If you have any leakage that’s not urine after the test, any pain, or any infection, seek urgent medical attention.
In less than 1% of procedures, the samples removed may not be suitable for testing, and a further procedure may need to be done.
Rarely, the tests might result in the baby’s blood cells entering the mother’s blood stream. If you have Rhesus negative blood, you’ll be given a drug to prevent you from producing antibodies against your baby’s blood cells.
If you have an infection, such as hepatitis C, human immunodeficiency, toxoplasmosis, the infection might be transferred to your baby.
A small amount of the amniotic fluid is taken, by your obstetricians using a fine needle, from the womb so it can be tested.
When the needle goes in, it might sting slightly. The progress of the needle going into your womb is then watched carefully on a scanner. Once the needle is in the right place, a syringe pulls up a little sample of amniotic fluid.
Afterwards, you’ll need to rest and you may have cramps or small blood spots.
You may be asked to come to the test with a full bladder, so the scan will work better.
If you feel feverish, bleed heavily, have watery fluid (not urine) coming out of your vagina, pain, or contractions, then you should seek urgent medical attention.
The risk of miscarriage is higher the earlier you take the test, so the minimum time it is usually carried out is around 15 weeks. It is usually done between 15-20 weeks.
Chorionic Villus Sampling can be used as an alternative to the amniocentesis test. The test involves taking a sample of cells from the inside wall (the chorionic villi) of the placenta. These cells contain information about the baby’s genes. The genetic information in the cells is then analysed to see if an abnormal genetic or biochemical condition is present. CVS is usually carried out earlier than an amniocentesis, around 11-13 weeks, but can be performed later if necessary. Again, you may be asked to come for the test with a full bladder, so the scan will work better.
For a transabdominal CVS – a needle is inserted through your tummy and guided using an ultrasound scan. A syringe takes a small sample of chorionic villi. This is the usual method since it’s easier to get to the placenta this way.
For a transcervical CVS, a thin tube is inserted through your vagina and cervix and guided to the placenta using an ultrasound scan.
The risk of miscarriage is about 1-2%. This is slightly higher than the risk for an amniocentesis. The risk is slightly higher for a transcervical CVS compared to a transabdominal CVS.
If you feel feverish, have heavy bleeding or discharge or contractions, afterwards, then contact the clinic or midwife where the procedure was carried out straight away.
The results, for either test, aren’t given straight away. Initial results are usually available within a few days. More detailed results take 2-3 weeks. Be aware that these diagnostic tests do not test for every abnormal condition.
If the tests show a problem, you’ll first discuss the specific results with your obstetrician. Some couples who discovered that the baby tested positive for a serious condition report that being forewarned helped them to plan and begin to comes to terms with the result. If it comes to the point where you need to decide whether to continue with the pregnancy, the decision will ultimately be yours, but you can seek advice and support from the specialists available and national support groups.