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If routine screenings or family history suggest a higher than average risk of your baby having an abnormality or birth defect, you may be offered additional tests to give you a more reliable indication of the health of your baby.
CVS is a test offered between week 10 and week 13 - the decision whether to have it is up to you.
Genetic abnormalities such as Down's syndrome
Inherited blood disorders such as sickle cell anaemia and thlassaemia
Metabolic disorders such as antitrypsin deficiency
Mental health conditions such as fragile X syndrome
CVS doesn't test for physical problems such as spina bifida.
What happens during CVS?
A sample of chorionic villi cells is taken from the placenta using a needle, which is either inserted through the abdomen or through the cervix.
The chorionic villi attach the placenta to the wall of the womb. They are made from part of the embryo that separates during early cell division and so they have exactly the same DNA as the foetus.
The test takes about five minutes and may cause discomfort but is not particularly painful. You will be told to take it easy for a couple of days and you may experience some cramping after the procedure.
The initial test results can come back in a few days but the full results can take up to a month.
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Is there any risk?
CVS is an invasive procedure and so carries risks. There is an increased risk of miscarriage, which has been estimated to be 2% more than the risk of miscarrying without a CVS procedure.
There is also a risk of infection, although according to NHS Choices, severe cases only occur in less than 1:1,000 procedures.
If you are rhesus negative, CVS carries a risk of sensitisation, which may trigger an immune response against your baby. You will be offered an Anti-D injection to counteract this.
CVS will not be carried out before 10 weeks gestation because there is thought to be an added risk of damaging the baby's limbs before this stage of development.
Remember - you're under no obligation to have antenatal tests.
What are your options?
CVS is said to be 99% accurate, although not every abnormality can be ruled out by the test.
If an abnormality is found, it can be very upsetting and throw all your future projections about your baby and your family into disarray. You should be offered counselling to help you cope.
If you decide to end your pregnancy, your GP and midwife will be able to go through your options with you. If you decide to carry on, it's worth finding out as much as you can about the disorder and how you can care for your baby before the birth.
Mumsnet can also help - umpteen women on the Talk boards have gone through similar circumstances and have advice and insights.
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