Diagnostic Tests

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Chorionic villus sampling (CVS)

CVS is used to detect Down's syndrome and other chromosomal disorders, and can be performed earlier in pregnancy than an amniocentesis test - from about 11 weeks. Like an amnio, it's an invasive test and involves taking a tiny amount of tissue from the placenta by passing a thin needle through the stomach wall, or inserting a small tube through the vagina and neck of the womb.

Some women describe it as uncomfortable but not painful and some hospitals offer a local anaesthetic. Ultrasound is used as a guide, and the procedure takes around 20 minutes. The placenta contains the same DNA as the baby, so it accurately detects major chromosomal problems. You'll get results within 10 to 14 days.

Amniocentesis

Amniocentesis is also an invasive test, but instead of taking tissue from the placenta, about two tablespoons of amniotic fluid from around the baby are removed and analysed. It can be done at, or after, 15 weeks (which is too late for CVS) and will be offered to mothers if a screening test flags up a higher-than-average risk of Down's syndrome; when a previous pregnancy has been affected by a chromosomal or genetic disorder; if a scan has shown up markers for Down's: or if there's a family history of chromosomal abnormalities.

The procedure is usually quick, although it may take 20 minutes or so to locate a pocket of fluid away from the baby or placenta. The process may also cause a little discomfort to the mum-to-be.

Ultrasound guides the doctor as she inserts a fine needle through the woman's stomach wall into the amniotic fluid to take the sample. A result for Down's syndrome, Edward's syndrome and Patau's syndrome may be available within three days, and results after looking at all the baby's chromosomes within two or three weeks.

If a CVS or amniocentesis result shows that there is a disability, depending on the nature and extent of the problem, you may be offered a termination of pregnancy. You may want to ask for information on what the baby's condition involves and how her life expectancy and quality of life will be affected.

Termination is a very personal decision and although hospital counsellors, doctors and self-help bodies, such as the ARC, can offer support and advice, only you and your partner can decide if it's the right thing for you.