Chromosome testing

Dr Trisha Macnair responds

I'm very sorry to hear about your daughter and wish you all the best with this pregnancy. Now that you have a positive pregnancy test, talk to your doctor who should be able to refer you at once to the local genetic counselling service. They will be able to:

• explain what the risks are that another of your children would have similar problems
• what tests can be done, if you choose, to look for chromosomal problems in your unborn child
• what the risks of these tests are
• what the options are if a test proves positive

It's important that you and your partner have the facts and have had a chance to think these things through before you go ahead with a test.

Cells are taken from inside the womb

In order to look at the chromosomes your baby has, and whether there's a problem with them, a few of your baby's cells are needed. There are two different methods usually used in the UK: chorionic villus sampling (CVS) or amniocentesis.

Both these tests involve taking samples from inside the womb, which involves some risk to the baby.

However, there are also some experimental methods being used in one or two research centres in the UK which involve looking at samples of the mother's blood, as a few of the baby's cells do cross the placenta and get mixed into the mother's blood. This would be much less risky to the baby, but as yet the results aren't so reliable and you're unlikely to be offered this sort of test yet.

In addition, one or two chromosomal problems, such as Down's syndrome, may be screened for using a combination of blood tests and scans, but the results are just an estimate of risk rather than a definite answer about the baby's chromosomal make-up.

CVS

In CVS a tiny piece of tissue is removed from the placenta. This is done by inserting a needle into the womb either through the cervix or through the abdominal wall, while an ultrasound scanner guides the doctor where to go. The cells in the tissue (which originates from the baby not the mother) can then be examined in the laboratory to check the chromosomes.

The test is usually done at nine to 11 weeks of pregnancy, and a result available a couple of days later. As well as a quick result, CVS has the advantage that should parents choose to terminate the pregnancy it's still relatively early days and so less traumatic.

The two main disadvantages are:

• The risk of inducing a miscarriage (this occurs in about two to four per cent of cases, depending on the experience and ability of the person doing the test, which is much higher than with amniocentesis).
• A possible small risk of damaging the baby. This risk isn't clear and may be linked to disruption of blood supply to the growing baby, leading to limb deformities and other problems.

Amniocentesis

In amniocentesis a needle is also inserted into the womb under ultrasound guidance, in order to extract some of the fluid which bathes the baby. This fluid contains fetal cells which can be studied in the laboratory.

In experienced hands the risk of miscarriage may be as low as 0.3 per cent, but it takes longer to get results than with CVS (as long as 21 days, which is a long time to fret for, although if the latest sophisticated techniques are used, a preliminary result may be available within 2 days). Since the test is also done later than CVS, at about 16 weeks, the pregnancy is much more advanced should a termination be done.