At present the only way during pregnancy to obtain definite information about a baby's chromosomes, (i.e. to rule out or confirm a chromosome abnormality), is by a diagnostic test, e.g. amniocentesis, chorionic villus sampling (CVS), or cordocentesis.
These are invasive tests that carry a risk of miscarriage for up to 3 weeks after the test. This risk is in addition to the natural background chance that a miscarriage may occur even if a test is not done. The exact cause of miscarriage following a CVS or amniocentesis is unknown. It is possibly due to ruptured amniotic membranes, infection or bleeding.
Depending on the sample and the laboratory technique used, these tests usually provide an initial provisional result within days and a final confirmatory result within 3 weeks.
In a twin or multiple pregnancy invasive tests can show whether each baby has normal chromosomes.
CHORIONIC VILLUS SAMPLING
CVS is performed from around 11 weeks of pregnancy onwards.
A small sample of the placenta (afterbirth) is obtained, which can give us information about the baby because the baby and placenta developed from the same cell.
Using ultrasound scan for guidance, a needle is inserted through the woman's abdomen to obtain a small sample from the placenta.
Occasionally a sample from the placenta can be obtained through the cervix, in a similar manner to having a cervical smear.
The risk of miscarriage is between 1 and 2%, and the test result is slightly more likely to be inconclusive than amniocentesis.
'I tried to think through the ethics of CVS. None of the journals I had on medical ethics discussed prenatal testing. If CVS could be of no benefit to this particular baby and might even harm her, surely we shouldn't have it. On the Thursday the consultant answered our long list of questions. He said that as we obviously wanted to continue with the pregnancy it would make no sense to have CVS, and he agreed to keep a close eye on me although he thought it most likely I would miscarry.'
'Our baby with trisomy 13 died and we decided to have CVS for my next pregnancy because you get the result earlier than with amniocentesis. The slightly higher quoted rate of miscarriage was worrying, but in the event our baby appeared to be fine and it was confirmed about a week later that her chromosomes were normal.'
Amniocentesis is performed from 15 weeks of pregnancy onwards. (Before this time there is not enough amniotic fluid to safely take a sample). Using ultrasound scan for guidance a needle is inserted through the woman's abdomen to obtain a small sample of amniotic fluid which contains cells from the baby.
The risk of miscarriage is about 1%.
'On the day I got the results I felt quite numb. Amniocentesis confirmed our baby has trisomy 13, an unbalanced translocation, and we were told the prognosis is extremely poor.'
This test is carried out after the 18th week of pregnancy and results take about a week. A needle is inserted into the mother's abdomen and a sample of the baby's blood is taken from the umbilical cord. The risk of miscarriage is between 1 and 4%.
'The specialist told me as gently as possible that all the defects linked up to a syndrome called trisomy 13, and asked for permission to take blood from the cord to confirm this diagnosis.'
'We had to make an immediate decision on whether to have cord blood taken. We asked for somewhere private where my husband and I could discuss what we had been told, and a room was provided for us.'