Alongside the general goodwill and cheerfully cynical comments about upstaging that have accompanied the news of Cherie Blair's pregnancy, there lie the less chit-chat-friendly facts about the risks of late motherhood.
Everyone knows that the chance of having a Down's Syndrome child increases as you get older, and a great many women have to decide whether to have an amniocentesis test. While this may tell you whether or not your child has the condition, it also puts the child at risk.
In an amniocentesis test the doctor inserts a needle through the mother's abdominal wall to extract cells from either the amniotic fluid or the outer membrane around the placenta. This carries the risk of introducing infection, or of disturbing the development of the foetus and the placenta. The risk of miscarriage is between 0.5 and 2%. (Chorionic villus sampling, in which cells from the placenta are drawn off, can be performed at 10-12 weeks, but involves an even higher risk of miscarriage.)
Plenty of women, and Cherie Blair may well be one, say sod it, I'll take what nature offers me. And fewer women need to have amniocentesis, as blood tests and ultrasound scanning techniques have been developed to identify women most at risk.
The first indicator is still age: women aged over 35 (or 36, or 40 - the age varies according to different health authorities) are seen to be more at risk, and should be recommended the double, triple or quadruple blood test - which look for abnormal levels of two, three or four hormones, including alpha-fetoprotein (AFP), oestriol and human chorionic gonadotrophin.
Examining the thickness of skin at the back of the foetus's neck - the nuchal fold - with an ultrasound scan is another way of identifying women at risk. In Down's pregnancies, the nuchal fold is enlarged and the scans can show this fairly early, at about 11-12 weeks. Professor Kyprianos Nicolaides, professor of foetal medicine at King's College Hospital, London, who developed this technique, says that using it in combination with the age of the mother and bloodtests could detect 90% of babies with Down's Syndrome.
As well as abnormality of the nuchal fold, an ultrasound scan can reveal anencephaly (absence of the brain) or missing limbs. A later ultrasound scan at 16-20 weeks can detect spina bifida, hare-lip and abnormalities of the intestines, diaphragm and limbs, but how much is revealed depends on the skill of the operator and position of the baby.
One problem, when trying to protect parents from the strain of undergoing tests needlessly, is that many detectable abnormalities are what are known as "soft markers" - they are associated with certain conditions but do not prove that they are present. An enlarged nuchal fold, for example, could also be a sign of a heart abnormality, and high levels of AFP (an indicator of Down's) may indicate spina bifida, although nine out of 10 women with raised AFP levels have a normal foetus.
On the plus side, chromosome analysis is now much faster than it used to be, and it is possible to get the results back from an amniocentesis in 48 hours, instead of the three weeks it used to take.
A blood test is being developed which would render amniocentesis obsolete by using bloodcells from the baby which are present in the mother's blood. The main problem in developing the test has been finding enough cells to provide accurate genetic information: a teaspoon of maternal blood contains around 20,000m maternal red blood cells, 50m white cells and five foetal cells. Not only are the foetus's cells scarce, they are hard to distinguish from the mother's cells.
Scientists at Imperial College School of Medicine in London are overcoming at least one problem: using growth factors, they are multiplying the foetal cells up to 900-fold. They can then find out the baby's sex, and many chromosomal abnormalities. The test is designed to be used as early as seven weeks.
Meanwhile, back in the real world, in 1998 10 British health authorities used neither blood tests nor ultrasound imaging to screen for Down's. Instead, they offered only amniocentesis on the single basis of age. Oh well.
• Antenatal Results and Choices (ARC) offers support and information to parents facing abnormalities diagnosed during pregnancy. Helpline: 0171 631 0285.
