Staring at the deep magenta spot on my pregnancy testing stick, my first thought was, ‘Pregnant at 41 is very unlike pregnant at 32.’ Just behind the doctor’s head, about level with her ear, was a tatty little chart of statistical probabilities of Down’s Syndrome births in relation to increasing maternal age. You couldn’t miss it.
Mine had gone up to 1 in 42. At first I thought I would cut out all the agonising and go straight to amniocentesis, an invasive diagnostic test offered routinely to mothers over 35. This test diagnoses Down’s Syndrome but has a 1 in 100 risk of miscarriage. Then I heard through friends about Professor Nicolaides at Kings College Hospital, Dulwich, and an ultrasound screening test called Nuchal Translucency or Nuchal Fold.
The test is carried out between 11 and 13 weeks from the date of the last menstrual period (LMP). The ultrasound measures at the back of the foetus’s neck, an area that appears as a black stripe down its back. This is strictly a screening test. It gives no concrete answers, only statistics. A combination of measuring the nuchal fold and the mother’s age will give a new statistical probability about the possibility of Down Syndrome or other chromosomal abnormalities. Nicolaides had found that 18% of foetuses with nuchal folds above 3mm had Down’s Syndrome.
The risk rose with the thickening of the fold – a measurement of 6mm meant 36 times the base risk. So, the overall risk for Down’s Syndrome at 35 is around 1 in 270. If the foetus has a nuchal fold of 6mm, then its risk becomes 1 in 7.5. This test turned out to be available at Queen Charlotte’s, my nearest maternity hospital.
Dreading even the small risk of a miscarriage that amniocentesis brings, I opted for the nuchal translucency ultrasound. The reality of the test is that it detects between 60 and 80% of Down Syndrome pregnancies, which means that between 20 and 40% of foetuses with a ‘normal’ measurement (currently below 2.5mm) can be born with Down’s Syndrome or another abnormality.
On the other hand, foetuses with abnormal measurement have been known to turn out perfectly healthy. The measurements, plus maternal age, are then fed into a computer, and the statistical probability comes out. Mine went down from 1 in 42 to 1 in 310 for that stage of the pregnancy. If the result is less than 1 in 250, then the pregnancy is judged to be high risk, and you are offered an amniocentesis.
I had just scraped through – a consequence of my age, rather than the modest 1.6mm measurement of the foetus’s nuchal fold. Approximately 30% of affected pregnancies are lost naturally, so the probability goes down further as the pregnancy goes on. The nuchal fold scan is followed up at 20 weeks by the routine anomaly scan offered to all pregnant mothers. This looks for ‘soft markers’ of chromosomal disorders such as heart defects, enlarged kidneys and shortened limbs. Again, there are no certainties. Doctors shower you with statistics. In the end, you have to take a cool view (not easy with all those hormones whizzing around your system) of the comparative risks, and make up your own mind.