Ultrasound scans can predict Down's syndrome. A nuchal scan measures the depth of the dark fluid-filled space at the back of the baby's neck at 10-13 weeks of pregnancy. If the space is thicker than usual, your baby may be at risk of Down's.
However, an analysis of more than 133,000 babies with and without the condition found that the nuchal scan throws up false positives, signalling Down's when it isn't. Up to 87,000 women would have to be scanned to find one accurate diagnosis (JAMA, 2001; 285: 1044-55). One study of more than 96,000 pregnancies found the scan to be only 80 per cent accurate, even among women at high risk of a Down's baby. These false positives may lead you on to invasive tests such as amniocentesis, which carries a risk of miscarriage (Lancet, 1998; 352: 343-6). Another study found a false-positive rate of 8 per cent (BMJ, 1998; 317: 368).