Midwife Nikki says...
The scan done between 11 to 14 weeks is called a 'nuchal translucency scan' and measures the fluid in the skin at the back of your baby's neck (the nuchal fold), giving an indication of the risk of Down's Syndrome. The antenatal blood tests screen further, measuring the levels of hCG (human chorionic gonadotropin) and a specific pregnancy protein in the blood.
The scan and the blood test combined give a detection rate of about 90 to 93%. However, some units use only one of the screening tests and a risk factor of less than one in 250 deemed to be negative. The detection rate and risk factor can be affected by the timings of the blood tests and scan and the woman's age is also taken into consideration. These tests can help you decide whether or not to have a diagnostic test such as an amniocentesis. Discuss any concerns with your midwife and/or obstetrician.
For advice, visit arc-uk.org