The test is accurate in what it is measuring. The NT scan measures the skin at baby's neck and the blood test measures certain chemicals in the blood.

The combination of the results give you a risk figure. This figure is also accurate. If the risk figure for DS is 1/250 it is saying that of 250 women who have those results, one woman will have be carrying a child with DS.

It is how we understand and interpret the results that leads people to say the screen is inaccurate. Some people will see 1/250 as extremely high, whereas others will see it as a low risk.

Before having the 1st trimester screen, the doctor should explain how the results work, and that the results are not definitive, just that they can tell you if you have a higher chance of carrying a child with a chromosomal variation. It should also be discussed what you think you might do / feel if the results came back as high risk, and the implications and risks associated with further testing.

The first trimester screen has come to be seen as routine, something that everyone should have. Everyone wants to see a low risk come back, but it is also important to think about what you might do if the results are not what you thought they might be.

The stated risk of miscarriage following CVS is 1/100, and in amnio is 1/200. It's a lot of maths and figures, but something that must also be considered when deciding whether you would have follow up testing.

Officially, they are not supposed to release the results of the blood test and NT scan separately because it can cause unneccessary stress. The risk figure is a combination of maternal age, NT measurement and the blood tests. Having all results together give you the most accurate information.