For example, the NT scan for Down's syndrome has a false-positive rate of five per cent.
This means that, out of 20 women who have the scan, one woman will be wrongly told she has a high chance of having a baby with Down's syndrome.
However, conception doesn't actually occur until around 14 days after your LMP, or later if your cycle is longer than 28 days. Multiple pregnancy: antenatal care for twin and triplet pregnancies CG129.
You may know when your LMP was, but not when you actually conceived.
The scan will usually show that your pregnancy is less far along than suggested by your LMP.