It involves a blood test and measuring the fluid at the back of the babys neck (nuchal translucency) with an ultrasound scan. This is sometimes called a nuchal translucency scan. The nuchal translucency measurement can be taken during the dating scan.

How do I prepare for a dating ultrasound?

For an abdominal dating ultrasound, you will need to have a full bladder. You may be asked to drink some water and not go to the toilet beforehand because a full bladder helps to push your womb up to give a better picture. For a vaginal scan, you can have an empty bladder. The scan will take 5 to 10 minutes to perform.