When you fall pregnant, you will undergo lots of tests, screenings and scans as part of your antenatal care. It can be a little daunting at times, but every check-up you go to is making sure that you and the baby are happy and healthy. Most of these will be done in the first and second trimester and one of the most important checks at this time is the nuchal translucency screening which assesses the risk of Down's syndrome.

What is nuchal translucency screening?

This is when the collection of fluid under the skin at the back of your baby's neck is measured using ultrasound. It is normal for babies to have this fluid but those with Down's syndrome have an increased amount.

When is it carried out?

The test is done between 11 weeks and 13 weeks plus six days of pregnancy (so around the start of your second trimester). It is usually done at the same appointment as your 12-week dating scan. This test is to establish whether you are at risk of having a Down's syndrome baby. It is done to help you decide if you want to have further tests to establish for sure if your baby is affected.

What happens?

The nuchal translucency screening combines a blood test and an ultrasound scan. This is usually done through the tummy, although a vaginal scan might be carried out if a better view is needed. The sonographer measures the thickness of the nuchal translucency (the pocket of fluid) at the back of the baby's neck, while the blood test measures two blood chemicals associated with pregnancy.

How do they calculate the risk?

The sonographer will enter your age and the measurements into a database to calculate the risk. Every woman has a chance of giving birth to a baby with Down's syndrome, although the risk does increase with age.

What happens if I am at a higher risk?

While this doesn't mean your baby definitely has the condition, you will be offered diagnostic tests to establish if she does. These will give an almost 100% accurate result on abnormalities as they test DNA taken from your baby's cells, either from the placenta or from amniotic fluid. You will be offered either chorionic villus sampling (CVS) at around 11 weeks of pregnancy or amniocentesis which is carried out in the second trimester, at around 15 weeks. Unfortunately these carry a small risk of miscarriage. A low-risk result from the screening means that you are at a low risk of having a baby with Down's syndrome, but it doesn't mean that there is no risk at all.