The main difference between screening tests and diagnostic tests is that screening is only meant to find out whether there is a higher chance that your baby has a condition or a lower chance, whereas diagnostic testing is designed to find out for sure whether or not a baby has a condition.

Although you can get a more definite result from diagnostic testing, there is a risk of misscariage each time a genetic diagnostic test is done. If everyone has screening first only those found to have a higher chance of carrying a baby with a condition are offered diagnostic testing. This should mean fewer babies have a diagnostic test and therefore fewer misscarriages because of them.

Ideally you should have screening during the first three months of your pregnancy. If you do plan to have screening and are under 14 weeks pregnant you will be offered First Trimester Combined Screening.

First trimester combined screening includes both a blood test and an ultrasound scan. The blood test is best taken at 10 weeks but can be taken any time between 9 weeks and 13 weeks and 6 days. The ultrasound scan (called a nuchal translucency), is best done at 12 weeks of pregnancy but can be done any time between 11 weeks and 2 days and 13 weeks and 6 days of pregnancy.

If you are 14 weeks pregnant or more when you decide to have screening, you will be offered Second Trimester Maternal Serum Screening. This is a blood test which is best taken between 14 and 18 weeks of pregnancy but can be taken up until 20 weeks.

Your doctor or midwife will give you your result. They will tell you whether the result shows that there is a low or increased chance that your baby has a condition. You will be given your screening results within 10 days of finishing screening.