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Understanding pregnancy tests

The most common tests offered are ultrasound, nuchal, chorionic villus sampling, and amniocentesis. Here we explain and help you to understand the pregnancy tests:

Test	Method	Screening or diagnostic	When it can be done
Ultrasound	Scan	Screeningand diagnostic	From about 4 weeks
Combined test Nuchal translucency test and blood test	Scan and blood test	Screening	Around 11-13 weeks+ 6 days of pregnancy
Chorionic villus sampling	Analysis of placental tissue	Diagnostic	Around 11- 13 weeks
Amniocentisis	Sample of amniotic fluid is tested	Diagnostic	13 + weeks
Triple Quadruple	Blood tests	Screening	15 -20 weeks

Before you can make any decisions about which antenatal tests to have (or whether to have any at all), the first thing to understand is the difference between a screening test and a diagnostic test.

Screening test - tells you what your risk is of having a baby with a particular health problem.

Diagnostic test - tells you whether your baby has or has not got a particular health problem.

Screening tests

A screening test estimates your risk of having a baby with a serious health problem such as Down’s syndrome or spina bifida or some rarer condition such as Edward’s syndrome. It can’t tell you for certain whether your baby has or has not got one of these conditions.

So, for example, you might have a blood test when you are 16 weeks pregnant and be told that your risk of Down’s syndrome is 1 in 140. This means that there is a one in 140 chance that your baby will have Down’s syndrome. Or, to look at it another way, you are far more likely to have a normal, healthy baby than a baby with Down's syndrome.

Name of screening test	When you can have the test
Nuchal translucency test	11 - 13 weeks + 6 days of pregnancy
Ultrasound scan (although this may be diagnostic in some cases)	Any time from 4 weeks of pregnancy
Blood tests	11-20 weeks of pregnancy

Screen negative

The result of your screening test may simply be described as screen negative. This means that your risk of having a baby with a problem is less than 1 in 250. Your risk is so small that it’s not even quantified. Nonetheless, a screen negative result does not mean that you definitely have a healthy baby.

Screen positive

If your test result is screen positive (ie, greater than 1 in 250) you will be quoted a figure such as 1 in 100 or 1 in 40. Only you can decide whether you are prepared to accept that risk or whether you want to have a diagnostic test to get more information - there’s more about making these difficult decisions later on.

Diagnostic tests

A diagnostic test can tell you whether your baby has Down’s syndrome or spina bifida or other major health problems. The result gives you a yes or no answer.

Name of diagnostic test	When you can have the test
Chorionic villus sampling	Around 11- 13 weeks of pregnancy
Amniocentesis	13 + weeks of pregnancy
Ultrasound scan (although this may only be a screening test in some cases)	Any time from 4 weeks of pregnancy