You may feel that you don't want to have any antenatal tests. This is a perfectly reasonable decision to make. However, if you do decide to go ahead, you need to understand the difference between screening and diagnostic tests.
A screening test tells you what your chances are of having a baby with a condition such as spina bifida or Down's syndrome, not whether your baby actually has these conditions. A diagnostic test can tell for certain whether your baby has a condition such as Down's syndrome or spina bifida. Diagnostic tests carry a slight risk of miscarriage.
At 10-13 weeks
• Chorionic Villus Sampling (diagnostic)
• Nuchal Translucency Scan (screening)
At 16-20 weeks
• Blood tests (screening)
The blood tests carried out between 16 and 18 weeks into your pregnancy screen for Down's syndrome and spina bifida. These will only be accurate if you know for certain how many weeks pregnant you are. Your blood may be tested for up to three different substances or 'markers'. Sometimes a fourth is added.
• Amniocentesis (diagnostic)
• Ultrasound scans (screening/diagnostic)
Scans are useful for dating pregnancies and for indicating whether your baby has developed normally. However, they don't pick up everything - at least 30% of babies with heart problems are not diagnosed on scans. They can also show unusual or unclear images that may be unimportant but can still cause unnecessary worry. The accuracy of scans depends on the skill of the sonographer, the quality of the scanner, and the length of time you are scanned.
See Antenatal testing 2 for more information.