How accurate are dating ultrasound scans - Bedec.
You will not be offered the combined screening test if your dating scan happens after 14 weeks. Instead, you will be offered another blood test between 14 and 20 weeks of pregnancy to screen for your chance of having a baby with Down's syndrome. This test is not quite as accurate as the combined test. Find out more about: the 20-week scan; ultrasound scans in pregnancy.