The 20-week scan is a screening for physical abnormalities.

You can have the 20-week scan from week 18 to week 21 of your pregnancy. This means up to 21 weeks and 0 days of your pregnancy. The best time to have the scan is in week 19 of your pregnancy. That is 19 weeks and 0 days to 19 weeks and 6 days of your pregnancy.

More about the second-trimester examination for structural physical abnormalities.