Pretty sure they must be safe as they wouldn't do them if not. If ur in the uk u'll have them at 12 weeks and 20 weeks some more if ur high dependancy xxx
They are safe. How often you get them depends on a few things: your insurance, your care provider, and your pregnancy. If your insurance doesn't cover more than 1 or 2, that's probably all you'll have. If your insurance covers more than that for a 'normal pregnancy', then it depends on your care provider. They may only require 1 or 2, or may like you to have more. And if you have a 'high risk' pregnancy, you'll have a lot more to closely monitor you and baby.