I had one at 12 wks, 19 wks, 32 wks and then at 37 wks, due to placenta praevia.

Also I had a 4D one at all of them from 19 wks as the SAH has the facility there, it was just a quick flick of the switch when they were looking at her face. No extra charge at all. I think if you go to a place specifically to have a 4D scan it costs extra, but if you're having your routine scan somewhere that can do 4D it's just included.

I would have preferred not to have had to have the last two u/s because then I wouldn't have had placenta praevia, wouldn't have needed a c/s and *ugh* wouldn't have had to have internal u/s done! I hated those!! Although it was good to see bub and it was cool to find out she had hair. She has a great video for when she's older too.

I believe the risks from u/s are so low they aren't worth worrying about. Probably more of a risk having a cup of tea every day! The heat from the ultrasound waves is so low it doesn't seem to have any effect on baby.