That it is best to wait till at least the 5 month mark.. Cause at least by then you can see everything.. Yet your baby isnt so big that all you see are angles!!!.. A friend of mine had one.. For her third baby.. And she said it was the best thing she've ever done!
She said it was such a nice way to see instead of imagining what the little one looks like with a 3 d one...
Wierd enough though my sis who is 26 2 weeks.. When she got her u.S done.. She had a 3 d done.. But instead of getting that little spit out of paper with the pics of baby.. They gave her an x-ray sheet!.. Like that when you break an arm!...
She was disapointed cause she wanted to send some pics to family back home and now she cant!...
That sucked!
I know I wont be getting my u. S done at the same place!
Enjoy!
Hope I helped!