25 -34 weeks is best, if you want to get um right away 24 weeks but if you want to see a big baby and more features 32-34.
They say between 26 weeks and 30 weeks are the best time, before 26 weeks the baby won't have much fat so you won't be able to see the baby's features as good as you would after 26 weeks. They also say not to after 30 weeks because the baby might be head down so you might not be able to see the face.
I had my first 4d with my first pregnancy at 14 weeks but the baby looks kinda scary looks like an alien she wasnt that develop yet lol I think by 20 weeks is good at least u will c the face more develop.yeh it's very exciting especially with first pregnancy. Good luck & have fun!:)