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YOu may have one done around 10 weeks to varify due date, that's really depends on the doctor. The most common and important one will be 18-20 weeks. As that determines the health of the baby and pregnancy and all. :) The doctor usually worries about that. If you are unsure when it will be you can ask him/her and they can tell you.
I had one when my pregnancy was confirmed, another at 19 weeks, and several in the weeks before giving birth because I had preeclampsia and they were keeping an eye on the baby. But as I understand it, there's one around 10-14 weeks and another at 18-20 weeks, and sometimes one at the hospital when you're in labor if your pregnancy is normal. If there are any health concerns or complications, though, I can tell you to expect them as often as every week after week 30.