How often should a pregnant woman have a ultrasound?

I can only speak from my experience and I am sure each dr has his own schedule but I had an US at 8 weeks to determine due date and then another at 20 weeks. I am sure though if there were any complications or problems, you would get them more frequently. That is about right. If you are certain of your dates you may not have the early ultrasound. If you are having Downs syndrome etc checks you may have one at 12-13 weeks. Almost everyone has one at 18-20 weeks as this is the best time for spotting abnormalities in the skull, heart, kidneys etc. You would not normally have any more unless there were concerns that the baby was not growing properly. It also depends on your doctor & their beliefs & practices & whether or not you are a high risk pregnancy. I myself had 30 ultrasounds in a 37 week pregnancy, but I was extremely high risk!