Trouble dating pregnancy?

I was wondering if anyone else had this problem and how accurate the measurements were when they were able to give a date. I was on depo before I got pregnant and knew the date of my one and only period but they say it doesnt match. Originally they thought I had an ectopic pregnancy or was going to miscarry due to low hcg levels but when they retested the levels more than doubled in less then 48 hours so now they want to do a repeat ultrasound to try to date the pregnancy. For anyone who had this done, was it accurate?