My doctor says that i'm only 12 weeks and 2 days pregnant, but my ultrasound said that i was 13 weeks and 1day. i know my periods are irregular, but i don't know with whomshould i go by?

Irregular: If your periods are irregular go with the ultrasound, however these dates are 8 days between them and the margin of error of the study in the first trimester can be up to 10 days.

Either is fine: At this stage, technically ultrasound is more accurate than clinical dating but unless it is off by more than 1 week, we usually don't change your dating. The reality is that both are an estimate and are only used as a guide to watch fetal growth and get a sense of when delivery will likely occur.

Need follow up: I do not think that this difference is significant. In follow up ultrasound you might get a little different measurement and numbers.

Less than a week : We often don't change due dates in the first trimester unless there is more than a week difference. Your due date is likely what was originally given to you.