Am I 14 or 15 weeks?

[newmama91]

I'm a bit unsure as to whether I am 14 or 15 weeks pregnant. I thought my due date was the 21st of December. But at my 12 week ultrasound the midwife told me the baby looked a little bigger and said I looked 13 weeks so she changed the due date to 17th December. For some reason this doesn't feel right because the first date of my last period was the 15th of March which would mean I ovulated around the 29th? So my due date should be the 21st?
I'm just a bit confused x

[GuybrushThreepwoodMP]

Your official due date is defined by your dating scan. As you could easily have ovulated a week earlier than you thought and most people don't have a 28 day cycle, dating based in LMP is not accurate.

If you are certain of when you ovulated (I.e through ovulation tests or similar), your due date stays at where they put it but you can take it into account if you'd go overdue. If they start wanting to induce you and you are not keen, you could decide to hold off.