I got a positive result on a home pregnancy test today. The first day of my last period was Jan. 10th but it wasnt a normal cycle, in fact it was just over a month long. Can I still use the fist day of my last period to determine how far along I am? Even though my last and previous cycles were irregular? I have an appointment with my family doc tor but not for another couple of weeks, I would like to know before then how far along I am.