4 weeks. A little confused.

lisa
So I just had a dr appt today. She confirmed I'm 4 weeks along. She said so probably right around Valentine's Day. Well, I wasn't paying attention til I left to realize, that's not 4 weeks ago, so how does she figure?
The first day of my last period was feb 1st. Any insight? Am I really 4 weeks along then?
I have another appt in 2 weeks so I guess I'll know more then.