This chart is courtesy of the Pregnancy Due Date Calculator on BabyZone.com.

I always wondered why some people counted pregnancy in weeks and others in months.  As in “I’m 12 weeks pregnant” or “She’s 7 months pregnant”.  It turns out that Doctors and other healthcare professionals only keep track of “weeks”.  This is from the Mayo Clinic:

It may seem strange, but the first week of your pregnancy is actually your last menstrual period before becoming pregnant.  Why is that? Doctors and other health care professionals calculate your due date by counting 40 weeks from the start of your last cycle.  That means they count your period as part of your pregnancy, even though your baby hasn’t been conceived yet.

Conception typically occurs about two weeks after the start of your last menstrual period.  When your baby arrives, it will have been about 38 weeks since he or she was conceived, but your pregnancy will have “officially” lasted 40 weeks.

So there you have it.  The mystery of “weeks” vs. “months” is solved in my mind.  For the record, I don’t use “months” anymore.  I always now say that we are “X” weeks pregnant since it is more accurate and the term “weeks” is what I see in evertyhing that I read about pregnancy.