from the 1st day of their last period. That is called "gestational age" . We are actually pregnant for the 2 weeks before we "really" are.

Apologies for my ignorance, but why 2 weeks?

Because you count from the first day of your last period. Having your period &ovulating is part of the reproduction process. It's just the way they count it.

Well I mst b 2 weeks overdue then :)) coz I didnt get a pierod as I was on the jag and had just came off it and got prego, I know the exact date in concived and the dates count up and my midwife counts from that date to see how far on I am

When a woman starts a new cycle she is considered pregnant .....So day 1 of her cycle would be 1st day of her period. If she engages in sex during her fertile time usually mid cycle and concieves by the time she is expected to get her next period she is already 1 month pregnant. That is the way all drs. Calculate it .......Example...Im 29 weeks pregnant. My last period was 29 weeks ago...My babys actual fetal age is 27 weeks.

Lol, natzcee, .I didn't have a period either.
I had just had a miscarriage &got preggo again
not even 2 weeks after .I miscarried.
I haven't had a period since .April!

yeah ..Your case is a little more difficult ..There are always exceptions ...

Ive had 4 periods since december of '04 lol.