Which timing should I go by?

It has come to my attention that there are two different timings during a pregnancy count down.
1. Count down beginning from the last day of your menstrual
2. Timing from the actual date of conceiving
My wonder is which timing is the authentic 40wks beginning from? I had a u/s today & I know the exact date my last menstrual was, but was confused by the nurse telling me I was only 4wks & not 5wks. Should I ignore the whole incident & just continue my regular count down?

It goes from the date of your last period.
You're technically 2wks pregnant when you conceive..

Quoting brittanyy™ [+M]:“ It goes from the date of your last period. You're technically 2wks pregnant when you conceive..”

Thanks for confirming my suspicions  

um, pp...not necessarily true.

yes your pregnancy is 40 weeks. HOWEVER...it is 38 weeks from when you ovulate. the whole 40 weeks from your lmp is assuming that you ovulate on a typical 28 day cycle (ovulating on day 14). if you do not ovulate exactly on that date though, as the majority of women don't, then that dating would be off. if your us was done in the first tri (which it obviously was) then it will generally be more accurate than your edd simply according to your period if you are not positive that you ovulated on day 14 of your cycle.

for example, with my last pregnancy, my lmp was dec 22nd, which would have given me a due date of sep 27th. however, i have long cycle's and did not ovulate for a week and half after the typical time the average woman would. i ovulated jan 14th, giving me an edd of oct 7. i knew when i ovulated, and multiple ultrasounds agreed with that date, or only a day off.

(although i will say, ironically my son was born the day before sep 27th, but tha is merely coincidence, my first was early too).

hope that makes sense...

Quoting NicoleKT *EOM*:“ um, pp...not necessarily true. yes your pregnancy is 40 weeks. HOWEVER...it is 38 weeks from when you ... [snip!] ... my son was born the day before sep 27th, but tha is merely coincidence, my first was early too). hope that makes sense...”

Yes it does, thnx   for all the info