How do obstetitrions determine how many weeks pregnant you are
Than can also tell from how big the baby is. Depending on weight and length. :)
I believe they count from your first missed period.
Obstetricians can determine how many weeks pregnant you are by using blood samples and dating scans. The scan gives a more accurate idea of how pregnant you are and can tell you if you're expecting more than one baby. For more information, visit http://www.babycenter.com.sg/pregnancy/antenatalhealth/testsandcare/bookingappointment/.
The can go by the date of your last period, they can do a vaginal ultrasound (if you are newly pregnant) or measure fundal height.
Usually at the very beginning they go by your last period but once you have a sonogram they can measure the baby and get how many weeks you are to the day (ex 7 wks 5 days)