Week one -- when we're talking about the typical 40 weeks of pregnancy -- starts on the first day of your period.
Yes, it's true that you're not actually pregnant yet, but most caregivers count everything from this day, anyhow.
The standard definition for gestational term is 266 days from conception to the date of the baby’s birth.
This is also defined as 280 days, or 40 weeks, from the first day of the mother’s last menstrual period, a definition which assumes that the mother ovulates on day 14 of a 28 day menstrual cycle. In the 1980’s, Mittendorf noticed that birth dates for women in his practice, primarily second-generation Irish-Americans, averaged seven days past their “due dates”.
For example, as childbirth educators, we frequently see women who have reached the 38 week of pregnancy, where they’re told the baby is ‘no longer premature’ and the moms are ready for the baby to come.