I was also always confused about that. According to my Dr. I'm 27 weeks tomorrow, but when I went to a different hospital because I had unexplained lower abdomen pain I panicked and went to the nearest hospital, they said I was almost 2 weeks ahead of what my Dr says. My nurse says I conceived in the middle of May.
You count from when your period begins, not ends.
So typically, around 14 days after your period ends is when you are fertile. And then 14 days after you are fertile your next period begins if you didn't get pregnant that cycle.
Oh OK I think I get it now so this means if my last period ended sept. 26th I got pregnant sometime within the week after
If a doctor looks at the baby by ultrasound and, due to its measurements, says "You are 6 weeks pregnant," he or she is counting from the first day of your last period, or the calculated first day of an average woman's last period who has an embryo that size in her uterus. That would mean, as RockRose says, if the doc said you're 6 weeks along, you are at that time 4 weeks from conception. Pregnancy is 266 days from conception to full term, but it is counted as being 280 days long in order to stretch the count back to the (presumed) first day of the last period the woman had. All medical counts are calibrated this way. I realize it means that from the first day of your last period to ovulation, when you are not in fact pregnant yet, is counted into the count. They do this because they need a definite start point, and the period is a big, obvious signal. The doctors don't think you are pregnant on day 1 of bleeding. They just need a place to begin the count.
Some people are different if they have irregular cycles some can conceive whenever out the month (of course certain dates)
If you are 6 weeks pregnant on Nov 2nd, you got pregnant 4 weeks prior, around Oct. 6th.
You get pregnant two weeks before you expect your period to start, and when you miss your period, at that point, you are 4 weeks pregnant because the pregnancy calculator countdown starts with the first day of your last menstrual cycle.
Clear as mud.