Most medical professionals calculate pregnancy from the date of your last menstrual period... in other words weeks start calculating from the time you start your period through the time you actually conceive.
Week 1 - First day of last period through 7 days.
Week 2 - 7 through 14 days (assuming at the end of this week, you ovulated and conceived.)
Week 3 - Most docs assume that the beginning of this week, or end of last week you conceived, and the baby is traveling down from fallopian tubes to your uterus.
Week 4 - Implantation into uterus this week or so. Baby is up to 14 days old. But you are considered a month pregnant by docs.
It is done this way because most women have no idea when they ovulate. The average woman will ovulate on day 14 (so they say) so this is an easy way for them to date an early pregnancy prior to scans.
In my case, I had my last menstrual period on August 25th. I ovulated September 7th and got my first BFP on September 17th (my cycles are very short at 25 days, so I tested the day I expected my period).
So even though I wasn't pregnant, docs are counting the 13 days before I ovulated as "you're pregnant".
Docs consider me being pregnant 96 days or 13 weeks 5 days. My baby inside me is about 13 days less than that or 83 days old. So fetal age is closer to 11 weeks.
I have found most internet sites go by the standard medical dating from lmp.
But some sites like
http://www.visembryo.com/baby/ use actual fetal age... from date of conception as the dating method.
Kinda confusing
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