OK I’m a little confused about something.
My wife is pregnant, and I’m positive that we conceived over Labor Day weekend, call it 8 weeks ago. Well that’s when we “did it” any way.
But even though we told the OB/GYN this, they still keep saying this is week 10 - they add two weeks to it.
We also have a book about week by week pregnancy details, and the chapters are labled as such: “Week 10 (fetus age 8 weeks)”
Why the discrepancy?
Weeks of “gestational age” are calculated from the date of onset of last menstrual period (LMP) - due date is 40 weeks from the LMP. Generally speaking, ovulation (and thus conception) occurs 2 weeks after LMP, hence the actual fetal age is 2 weeks less than the gestational age.
Only once in recent memory?
Thanks OslerKnew, I knew I could count on the Dope. So when somebody asks, “How far along?”, I should say 10 weeks, right?
Only once within a period of a few weeks around that time. Unfortunately, we don’t do it 20 times a day like we used to. 
This drives me nuts! I’m 25 weeks pregnant. Or 23. Or 5 months.
When someone asks you how far along, you can tell them the due date. That gives them an idea of how much longer.
Or you can tell them 8 or 10 weeks, or 2 months. They’ll never remember and ask you again next time you see them.
Except for the happy couple, and maybe her mom and your mom, almost nobody can keep track of other people’s pregnancies. I’m always shocked to find out how far along a friend is. And then I overcompensate and think they’re much farther along than they are.
Anyway, congratulations!
You aren’t the only one confused by this. My wife and I knew the date of our daughter’s conception. The OB kept ignoring my wife when she told her this and assigned another date much like yours did.
I am glad to hear there is a real reason for this.
Just remember that even if you know beyond the shadow of a doubt the exact moment of conception, your projected due date is still only an estimate. Some babies and/or mothers just take longer than others. And nobody knows why. (Or, maybe somebody does, but I’ve never heard it.)
The ‘gestation period’ for a human is (on average) 40 weeks. But like others have said that’s based on the beginning of the last menstrual period. The reason that date is chosen is because it’s an obvious beginning time. The date of ovulation can vary several days but it is pretty much always 2 weeks before the next period starts. So the conceptus/fetus is not actually formed until gestation is 2 weeks along.
This usually means that you notice you’re pregnant about 2 weeks after it’s happened when you miss next month’s period. Most home pregnancy tests are pretty good at detecting an elevated hCG at that point. Some of the newer HPT’s and pregnancy tests available at the doctor’s office are more sensitive. This means that if you are trying to get pregnant, you may be able to get a positive pregnancy test at between 7-10 days after conception as opposed to 14. Not a big differenct, but to some couples it really matters.
As an aside… In the ER we see a lot (LOT) of young ladies who come in with complaints of ‘abdominal pain’ because they know they’ll get a pregnancy test and if it’s positive they’ll get an ultrasound. All they really want is the ‘free’ pregnancy test and a picture of their baby.