It depends on if you are given your gestational age (how long the baby has been growing) or your pregnancy weeks. You have 40 pregnancy weeks because they count the (approximately) 2 weeks between the first day of your last period and the day you conceived...so you're only pregnant for 38 of those 40 weeks. So if they tell you "You're 7 weeks pregnant" that typically means that you conceived 5 weeks ago.
If you know the date you started your last period, you can add two weeks to that to find your approximate date of conception. You can always ask for your date of conception at the ultrasound or doctor appointments too.
at 7:22 PM on Apr. 28, 2011