I see a lot of women confused, angry and upset over the dating of their pregnancy. Just wanna help out a little-
Pregnancy is measured from the first day of your last menstrual cycle. This is called the 'gestational age'. If your cycles are irregular, the doctor will measure the fetal age (from conception, usually by scan) and add 2 weeks. Don't freak out because your doctor says you are 2 weeks further along and you KNOW you conceived such and such date. Because conception is so tricky and even once you've had sex, could be 24-48 hours before it's completely 'sparked', GESTATIONAL dating is the most accurate. Yes, all pregnancies are different and some babies are early and some are late. Some measure small, some large, some bounce ahead and then fall back...
Just don't freak out when the doc says 9 weeks but you know you conceived 7 weeks ago so he/she is wrong. And if your period and or ovulation is irregular, the first scan within the first 4-10 weeks is going to be the best shot of how far along you are. (And yes, the dr will still add 2 weeks to the fetal age so you're measuring along with every other pregnant woman lol) Good luck and healthy pregnancies to you all!!!