It sounds like you think that when a doctor says you are 12 weeks gestational age (GA) or just 12 weeks "pregnant," he or she means you are 12 weeks from conception. However, the doctor is counting from the first day of your last period. If you had said to the doc, "Do you mean I conceived 12 weeks ago?" on the day he or she said that, the doctor would have said, "No, you probably conceived around 10 weeks ago."
The reason doctors count all the way back to the first day of the last period, when they know you weren't pregnant then, is that this is something that can be seen. A period is noticeable, ovulation is hidden and not happening in a noticeable way. They calculate 40 weeks for the pregnancy time period, when it really takes only 38 weeks from conception to birth. The two weeks tacked onto the front end of the pregnancy count is to calibrate everything to the day the woman began bleeding with her last period she had before getting pregnant. All pregnancies are counted that way, even in this day and age of ultrasounds.
Next time you are at the doctor's, ask for your due date based on an ultrasound (if you have had one yet) and then look it up on an online conception calculator, or ask when you would have conceived, using that exact word. Don't just take home a number of weeks "along" and think you know what the doctor meant.
Not sure I made that post very clear, dr says I'm 12 weeks to the day I started my period the 14th normal day and length but I had sex earlier that day. I thought I became pregnant a week later when current bf and I were together but now I'm not sure