I’m pregnant and know the day of conception 8/1/04 (this is the only day we had intercourse) and I had been keeping track of my period so I know I was ovulating between 7/30 to 8/1. Assuming I got pregnant on 8/1 I would now be 8 weeks pregnant. I just had an ultrasound that measures the baby at 7 weeks and 2 days. I couldn't be a week off in my calculations. Is my baby measuring small and is there something wrong with it if it is so small?
Keep in mind that the calculations are based on averages of thousands of babies, so your particular baby will show some difference from the absolute center of the bell curve. In ultrsound, if it's within a week of your dates, your dates are kept. If it's off by more than a week, that's when a due date can be changed, assuming you are wrong about your ovulation. So it sounds like you're right on the button here.