An ultrasound done during the first 12 weeks is the best way to estimate your due date. At that point, unless there is a problem, all babies grow at the same rate, which is why it's the best judge of your baby's age. Later on in pregnancy, a number of different things can cause variations in a baby's size, so it's not really possible to judge age by ultrasound. When I was pregnant with my second baby, I had a threatened miscarriage at what we thought was 10 weeks. But, after the ultrasound, we found out that I was only 7 weeks. Using your period/menstrual cycle as a guide isn't always the best way to detemine how far along you are. Crazy things can happen. Like for me, I ovuated about a week later than normal during my last cycle, and when my expected period was a couple of days late, we got surprised.So, go with the new due date. I know how frustrating it can be to find out you're not as far along as you thought/hoped. But really, it's better to find out now than to worry that there is something wrong later when your baby isn't as big as it should be, or to have to be induced because you pass your due date.