You may have simply conceived earlier or later than you thought you did (which can happen if your cycle is at all irregular or if you remembered your last menstrual period date incorrectly).
You may have calculated the length of your pregnancy from the first day of your last menstrual period (LMP).
Don't worry: This type of exam is painless and risk-free, and seeing your baby for the first time will likely be an enjoyable experience.
Naturally, one of the most common questions asked about ultrasound accuracy is: Just how accurate are the due dates that are predicted by an ultrasound?
So, my question is, when I went to have my ultrasound, the sonographer did 2 measurements for the conception date: one was 6 weeks and 3 days, and the other one (just to be sure) was 6 weeks 5 days.
So, we split the difference and said 6 weeks 4 days.