i went last night for my scan. i have very irregular periods, i know i conceived may 24, but my LMP was may 2. so even though i am 100% sure on conception, he still marked me as 6 w3d, but i am only 5w2d. you can see a heartbeat over 6 weeks, so when the girl looked, she was like "oh, you are just pregnant, like 5 weeks, no heartbeat". now if i wasnt aware that i am irregular, that could have been concerning by looking at my LMP, measuring over a week smaller with no heartbeat. and to top it all off, measuring the gestational sac was 6 weeks, which if i didnt know the date, maybe i would have been concerned with no heartbeat. in these early weeks, even a few days is crucial with measurments.
at 5 w2d all i have is the black gestational sac with a light circle which is the yolk sac. now most 6 1/2w scans you can see something more than just the circle of a yolk sac. do you have a pic? did you see a yolk sac? when did you test in terms of getting your period? was it late or was it before? are you sure your cycle is always exact?
maybe you ovulated later than you thought, even if your periods are 28 days apart, you might ovulate later in your cycle. were you trying? some of those tests can detect pregnancy quite early. my test strips can test as early 7-10 days after ovulation. is it possible you took it earlier than your missed period? that could give you a few days of error. when you are using these "weeks", is it exact? is he saying it measures 3w5d rounding up to 4 w, or is it 4 w3d rounding down. even a few days can make a huge difference.
i dont know if this helps at all, just know that you shouldnt worry just yet. just take it easy and see what happens. and if it does turn out to be worst case scenario, at least you have realized just how much you want another baby so you can focus on that. good luck