I had a similar thing happen to me, but it was in the begining of my pregnancy, I wasnt sure about when my last period was but I didnt remember having one in december, but I could remember having one right before thanksgiving... Before I went to the doc I had estimated a due date, so I thought I was 13 weeks... Well the doc was feeling, and pressing and all that good stuff and he told me I was only 6 weeks... I was like how is that possible?!? So I missed a period in december, and then got pregnant, and I had found out I was pregnant before I should have even suspected my period was late? Well then at my next appt. I would have been 10 weeks from what the doctor had said from before, and he asked me if I wanted the triple screening thing done, I said yea so then hes like ok then we need to find out how far you exactly are, so I got an u/s and jumped up 5 weeks... So from the begining to end, I fell back 7 weeks, and then jumped up 5... Who knows, I might jump up another 2 sometime and be right where I originally was. I havent had an explaination for all this... I really just think different babys grow at different times, some are bigger, some are smaller, so I dont think that the doctors are right when they do all the estimating crap... And the size crap with the u/s... They just assume, with the average size and everything. Dont stress, its more than likely nothing...When the doc told me I was only 6 weeks, I was worried that the baby had died and I had a missed m/c... And that the baby was 7 weeks too small cuz I had smoked in the begining before I knew I was pregnant.