I was pregnant with twins I lost one...im 6 weeks 1 day
One could just be a little smaller than the other and I'm only talking about the sacs they'll measure the babys when you're further along don't stress about it, it makes you think about things Much worse than it probably is I hope I helped a little, although I'm not pregnant with twins, I am a twin myself and my mom told me my sac was smaller than my brothers and he was smaller than I was when we were born and I was born first :) Goodluck sweetie. Think positive!! :)