My 7 wk ultrasound is only measuring at 4 wks 4 days? Could the baby just be starting out small?

I'm pregnant with my second child and I went for my first ultrasound at 6 wks 6 days and she said the baby was only measuring at 4 wks 4 days. I took a preg test 3 weeks ago tomorrow and that's when I got a positive so I know I'm 7 weeks and 1 day as of now. I do have a 33 days cycle so at the least I might be around 6 wks and a couple of days. With my first daughter her ultrasound showed that she was about 5 days behind but that was more thank likely cause of the cycle length not being normal and by 8 weeks she grew to be right on target and she ended up being 9.6 lbs at birth. I'm having the exact same pregnancy as I did with my first daughter no morning sickness nothing. I'm also not cramping or bleeding. The doctor said that the ultrasound looks good that the sac is up high and not dropping or anything and that it should be ok but they did blood work to check my hcg levels to see where I'm at. Has this happened to anybody else and everything turned out ok. I'm just worried that maybe the baby stopped developing at 4 1/2 wks or maybe it's just taking it's time to have a growth spurt like my first daughter did?