Fraternal Twins Measuring 7-8 Days Behind

Our twins :)

Hi, I'm due August 4th 2012, normal twin pregancy so far (thank God). I'm 19 weeks 2 days and today at the anatomical ultrasound scan, they said the twins are measuring about 7 and 8 days behind my gestation. I go back in 3 weeks for another scan but I was just wondering if this has happened to anyone else?

Twin A (girl) hb is 167 and Twin B hb is 145. Everything else looks perfect my doctor said--cervical length nice and long and no other issues. I do have a bicornuate uterus (heartshaped uterus) but have delivered two previous children, one in 98--2 months early (preeclampsia) weighing 3.6 lbs (he's 13 and fine now :) and another son born in 2005, at 39 weeks (no preeclampsia, yay) and weighing in at 8.6 lbs. :)

My blood pressure and everything is great and low but I haven't really gained any substantial weight with this pregnancy so far--I was sooo sick in the first trimester and now feel like they are right under my stomach so I get full so quickly. Anyway...any words of comfort/experience would be so so welcome. Thank you!! Melissa