Just found out!!

Ok so this was a shock for sure considering there are no twins in either one of our families. So I hate to even say that I know nothing at all about twins except that I'm grateful for two heartbeats. According to my last missed period I'm 8 weeks and 1 day today but when I had my ultrasound done Thursday the babies were measuring different?? Baby A was 7 weeks 3 days and baby b was 7 weeks 1 day??? Is that normal heart rates were 143 and 168. Are they measuring a week behind bc there are two? Has anyone else had this happen