Confused!!! :((

Laura • Hello. I'm 21 I have a 14 month old little boy and I have just found out me and my partner are expecting our second :)
Hello, from the first date of my last period I would be exactly 7 weeks today, making me 6w4d on Friday, but when I went for the early ultrasound I was measuring 5 weeks :S has anyone else had this?x