Help ! Please !

Mia

My Last Period Was 1/21. I Got My BFP 2/27. Making Me 7w and 3d . I Went In For An Ultrasound and They Measured Me 5w. Is This Possible ? Should I Be Worried ? Has Anybody Been Through This and Had A Successful Pregnancy ? I’m TERRIFIED That I’m Gonna Miscarry. Help !