Is this normal??

So last week we had our first appt. Counting from the first day of my last period baby should have been around 7w 2d but he measured at 6w 1d with a heart rate of 116. Dr said i probably ovulated late and heart beat was low but that was normal since it had probably only been beating for acouple of days. Has this happened to anybody? Had a mc last year and im super uneasy... Pls advise!