Because a pregnancy is counted from the first day of the last menstrual period, but the actual conception doesn't take place til about the 14th day after the start of the last period. So while you're physically 5 weeks pregnant, the medical field considers you 7 weeks pregnant. It's for accuracy of dating.
Determining pregnancy start dates has a bit of guess work involved. At seven weeks I wouldn't be overly concerned unless you are a heavy smoker/drinker, which could easily show as slowing the growth process of the baby. Talk to your doctor!