The earlier a dating ultrasound is done, the more accurate it is. If you are 9 weeks pregnant, you conceived 7 weeks ago. Your doctor will explain how this works to your husband if you ask.
Your period is considered the FIRST day of your cycle. Women ovulate somewhere in the middle of their cycle - usually 14 days PRIOR to the FIRST day of their next period. But if you are pregnant, you never get that next period. When you miss your period, you should 'technically' be 2 weeks pregnant, right? No. For dating purposes, doctors use the FIRST day of your last menstrual period (LMP).