Can someone explain to me why I would be measuring 7 weeks

desi • mommy to mason ray born july 27th 2017 💙 mommy to an angel baby
I got my first ultrasound today, according to my last period i would be 8 weeks. My doctor said I'm only measuring 7 weeks. I'm so confused & worried something could be wrong. I worry about everything. heartbeat was 155 nomm. someone please ease my mind lol.