Help, Ultrasound

Ok, so I went in for my first ultrasound today. My last period was on March 8th so I am 7 weeks 3 days pregnant. When I had my US today they didn’t hear a heartbeat and said I’m measuring 5 weeks not 7. They looked worried and said that I’m possibly going through a miscarriage and they want me to have an US next week. My question is, wouldn’t baby be measuring 5 weeks because I conceived 2 weeks after the first day of my last period? Looking for hope I guess.