First ultrasound no fetal pole, no heartbeat

According to my last menstrual I am 6 weeks today. I went for my first ultrasound today and all we could see was a sac, no fetal pole, no heartbeat. My doctor said according to the sac measurements I'm approximately 5 weeks and change. Has this ever happened to anyone? Did I miscalculate? Please help! I've had 2 miscarriages in the last, and it's so hard to stay positive!