7 weeks and 3 days measuring 6 weeks

This is my fifth pregnancy and I just turned 40. The last two pregnancies were miscarriages at 5 weeks... the last healthy pregnancy was in 2010. I went for my 7 week appt and ultrasound yesterday but baby only measured 6 weeks. The tech thought she detected a heartbeat but it’s lower than they want right now. I go back on Monday for another ultrasound to see if baby is growing.

Obviously, I’m freaking out. The dr says it’s possible that I just ovulated later than I thought and things are on track but that seems so strange considering my lmp was Dec. 10 and I tested positive for pregnancy on Jan. 1. So, has anyone else experienced this with baby measuring smaller than should be but still gone on to have a healthy, normal pregnancy and baby? I’m driving myself crazy with worry and I know I should relax but can’t. The doctor did say that if it would give me peace of mind; I could come in for the ultrasound on Friday instead of Monday. Any thoughts, positive stories and good vibes are appreciated.