Measuring 2 weeks behind, no heartbeat.

Can anyone share their experiences with me if similar? I am supposed to be 9w2d today, I used an opk- and only had sex on the days it was positive and the day after (3 days).

Today at my ultrasound the baby(w/fetal pole) measured 7W but they could not find a heart beat. The doctor assured my husband and I that things still may be fine, right now the fetus measures .64 cm and the cutoff if .7 cm without a heartbeat = failed pregnancy. So there’s still a chance that next week at my follow up US there could be a heartbeat and the pregnancy would continue.

I’ve heard a lot about “ovulating later than you thought” and sleep living for 5 days... but I don’t think those apply to me knowing that I used an opk and only had sex on those days. Please let me know if you’ve ever heard anything like this or experienced it- good or bad.

Side note- has a blighted ovum, just a gestational sac in Sept.