I had a tvscan today, ideally i am at 6w4d acc to lmp(32d cycle) but 6 weeks according to crl:3.9mm.heartbeat was very low at 90b/min.should i worry?

Probably not: Heart rate is very variable that early. Your u/s dates are fine relative to your lmp. All you can do is wait, and worrying is going to make the time pass slower.

Yes: What is tv? Lmp? D? W? Crl? B? Too many abbreviations, too many errors. Avoid all abbreviations! discuss with your doc.

Repeat : At six weeks a fetal heart rate be transvaginal ultrasound is often visible but slow. Over the next week the fetal heart rate begins increasing to a more normal rate of hundred and 110-170. In my practice when i see these low heart rates at 6 weeks i simply repeat the ultrasound 1 to 2 weeks later. The crown rump length measurement you mentioned is normal.