6 week US for heartbeat?

Zahra
Hi, 
I am not pregnant, i am new to all of this ttc stuff. I heard an ultrasound is done about 6-8 weeks to see how many sacs, baby's heartbeat etc. 
Is ithe scan done, say, 6 weeks from conception or 6 weeks from last period?
I apprecuate any answers. Thank you ❤️