When doctors say that u can hear a baby's heartbeat through vaginal ultrasound around 6 weeks, is that 6 weeks past last menstraul cycle or conception?

LMP: It is standard to discuss pregnancy dates from the 1st day of last period. In addition, 6 weeks would be about the earliest for heart detection. Many offices won't look until at least 8 weeks, as cycle lengths vary between women and there is mild variability in fetal dating. While this is not a big deal later on, a few days makes a big difference early on!