The earliest time that a sac can be seen is at 4 weeks 3 days, an embryo at 4 w 5 days. The heart activity may be visualized from 4 weeks and 5 days pregnancy in a large proportion of pregnancy but in all pregnancies at 6 weeks from the last period.
At the Fertility Academy we recommend 1st ultrasound at 6 weeks when heart activity is expected to be seen.