Most accurate dating pregnancy ultrasound
Remember as soon as we see an embryo we can measure a heartbeat, normal ranges are from 115 to 160 beats per minute, varies thought out the pregnancy, it increases at around 10 weeks to 150 to 175 beats per minute and decreases again approaching to term to 130 beats per minute.
Most hospitals or ultrasound clinics ask you to arrive with a full bladder.
Pregnancy weeks are counted from the first day of your last menstrual period (LMP).
But that won't tell you when you actually conceived.
In the previous post I explained more in deep about HCG levels in the first trimester and when are you expected to see an intrauterine pregnancy, this topic of HCG is also important on the process of dating your pregnancy and give you an accurate due date.
So assuming that on your first scan only the Gestational SAC was seen which is normal at a 5 week ultrasound, then the gestational sac dimensions are taken and that will give us an idea of how far you are.