I agree with chrissymom. At 5 weeks you'll only see a baby and a yoke sac. I didn't see my daughters heartbeat until 6w1d with the first and 6w3d with the second.
the gestational age of the baby is taken from the date of your LMP. However I would point ou that an abdominal scan at 5 weeks wouldn't show a heartbeat, the earliest a heartbeat can be seen is around 6 weeks vaginally and VERY rarely abdominally. For a good picture you generally have to be about 6 to 6 and a half weeks to see the heartbeat abdominally. However it CAN sometimes rarely be seen just before 6 weeks vaginally, but not very often. Hope this was helpful and all the best :)