I'm taking my OB rotation now as a student nurse, so, according to text books and such:
Heartbeat can sometimes be seen at 6 weeks (age of embryo would be 4 weeks), but it's a long shot.
Problems may be suspected if not seen by 7 and a half-8 weeks.
Keep in mind-at 6 wks, the 4 week old embroy's heart isn't really a heart, but a hollow tube that makes rhythmic contractions....it will later form chambers and become what we know of as a heart.
Personally-i did not get prenatal care that early in my pregnancy, so they saw mine at my 12 week u/s.
With this new pregnancy (I'm about 5 and a half weeks), I'm getting a trans vaginal sono tonight, so we don't expect to see much cept a sack.