As pregnancy progresses you can make the pregnancy diagnoses seeing several landmarks where you would normally expect to see the pregnancy development. Pregnancy landmarks follow very clear rules and you would expect to identify them as pregnancy progresses.

Below are the typical landmarks and when you would expect to see them:

Typical Timing of Pregnancy Landmarks
     
Landmark Days Past Ovulation Days after Last Period
Blood hCG >25 mIU/cc 10 days 24 days
Positive Urine HPT 12-13 days 26-27 days
10 mm Gestational Sac on Sonogram
Usually hCG above 2,500 mIU/cc
21 days (3w+0d) 35 days (5w+0d)
Yolk Sac/Embryo
Usually hCG above 5,000 mIU/cc
24 days (3w+3d) 38 days (5w+3d)
Crown-Rump Length 4.5 mm 28 days (4w+0d) 42 days (6w+0d)
Fetal heartbeat seen 31 days (4w+3d) 45 days (6w+3d)

Usually the fetal heart beat is clearly seen by transvaginal sonogram by about 6 and 3 days after the first day of the last period if the cycle length was 28 days and ovulation happened around CD 14. That is 4 weeks and 3 days after ovulation/fertilization. In a woman with irregular and slightly longish cycles (say, 35 days verses a usual 28 days), the day of ovulation would actually be postponed to 1 week on top of the 14 days in the usual case, that is, she ovulates on Day 21.