I had a positive pregnancy test and went to do a vaginal ultrasound but its shows clear, my last normal period was august 29. why?

Too Early: It is too early to see a definite gestational sac. Between 5 to 6 weeks it may be possible to see an early gestational sac, an early manifestation of pregnancy. The fetal heartbeat will usually be seen after 6 weeks on a transvaginal ultrasound. Be aware that there is a fairly high loss rate of very early pregnancies so get another sono after the middle of october to confirm viability.

Agree with Dr. S: You may wish to get a blood test for HCG level to confirm pregnancy.