Is it possible to hav only gestational sac in 5th week pregnancy ultrasound?

my last period was on 5may 2011 ..i had two hpt test which were positive and then did urine test to confrim it and it was positive too...i went to dr he didnt ask me for u/s just gave me folic acid tab and ask me to com again after 1 month...on 20 june i did vomit almost 9 tymes so next day i went for u/s just to check every thing was fine...but thy saw only ges sac nthing else ..and ask me to come again for another u/s after 21 days...is some one hav gone throug the same situation and had a healthy preg,,,plzzzz do reply im soooo worrieddd .