Small gestational sac, slow hcg

Hi all I am 6 wks 3 day preg today. My hcg levels were rising but not doubling previously. I did a scan at wk 5 and doc mention could see gestational sac but were smaller than expected. Another scan at wk 6 saw gestational sac(6mm) + yolk. The gestational sac measures 5wk 2 day old even though it was 6wk. Doc still say my gestational sac is too small. And is pessimistic about the situation saying I have less than 50 percent chance for a viable pregnancy. May I know if anyone has similar situation in the past with a sucessful pregnancy? Or anyone similar situation resulted in miscarriage? #pleasehelp #firstbaby #1stimemom #ivf #ivfquestions #advicepls #ivf #gestationalsac