Gestational sac measuring smaller

I did a frozen embryo transfer of a day 5 blastocyst and during my first scan at week 6 day 4, my gestational sac measured smaller at week 5 day 3. There was also no yolk sac and heartbeat visible. My understanding is that the dates would be very accurate for ivf, and measuring behind on my scan made me very worried. Does this mean the pregnancy is not viable? Any positive stories of similar situation that anyone could share?