living embryo but too small gestational sac

Good morning everyone, I have a very important question and I’d like to know if anyone here has gone through the same thing? I had my first ultrasound at 6 weeks and 1 day, and now the gestational sac where the baby is developing seems small. The embryo is on the smaller side, but there’s a good heartbeat. Who, oh who can tell me more about this, and who, oh who has experienced this too? Best wishes, Wendy