Small Sac

Hi I’m 8 weeks and 2 days pregnant and at 7 weeks and 6 days I finally got to see the baby and heartbeat. Babies heartbeat was 145bpm and size was good but only thing a little concerning to the dr was my sac being 11 days behind. Has anyone else had this issue and everything was fine? Should I be worried? The Dr. on call that saw me mentioned could cause possible miscarriage if it doesn’t grow. Clearly that doesn’t help with the stress right now. She told me to drink lots of fluids but I’ve been doing exactly that for the last two months at least 2 litres or a little more each day. This is my first pregnancy after many years of trying to conceive. Maybe hearing some positive stories from any of you who has experienced this would help me get through the week. My next ultrasound is this Friday.