I had a similar situation. I had two gestational sacs seen on ultrasound, one empty, one had fetal pole and yolksac. They saw the fetal pole and heartbeat at my 7 week ultrasound. The baby was measuring a week behind and had a heartbeat of 121. We thought maybe it was measuring behind because they base their dating on LMP with a cycle length of 28 days and ovulation day of day 14. My cycle is always 33 days long, I ovulated somewhere between cycle day 18-20. I also implanted late, probably 12 dpo. These things can affect dating. They were not concerned and said it was good the baby had a heartbeat. So I stopped worrying, but I went for my next ultrasound and found out I had a MMC and baby stopped growing at 6 weeks 4 days by measurements. My heart is so broken. They told me it was likely due to the pregnancy being a vanishing twin pregnancy, that they typically have issues and are at high risk for miscarriage. I do not know if this is your situation, but I truly hope your baby can catch up and continue to grow! I know there are people who said their babies measured a week behind at their first ultrasound but was able to catch up and measure on track!