amniotic sac pinhole

Hi all! I went in for my bi-weekly ultrasound yesterday to check up on the baby and the baby is measuring 9.6 weeks, however the technician measured the amniotic sac as 8.5 weeks. The nurse told me to not be concerned and based just on looking at the ultrasound, the baby/sac looks ok, "but she's not an ultrasound tech." Understood. I'm on an antibiotic now, I'm assuming in case there is a pinhole in the amniotic sac, it will deter an infection. I go back next week for an ultrasound to check the progress. I was just wondering if there has been other mom's that have experienced something similar? And maybe what to expect moving forward?