Hi, I Had An Ultrasound At 7 Weeks + 1

Hi, I had an ultrasound at 7 weeks + 1 day. It revealed a live intrauterine pregnancy, with a yolk sac and fetal pole measuring perfectly and a heartbeat of 136. It also said "amnion collapsing around fetal pole". I had a second ultrasound at 9 weeks + 2 days, and everything looked fine. Can you explain what could have caused a brief collapse? Or if there is anything to worry about moving forward with this pregnancy?