Yolk sac remained same size

Hi all! So, I had a 7.1 week ultrasound and everything looked great! I did a repeat ultrasound at 9.1 weeks and both the crown rump length and gestational sac grew appropriately, the heartbeat was also strong. However, my yolk sac was 3.1mm at 7.1 weeks and measured 3.1mm at 9.1 weeks. The umbilical cord was also visible and looked great. Doctor didn’t say anything about the yolk sac, just said my ultrasound looks great and graduated me from the fertility clinic. I only realized that the yolk sac remained the same size when I read the results at home. Is this normal?