Worried

Gabrielle • Mother of one\Want twins 💁🏻🙏
So I got a call from my doctor today from the results of my ultrasound I had to get due to my stomach measuring small. 
Her heart was beating 116 beats per minute an is measuring in the 9th percentile. I have no idea what this means! They are scheduling another one to get a second opinion. I am so worried, I am 34 weeks today can anyone explain this percentile thing to me? On the ultrasound they said she weighed about 4.5 pounds.