Why Not Restylane or Juvederm While Pregnant?

I'm wondering if hyaluronic fillers can harm my unborn child? If so, how? From what I know about Restylane/Juvederm they are very natural and they stay in place in the area where they are injected. I have had treatment with Juvederm 3 times before, and I would really like to have another one on 1 ml while I'm pregnant. I know it's not recommended with any unecessary medications on pregnant women, but how could such fillers hurt my baby?