Why is hyaluronic acid filler considered unsafe during pregnancy?

Hello RLF. You are correct. There is no evidence that a Hyaluronic Acid based dermal filler during pregnancy would be a problem. And you are also correct that there is no research on the subject.

The reason that no research has been done is that, generally speaking, most practitioners will limit the medications and procedures performed on pregnant women to only what's necessary for good health. This is primarily because we live in a litigious society where a cosmetic procedure could be blamed for the health of an unborn child. From an insurance and liability perspective, it's just better for us and practices like ours not to have to worry about the potential liability.

The reason why Hyaluronic acids are not advised on pregnant women is because it has not been studied on pregnant women. There may not be a risk, however, because it is unknown, it is safer not to administer on pregnant women.