Hi - from what I understand the jury is still out... it seems that there is really not enough evidence either way. As you can imagine, it is not the type of thing that it is easy to run a clinical trial on as one would not want to cause harm to any child.
My memory on the potential affects of retinoids (oral) tells me that there was a concern about the proper bone development.
Overall, I would say that if you can do without, that is best. If you have prescription redinoid you can likely just call your dermatologist & ask... or, maybe you can just call anyway - it is not like they need to see you to answer if the drug could harm your child through your nursing.