There are several synonyms for the word "amnionic" which is used to describe something related to the amniotic sac. One synonym is "amniotic," which directly references the sac itself. Another synonym is "embryonic," which can be used to describe something related to the early stages of development. "Fetal" is another possible synonym, as it is commonly used to refer to the stage of pregnancy where, thanks to the amniotic sac, the fetus is protected from external conditions. Lastly, "prenatal" is another synonym that is often used to describe the various stages of development that occur before birth.