Pregnancy, most likely

I think it really was because she was pregnant. She probably did not see how it would be feasible in an action show to hide her pregnancy. I also think it was because she was pregnant because she obviously agreed to do the subsequent Hunter movies and Tv episodes (including Hunter in China which obviously never got filmed). Thus, it demonstrates that she was still interested in the show Hunter. So, I think she was concerned about the stress of filming while she was very pregnant. Plus, being a first-time mommy, she was probably very nervous about pregnancy and did not know what to expect, and she did not want anything to jeopardize it.