The following theory assumes that
- Beth was actually living in Pennsylvania or a nearby state when she was killed
- The isotope tests are somewhat accurate
Beth's baby gestated when she was in the southeast. When Beth moved from the southeast to the northeast at some point between February 1976 and her death, she might not have taken her baby's father with her. If she left her original place of residence early on in her pregnancy, she might not have even been aware that she was pregnant at the time, and the father of her baby might not even know that her child existed.
I'm thinking that what could have happened is Beth, after realizing she was pregnant and alone in a completely foreign part of the country (a country to which she might not be native in general), met somebody who ostensibly sympathized with her. This person could have offered to help her, perhaps going so far as to offer help with raising Beth's unborn child, but in the end, the relationship took a very dramatic turn for the worse (perhaps he turned out to be abusive), and the rest is history.
I'm not sure, though, that Beth's killer was abusive or controlling towards her as much as I think they were scornful or disdainful. It seems to me that whoever did this had a deep-seated hatred and resentment towards Beth, and they viewed her as a nuisance or the source of everything wrong in their life. Taking that into consideration, the fact that she was 9 months pregnant doesn't seem like a coincidence to me. I would not rule multiple killers or even female involvement in this case - yes, we know Beth suffered genital trauma and that she was raped, but I'm not sure she was raped necessarily in the "conventional sense". The chopping off of her hair, mutilation of her face, and removal of her breasts might be an attempt on the part of her killer to make her "ugly" or undesirable.
Sorry for rambling.