What if the baby is already born ? Sid could have been pregnant 2 years ago when she disappeared, had baby, left him/her with her Momma, volunteered to go back with memory erased in order to protect baby from bad Sloan or whoever.
However, my favorite plot would be the Rambaldi angle, Sid has the scar on her abdomen from where they harvested her ovaries BECAUSE didn't the book say she was the ONE to destroy the world or something. And didn't they say it matched her DNA ? So grab an ovary and clone her. Baby Sid running around, Sid trying to find her and save her. Nuf said.