If what she has is life threatening and she has exhausted all possible options for treating it that does not involve entering the US, and entering the US is possibly the only way to save her life, and you can prove all this to USCIS, you may have a chance at expediting your case.
USCIS usually does not expedite I-129F cases unless the petitioner is a member of the US military and they are getting deployed somewhere or they could at anytime.
It's really difficult to expedite I-129F... pregnancy don't even quality as a reason for expediting the petition and gee, I don't even want to imagine handling pregnancy without a partner looking after me. I feel really bad for people who are/were pregnant during the I-129F process... That is so much pain and discomfort you have to handle all by yourself.