How far along is the cow?
Do you have access to ultrasound and can you have her palpated if she is close. That is a bad deal. If it is a PHA calf she won't have it on her own more than likely. Even cutting them out you can still loose the cow. if you ahve some time you could always pull blood on the cow and send in for testing. You have 1 in 4 of it being PHA calf.
PS sorry if the cow is Carrier then 1 in 4 chance of dead calf. Two calves carriers and one free.