What do you mean she has no current Visa?
US Citizens are normally granted a 6 month visitors Visa at the port of entry.
I am assuming that she had a 6 month visitors Visa.
If this is the case, she may not be able to fly in any event as she would not be permitted to do so in the last trimester.
I would suggest that you first obtain a medical report and then apply for an extension of the visitors Visa to allow the birth to take place.
Once the child is born, she can apply for access to a child. Further details can be found at the following link:
I hope this answers your question. If so, kindly rate the answer and provide feedback.
If you have any further questions, please do not hesitate to ask.