The verb "be" is tricky and troublesome. As it sometimes could be changed to a "present participle" when it means "the state of being X" is temporary.
He is being rude.
You want to express that he behaves rudely at this moment while you know he doesn't behave that way very often. It conveys a different meaning from "he is rude."
"She is being in labor" is quite different. Women have a range of time that they take to give birth to a baby. It depends, however, it doesn't take as long as "a week" for example (of course, in an extreme case, it might take longer).
Therefore, it is not necessary to change "be" to "being" when you want to express "someone is about to give birth to a baby".
This applies to other phrases using a preposition after the verb "be".
He is at school.
We know he will be back from school when school is finished. In the same way, we know the labor will be finished after a certain amount of time.
The below Ngram Viewer shows no usage for "is being in labor/was being in labor"