Whelp help

I want to use the term "bitch in whelp" to mean a female dog that is pregnant but has not given birth. Is this the correct usage? I cannot find the phrase in my reference books. Thanks.

The Oxford English Dictionary says "in whelp," meaning pregnant, is obsolete, so it won't do unless, for example, you're writing a historical novel and you have a character use the expression.

This is exactly the situation. Thanks.