Are you sure it was used to mean “pregnant”? Jonathon Green’s Cassell’s Dictionary of Slang (2e) has as sense 3 under “doughnut” n. “[1980s] of a woman, a state of sexual excitement, supposedly the vaginal equivalent of the penile erection.”
Green’s work is not always reliable, though. If you want to get an idea of how he can go wrong, look under all the entries for “doughnut” and you’ll find “doughnut” v. “[1970s+] (Aus./US teen) to make an automobile spin by pulling on the hand-brake. The vehicle in question is usu. stolen.”
There are at least three things wrong with that. One, there’s no entry for the noun form of the verb, which is far more common in the US. We’d say “do a doughnut/make doughnuts/do doughnuts.”
Two, it’s not always nor, in my experience, most often done with the hand-brake. The definition is overly specific and needs to be more general.
Three, “usu. stolen” is ridiculously overly specific and most likely wrong, too boot. Again, the definition needs to be more general.
I’ll have to ask Green the next time I see him if his entry is merely a memory of his own youthful shenanigans (though he’s British) and not gathered from citation evidence. Joshing between lexicographers makes dictionaries better. :)