"Knocked up" to mean "woken up"

The Google n-grams viewer suggests that the “impregnate” sense became dominant in the US around the 1940’s, but that in British English, other meanings were more common until at least the 1990’s.

This is based on comparing the relative frequencies of knocked her up vs. knocked him up. It seems reasonable to conclude that when the “impregnate” meaning becomes dominant, knocked her up should become much more frequent than knocked him up. The results for this in the “American English” corpus show this shift happening between the ’30s and ’60s, since which time knocked him up has been much less common:

(The predominance of knocked him up in earlier years presumably just due to the preponderance of male characters in general.) As one would expect, the British English corpus shows a very different pattern, with no such notable switch:

It looks like knocked her up may have been becoming predominant in British English in the ’90s, but this is such a small interval of the data that I’m not sure how much significance one can attach to it.

This is a difference between American and British English. In England if you knock someone up you get them out of bed. In America you usually have to get them into bed to knock them up (unless you're both in high school, in which case you will have to tilt the seat back in your parents' car). And by "them" I mean women only, because it's still the case that only women can get pregnant. But you can knock up anybody you want in Britain — how awesome is that?

Knock up is 1660s in sense of "arouse by knocking at the door;" however it is little used in this sense in Amer.Eng., where the phrase means "get a woman pregnant" (1813)....

—Online Etymology Dictionary

So I guess it's a safe bet that, by the early-mid-1800s, knock up lacked the "wake up" meaning in the States. Google Books and the BYU corpora are your friends, though, if you want to look for specific dates of usage.

Update: Another answer, q.v., has examined Google Books, with the result that my guessed safe bet was a bad guess after all.