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.