For them as don't know when workers had to get up in the middle of night a bloke with a long pole was employed to go round and knock on their bedroom window in time for them to work the early shift. This bloke was, of course, known as the 'knocker-up'. It may well be that this meaning pre-dates the pregnancy meaning, at least in the UK.

Knock up also refers to something hastily or easily assembled. You can knock up a meal from a few random ingredients, for instance.

These phrasal verbs (a verb with a preposition or two attached) cause great difficulties for people learing English, and also for English speakers from different cultures, like the difference between pissed off and pissed up.