Criminal Sentence 306: Not Expecting

From a book I'm reading:

"four months' pregnant"

That's clever (I mean, incorrect) apostrophe usage. It might at first seem to be similar to "four weeks' worth," which does require an apostrophe. However, if you put each of these in the singular, it's easy to tell where an apostrophe is warranted:

one month pregnant
one week's worth

You wouldn't write one month's pregnant, nor would you write one week worth.