My previous blog post noted a missing apostrophe on the front page of Yahoo News. I think I’ve found it. It turned up in a headline on Yahoo Lifestyle:
The apostrophe simply doesn’t belong there. When faced with a similar situation — a length of time preceding an adjective — don’t include the apostrophe. (But if the time period modifies a noun, it gets an apostrophe: one day’s pay, ten years’ experience.
If that’s too grammar-geeky a rule for you, try this: Replace the length of time with the singular: one month pregnant sounds right; one month’s pregnant doesn’t. So, no apostrophe in the plural. Of course this method requires that you have an “ear” for correct English.