Q. In formal writing, it is always recommended not to use contractions. But what about the expression “what’s more”?
A. We wouldn’t say always. In writing that is both formal and technical, contractions are still generally discouraged (as you will find, for example, in the latest editions of Scientific Style and Format and the style manual of the American Medical Association). But in nontechnical contexts, any rule against using contractions works against writing that sounds natural and is therefore easy (or at least pleasant) to read. Chicago therefore doesn’t prohibit them. What’s more, the first edition (published in 1906, in the era of spats) included a few (and not only as examples demonstrating how an apostrophe is used). Here’s one: “Don’t stultify yourself and discredit the office by asking foolish questions on the proof” (p. 99). That advice might just as well apply to contractions: “Don’t stultify yourself by avoiding the apostrophe.” As for the phrase “what’s more,” if the apostrophe bothers you (or if it’s forbidden by your style guide), try “furthermore” instead.