Contact lens maker Cooper Companies just sprung a surprise that no one saw coming.

The market had been looking ahead to the potential sale of its women's health and fertility business CooperSurgical. But the company said late Wednesday it has decided to keep the business following a strategic review.

The stock tumbled 18% in early trading Thursday, by far the biggest faller in the S&P 500.

Cooper's earnings also aren't helping. The company did beat earnings estimates but fell short of revenue expectations in its fiscal third quarter. Its decision to reduce U.S. inventory for its CooperVision contact lens business, as part of its strategic review, weighed on results.

Those inventory impacts will persist into the fourth quarter, it said. Cooper also cut full-year adjusted earnings-per-share guidance to a range of $4.51 to $4.55, down from between $4.58 and $4.66.

"Investors will likely need some time to digest the combination of an incremental reduction to guidance, as well as the conclusion of its strategic review resulting in no transaction," KeyBanc analyst Brett Fishbin said in a note late Wednesday. He has a Sector Weight rating on the stock.

Cooper is the No.2 maker of contact lenses behind Johnson & Johnson, which makes Acuvue.

The company was a Barron's stock pick last month and the potential sale of its surgical unit was part of our argument. The sale would have created a pure-play contact-lens company that could itself become a takeover target.

We can't always see into the future.