Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of diversified health-care benefits company Aetna (NYSE: AET) dipped as much as 12% earlier today after the company reported disappointing first-quarter results.

So what: For the quarter, Aetna reported a drop in quarterly net income to $1.34 (excluding one-time items) which is $0.06 worse than Wall Street had expected. Revenue, however, did rise 6% to $8.92 billion as enrollments ticked higher by 0.6%. The company blamed rising medical claim costs and higher administrative expenses as the primary culprit for the earnings miss. Specifically, premium revenue spent on medical costs increased to 81.5% from 79.2% in the previous year.

Now what: What's particularly disturbing about Aetna's miss is that larger peers UnitedHealth Group (NYSE: UNH) and WellPoint (NYSE: WLP) both surpassed Wall Street's forecasts and upped their full-year guidance. Being that this seems isolated just to Aetna isn't good news for Aetna shareholders. Although it kept its full-year guidance unchanged and does expect overall enrollment to grow from 17.92 million to 18.2 million by year's end, Aetna's inability to curb rising medical costs is a worrisome trend that I'd just as soon avoid.

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