Stock markets marched higher in Wednesday trading, helped by positive comments from Federal Reserve Chairwoman Janet Yellen. Speaking at the Economic Club of New York, the central bank chief said a U.S. economy with full employment and stable pricing is on the horizon. Yellen added that the central bank and many economists are expecting this healthy level of activity by the end of 2016, but we all know nearly three years is a long time frame for such a projection.  

Still, investors took her comments as a buying sign and the Dow Jones Industrial Average (DJINDICES:^DJI) was up 0.85% in late trading.

UnitedHealth Group sits out the rally
UnitedHealth Group (NYSE:UNH)failed to join the rally, instead falling 1.6% by 3:30 p.m. EDT. The driver of the drop was a note from Citigroup analyst Carl McDonald, who downgraded the stock from buy to neutral.  

Insurers are covering more people going to hospitals this year, which is good for earnings.

McDonald thinks there's little opportunity for earnings growth this year and next as the government pinches margins on Medicare Advantage and customers adapt to new health-care requirements.

The challenges for UnitedHealth are easy to see, but there are also opportunities for growth going forward. The insurer didn't work aggressively to get customers via the Obamacare health-insurance exchanges that opened for consumers last year; but there's an opportunity to gain customers there if it chooses. Also, while margin pressure for Medicare customers has been discussed for years, UnitedHealth continues to post strong numbers grow year after year.

We'll get a better picture of what UnitedHealth's management sees for 2014 and beyond tomorrow morning when first-quarter earnings are released. Analysts expect $31.99 billion in revenue and $1.09 per share in earnings; it's worth noting the company exceeded Wall Street's expectations three of four quarters last year. Look for the earnings trend and how Obamacare is impacting results; I don't think it'll be as bad for UnitedHealth as people think.