America's top insurance companies and health-care investors have prepared for Obamacare for months, and reform's finally here. This pivotal event, though, hasn't stopped the gears of UnitedHealth (UNH 0.23%) and other major insurers from churning along.

Wall Street is watching to see how these companies perform. On Thursday, UnitedHealth gave us the first taste of how the industry has fared over the last few months with the release of its third-quarter earnings. Did UnitedHealth's earnings hide a dark side to this giant insurer's recent run-up? And what should you prepare for as other leading insurers, including Aetna (AET) and Humana (HUM -3.66%), head into the teeth of earnings season?

Keeping up the momentum
UnitedHealth didn't slow down from its recent upbeat pace -- at least, for the most recent quarter. The company's diluted earnings per share edged up by roughly 2% to $1.53 from the year-ago $1.50 mark. That eked out just enough to meet analysts' expectations for the quarter.

The insurer's revenue has been a particular bright spot for investors recently, as UnitedHealth's total sales climbed more than 11% over the first half of 2013. The third quarter continued that torrid pace, as UnitedHealth boosted revenue by 12% year over year to reach $30.62 billion. That missed Wall Street's projections by a hair, but it hardly was a mark for investors to fret over.

With top insurance companies, however, it's all about bringing in new members -- particularly before Obamacare picks up steam. UnitedHealth hasn't had much problem with that in recent years as America's largest publicly traded insurer, and the company boasted of 45.3 million enrollees as of Sept. 30. That's not bad by any measure, but it's only a small increase from the 45 million total medical members that the company showed off as of June 30.

Despite the positive note set by the company's earnings, investors hammered the stock furiously in the wake of the release. Wall Street didn't like the company's results one bit, as UnitedHealth's stock fell by more than 4% at the opening bell. What caused the big drop?

An industry facing change
UnitedHealth's earnings and revenue forecast for the full year both came in under analyst expectations and, even worse, the company expects that cuts to government-run insurance plans, especially Medicare Advantage, could hit its finances in the future.

UnitedHealth is the largest Medicare Advantage provider in the country, so any larger-than-advertised cuts here could leave investors disappointed. It's hardly the only insurance provider at risk of government cuts, however.

Rival Humana, the second-largest insurer in the country, boasted of more than 2 million Medicare Advantage subscribers in its retail segment alone as of June 30. That was a big growth segment for Humana as well, as enrollees jumped more than 7% year over year through 2013's first half. In all, Humana had more than 5.2 million retail Medicare enrollees by the year's midpoint.

Big cuts there could threaten Humana's momentum. The company's retail group posted its fastest-growing premium revenue, jumping 9% year over year through 2013's first six months. While Humana won't likely feel much of a hit from government cuts by the time it reports earnings in early November, they could add up over the long run -- particularly if Obamacare forces a measure of frugality in Washington.

Aetna, the third-largest publicly traded insurer, also could face a hit from Medicare and Medicaid cutbacks. The company gambled on the growth of the Medicare segment when it snapped up rival insurer Coventry for $5.7 billion last year. Through the first six months, Aetna grew its total Medicare premium revenue by a sizzling 52.6%. Investors won't want to see any slump there, as the segment's far outstripped the growth in Aetna's commercial premium revenue.

Looking at the long term
Even with the risks, it's important to keep UnitedHealth's earnings in perspective. The company's in strong position to weather Obamacare's rollout as the largest publicly traded insurer in the country, and its plan of selectively choosing which state insurance exchanges to enter will keep it from counting too much on a successful launch of the reform law.

For large organizations like UnitedHealth, Aetna, and Humana, Obamacare and earnings are another bump in the road. While Aetna and Humana might see a hit this quarter on their earnings if they follow in UnitedHealth's footsteps, these three companies are among the strongest picks in an industry where size matters. They're strong foundations for any health-care portfolio.