We're now just two days away from Google's (Nasdaq: GOOG) second-quarter report.

The world's leading search engine certainly has a lot of things working in its favor. Despite a few territorial misses -- losing out to Yandex (Nasdaq: YNDX) in Russia and Baidu (Nasdaq: BIDU) in China -- Big G is the undisputed global champ.

The high-margin nature of paid search, and the growing number of Internet users worldwide, have played out well for Google in the past. The company kept growing its top line even during the darkest recessionary stretches. Shareholders have also made out nicely; the stock has risen sixfold since going public seven summers ago.

However, there are a few reasons to be concerned heading into Thursday afternoon's report. Let's go over three things in particular that should trouble investors.

1. The trend isn't Google's friend
Google has beaten Wall Street's profit expectations in nine of its past 11 quarters. Unfortunately, the two misses have come over the past year, including its most recent first-quarter whiff.

There's another trend working against the dot-com giant. Forward estimates -- which historically inch higher for perpetual market thumpers -- are going backward.

Prediction

2011 Estimates

2012 Estimates

Current $33.86 $39.31
7 Days Ago $33.83 $39.33
30 Days Ago $33.93 $39.53
60 Days Ago $33.90 $39.44
90 Days Ago $34.61 $40.09

Source: Yahoo! Finance.

Analysts are expecting Google to earn 2% less this year and next year than they did three months ago. This may not seem like a big deal, but these figures used to inch higher week after week during the company's early years as a public company.

2. Pessimism is growing
Morgan Stanley downgraded shares of Google on Friday.

Who does that? Who would talk down a company less than a week before a critical quarterly report? Analysts do some pretty stupid things, but only those with the strongest convictions usually do something as brazen as this.

Morgan Stanley's Scott Devitt also slashed his profit forecast on the company. He now sees Google earning $31.44 a share this year and $34.43 a share come 2012. That falls well short of the wider market's diminishing estimates, not to mention the $32.73 and $37.58 in per-share profitability, respectively, that Devitt previously predicted.

He's concerned that the company's margins are in for a bruising, given corporate spending run amok. Ambitious hiring and new product launches will definitely take a big bite out of earnings. Is it any wonder the pros are treating Google estimates like a limbo stick?

3. Good news may be bad news on the bottom line
There's no shortage of buzz for Google+ as a potential Facebook killer. Android continues to widen its lead over Apple's (Nasdaq: AAPL) iOS as the smartphone operating system of choice. Google Offers is no Groupon, but it's hard to underestimate Big G's Rolodex.

Think about the companies leading these industries. We don't have Facebook's financials, though China's leading social-networking site -- Renren (NYSE: RENN) -- is losing money. Groupon is gushing red ink. Android is being given away, so it will never be the profit beast that Apple harvests with its proprietary platform.

There are also some likely misses in Google's product bag of introductions over the past year.

The Chromebook will have an uphill battle in its quest for relevance. Despite big-name hardware partners like Sony (NYSE: SNE) and Logitech (Nasdaq: LOGI), Google TV hasn't moved the needle.

All of these winners and potential losers must be costing Google a lot of money now, even though they may never make Google a whole lot of money later.

Google's best play was its first play -- paid search. Everything else will likely eat at its once-lofty margins. Why else would analysts target revenue growth of 26% this year and 21% next year, yet hold out for profit growth in the midteens both years?

Google will remain a great company for a long time, but its bottom line will face challenges in the coming quarters -- and those challenges start Thursday.

Would you buy or sell Google ahead of its quarterly report? Share your thoughts in the comment box below.

The Motley Fool owns shares of Apple, Google, and Logitech International SA. Motley Fool newsletter services have recommended buying shares of Google, Apple, Baidu, and Logitech International SA. Motley Fool newsletter services have recommended creating a bull call spread position in Apple. Motley Fool newsletter services have recommended creating a write covered call position in Logitech International SA. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.

Longtime Fool contributor Rick Munarriz still uses Google a lot in his daily life. He does not own shares in any of the companies in this story. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool has a disclosure policy.