Next for Apple: How much will iPhone demand drop?

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Investors are optimistic that Apple's earnings report coming after the closing bell Tuesday will beat expectations, but what happens to iPhone demand in the next quarter is the big question.

Apple's stock has enjoyed a very strong run. In the past 12 months, it has surged some 45 percent. And it's coming in red hot for this earnings report: In just the past five days, the stock is up over 6 percent. (It's down slight in premarket trading Tuesday.)

Of course, a lot of the enthusiasm has to do with iPhone demand.

The iPhone still generates over half of the company's revenues and accounts for an estimated two-thirds of its gross profits.

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Analysts at RBC think that Apple shipped nearly 64 million iPhones in the last quarter. That would imply 25 percent year-over-year growth.

But what happens in the months ahead, specifically during the first and second quarters?

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Analysts at RBC think iPhone shipments will slow down hard from 64 million to 50 million units, a 22 percent drop.

"The deceleration is really a reflection of the fact that you get some seasonal patterns that kick into the March quarter," said RBC's Amit Daryanani. "Also, supply starts to get in line with demand. And the initial rush of the iPhone demand is behind you."

If RBC is right, that deceleration in iPhone shipments would be worse than last year, when it dropped 16 percent from the first quarter to the second quarter.

But even if iPhone demand slips in the next few months, several near-term catalysts could move the stock higher, Apple bulls say.

For one, Apple historically updates its capital return program in the spring. Analysts think the tech titan will return more money to shareholders, and the only debate is about how much. Analysts at Credit Suisse think a $200 billion return program is coming soon.

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There's also a lot of interest in the Apple Watch, which is expected to launch in the spring, though the Street remains divided on how much of an immediate hit that wearable will prove among consumers.

In the first 12 months after its launch, Apple will ship 30 million of its new watches, Morgan Stanley says. Piper Jaffray says 10 million.

Of course, after such a strong run, investors need to decide if the stock remains attractively valued, and whether many of these near-term catalysts might already be priced in.

Apple didn't respond to a request for comment.