Investors will be watching a number of metrics when Apple (AAPL 0.16%) reports its fiscal fourth-quarter results later this month. iPhone, services, and wearables revenue for the period will be particularly insightful, giving investors a good pulse on some of the company's most important segments. Of course, investors will also be looking to see if Apple's earnings per share can end its recent downward trend.

While the quarterly results themselves will certainly be interesting, investors should pay special attention to management's revenue guidance for its first quarter of fiscal 2020. Fiscal Q1 is Apple's biggest quarter every year. Not only does the period include the holidays, but it is always the company's first full quarter of new iPhone sales, as the company releases new iPhones a few weeks before the quarter starts.

Apple's guidance for its holiday quarter, therefore, provides the first look at what management expects from the important period.

Six iPhone 11 devices being splashed by water

iPhone 11. Image source: Apple.

Will Apple guide for growth?

As a result of underwhelming sales from iPhone XR and XS and tough year-ago revenue comparisons, Apple's fiscal 2019 top line has seen a headwind. Revenue fell 5% year over year in both fiscal Q1 and Q2. Fortunately, however, Apple returned to growth in fiscal Q3. But growth was only slight, with total revenue rising just 1% year over year.

Going into Apple's fiscal fourth quarter, the high end of management's guidance range suggests the company could report modest growth once again.

While it would be encouraging to see Apple report another quarter of growth in fiscal Q4, investors are likely more interested in the tech giant growing its revenue in its fiscal first quarter. The seasonality of Apple's business means that the period often accounts for around a third of the company's total revenue during the year. And the quarter's impact on profitability is usually even more pronounced.

The number to beat when Apple provides guidance for its first quarter of fiscal 2020 will be $84.3 billion. This was Apple's first-quarter revenue in fiscal 2019.

Most analysts certainly expect Apple to guide for growth in fiscal Q1. The consensus forecast for its fiscal first quarter currently calls for revenue of $87.2 billion, or 3% year-over-year growth.

Understanding Apple's revenue drivers

The big drag on Apple's top line recently has been its iPhone business. In fiscal Q3, iPhone revenue fell 12% year over year. To report consolidated top-line growth in the holiday quarter, the company will likely need a smaller year-over-year decline in this important segment.

But other segments can help pick up the slack if weakness in iPhone persists. Apple's second-largest segment, services, is seeing strong double-digit revenue growth -- and there's no reason this won't continue throughout the holidays. Furthermore, revenue from the wearables, home, and accessories segment is soaring, rising 48% year over year in fiscal Q3.

If Apple's newest iPhones are faring well and if services and wearables, home, and accessories continue on their current trajectories, the tech giant's fiscal first-quarter revenue could not only grow but even accelerate.

Investors will get to see what management expects from its holiday quarter by looking at the company's guidance when it reports its fiscal fourth-quarter results on Wednesday, Oct. 30.