While health features will be a driving force for the next few years, it's fashion that could actually be the true driving force behind the category's growth. Jitesh Ubrani senior research analyst for IDC Mobile Device Trackers says that "the struggle to move beyond health" towards fashion. He added that "This is where fashion-forward brands have a chance to shine as their customer base don't tend to prioritize features."

Beyond the smartwatch category, IDC sees traditional earphones giving way to smart earwear that feature fitness tracking, audio augmentation, or personal assistants.

Another are of growth could be smart clothing with health and fitness tracking, particularly for professional athletes." Last month Patently Apple covered this market segment in a report titled "Smart Interactive Garments may be closer to Market than we think and Google may have the Jump on Apple."

IDC's chart noted above breaks out the wearables market by known categories. Watches will account for 49.8% of the wearable over the next four years in terms of volume shipments. Clothing will take a giant leap forward in terms of growth by having shipment growing from 2.4 million units to 11.5 million units.

IDC added that "Smart Watches, led primarily by the Apple Watch, are expected to ship 71.5 million units by 2021, up from 31.6 million in 2017.

Other contributing factors for the watch category to grow include the adoption of cellular connectivity, additional SKUs from fashion brands, and the transition of kids watches (a phenomenon largely relegated to China) from basic location tracking watches to more sophisticated watches that allow kids to play games, run apps, and communicate with friends/family.

Next on IDC's radar is the emerging category of sensor-laden Clothing, led mainly by step-counting shoes, though newer products like the Levi's Commuter Jacket (made in partnership with Google) show promise, their high price points and limited use cases make them a tough sell for the broader market over the next few years.

IDC forecasts Earwear wearables to ship 10.6 million units by the end of 2021 with a 58.5% compound annual growth (CAGR) from 2017–2021. "It's important to note that this category excludes traditional Bluetooth headsets. Instead, it is comprised of wireless headphones that offer additional features such as fitness tracking or audio augmentation." With AirPods selling like hot cakes, I think that Apple could probably blow IDC's forecast out of the water by themselves.

IDC considers headsets or head mounted displays, Augmented Reality or Virtual Reality a different wearable class by itself and reported on that category earlier this month.

Our cover graphic and the graphic below covers the upcoming AR headset from Magic Leap called Magic Leap One.

While Magic Leap may be first with AR Glasses, no one will want to wear these outside the home. The new headset shows an attempt to take a different approach to designing a headset, but it's still way too geeky.

That's why Apple's CEO told the UK's Independent that "today I can tell you the technology itself doesn't exist to do that in a quality way. The display technology required, as well as putting enough stuff around your face – there's huge challenges with that."

The Magic Leap One uses an attachable unit called a 'Lightpack' as noted below which acts as the engine for the glasses. This is why Apple's CEO said the technology isn't ready yet. It's just too bulky and too geeky at this point in time.

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