Fitbit (FIT) shares plunged nearly 15% after the fitness and health company said first quarter revenue would fall well short of what Wall Street was expecting.

For the first quarter, Fitbit said it expects revenue to be between $420 million and $440 million, well below the $484.4 million analysts surveyed by Thomson Reuters expected. For the full year, Fitbit said it expects full-year revenue to be between $2.4 billion and $2.5 billion, with gross margins between 48% and 49%.

"We're ramping up production for both the Blaze and the Alta," CFO Bill Zerella said over the phone, noting that while the additional costs are hurting gross margins in the near term, full year revenue guidance was actually above what Wall Street was expecting. "It's a production transition that's more than anything else and since we're doing a global launch, which we've never had the capability to do before, it's driving operating expense levels up a bit."

When asked about the disappointing revenue outlook for the first quarter, Zerella didn't respond directly.

The Alta, Fitbit's latest fitness tracker, is slated to be available in North America in March and globally in April. The Blaze, Fitbit's smartwatch, will be available globally next month.

Full year adjusted EBITDA is expected to be between $400 million and $480 million, and non-GAAP earnings are expected to be between $1.08 and $1.20 a share, throughout 2016.

San Francisco-based Fitbit earned 26 cents a share on $711.6 million in the fourth quarter, up 92% year-over-year as it sold 8.2 million devices in the quarter. Analysts surveyed by Thomson Reuters expected the company to earn 25 cents a share on $647.82 million in revenue.

Shares were falling sharply in after-hours trading, down 16.3% to $13.80, near the company's 52-week low. Fitbit ended the regular session higher, gaining 5.9% to close at $16.52.