Fitbit, which makes wearable fitness tracking devices, said margins fell in the second quarter, mainly due to higher spending on new products.

The company also said it does not expect margins to improve for the rest of the year, pushing shares down significantly in after-market trading.

Fitbit said adjusted margins fell to 47% in the second quarter ended 30 June from 52% a year earlier.

"The stress of the US dollar ... accounted for approximately one-third of the decline in gross margin, with the balance due to product mix," chief financial officer Bill Zerella said on a post-earnings call, referring to three new device launches.

These new products accounted for 78% of revenue in the second quarter, Mr Zerella said in an interview.

"We just started shipping them (new products) in Q1 ... as a result, we have not had that much time to get to an optimised cost structure," he said.

Fitbit said it expects non-GAAP margins to stay between 47% to 48% for the current quarter and the full year as it increases advertising and R&D spending.

The declining gross margins overshadowed Fitbit's better-than-expected profit and revenue in its first quarterly report as a public company.

The company's revenue more than tripled in the second quarter due to strong demand for its colourful wireless wrist bands and clippable devices that track heart rate, calories, sleeping patterns and steps.

Sales have been benefiting from an expansion into Asian and European markets, with most international revenue coming in from Britain.

Revenue soared to $400.4m from $113.6m and handily beat analysts' average estimate of $319.4m, according to Thomson Reuters IBES.

"The revenue is outstanding. It very clearly provides a counter argument to anyone who would have thought Apple Watch had any impact," Dougherty & Co analyst Charles Anderson said, referring to Apple's smart watch, which also sports health-related features and apps and is seen as a challenger to Fitbit.

Fitbit, which also competes with Garmin, reported adjusted profit of 21 cents per share in the quarter, above the average analyst estimate of 8 cents.

The company's IPO was priced at $20 per share in June and since then shares have soared, hitting a record high of $51.90 yesterday.