In January 2012 Fitbit expanded beyond wearable activity tracking devices when it launched its Aria WiFi Smart Scale at the Consumer Electronics Show (CES). While it offers much of the same functionality -- if not identical features -- to many of its peers, it is now one of the only connected weight scales that does not have FDA 510(k) clearance as a medical device.

Perhaps the best known WiFi-enabled weight scale is Withings', which for a number of years sold through various retail channels, including Apple Stores, without a 510(k) clearance. Interestingly, the company secured FDA 510(k) clearance for its original weight scale (WBS-01) -- just a few months ago -- in September 2012. Like other 510(k) clearance documents for connected weight scales, the WBS-01 device's clearance is focused on its method for determining body fat percentage and lean mass, bioelectrical impedance analysis or BIA.

As Withings explains in its 510(k): "This method measures body composition by sending a low, safe electrical current through the body. The current passes freely through the fluids contained in muscle tissue, but encounters difficulty/resistance when it passes through fat tissue. This resistance of the fat tissue to the current is termed 'bioelectrical impedance', and is accurately measured by WBS01 Smart Body Scale."

Because of the low electrical current used in BIA, Withings and others also note that their weight scales are not intended for use by pregnant women, children under the age of 18, or people who have implanted heart devices.

Withings' two to three year wait to securing 510(k) clearance is also notable. Others in the connected weight scale space have had clearance since before they began selling their wares. Tanita and iHealth began selling their connected, BIA-equipped weight scales only after going through the FDA process.

Fitbit does warn in its user guide and (thanks to questions just last week) now in its help section that the Aria weight scale should not be used by people with pacemakers and that pregnant women should consult with their doctor first. The user guide further notes that: "Use of this device by people with an electrical implant, such as a heart pacemaker, is not recommended. Please consult with your doctor if you have questions regarding use of this device."

Fitbit has so far refused to comment despite repeated inquiries.