Fall detection, a slightly larger face, and a new processor were all welcome additions to the Apple Watch Series 4. But all of its new features and improvements took a backseat to its potentially game-changing electrocardiogram (ECG) heart monitor.

The upgrade replaced the photoplethysmography (PPG) sensors from previous Apple Watches with a medical-grade detector that can sense irregular heart rates. There’s one problem though, the ECG feature is still unavailable. But this will change very soon.

The upcoming watchOS 5.1.2 update will reportedly bring the feature to all Apple Watch Series 4s in the United States. Apple has released an exclusive beta for ECG to developers and an internal Apple Store document obtained by MacRumors claims it could be ready by the next patch.

Unfortunately, there’s no exact date as to when this will happen. The most recent watchOS 5.1.1 was just rolled out early in November so it’s too early to tell when the subsequent update will ship. However, expect it to land before 2018 is over.

apple watch series 4 heart rate monitor
Have you're Series 4 ping you when you're heart rate gets too high or too low.

Currently, the Series 4 page on Apple’s web store states, “ECG app coming later this year.” The first watchOS 5.1.1 developer beta was released on November 7, so it’s likely after a month or so of testing before Apple will deem the feature ready for the public.

ECG will only be available on the Series 4 and will only ship to devices in the U.S. That’s because Apple only got the feature cleared with the U.S. Food and Drug Administration and not with international health departments.

The initial release will be limited, but it will finally deliver on what is arguably the Series 4’s biggest selling point.