Apple Inc.'s plan to add a highly anticipated blood-pressure monitor to its smartwatch has hit some snags and the technology isn't expected to be ready until 2024 at the earliest, according to people with knowledge of the matter.

The company has teams working on an updated sensor and software for the Apple Watch that would determine if a user has high blood pressure, but accuracy has been a challenge during testing, said the people, who asked not to be identified because the matter is private. The feature has been planned for at least four years, but it's probably two years away from hitting the market and may slip until 2025, they said.

Blood-pressure features may become a key selling point for smartwatches in coming years, but the technology hasn't been easy to master. Though Apple rivals such as Samsung Electronics Co. have launched watches with the capability, they require monthly calibration with a traditional monitor. Last year, Alphabet Inc.-owned Fitbit launched a public study to test wrist-based blood-pressure measurement.

The iPhone maker is running trials of its blood-pressure technology on employees. Its planned approach won't tell users their specific systolic and diastolic readings — the numbers used to assess blood pressure — but would warn those wearing the watch that they may have hypertension, which is high-blood pressure, and should consult a doctor or use a standard blood pressure checker.

The company also has teams working to add noninvasive blood sugar monitoring to the watch, a move that could be a boon to diabetics. That feature is still several years away and hasn't been assigned a target year of release yet. In the interim, the company has discussed adding improved support for third-party glucose meters to the watch and the iPhone's Health app.

While Apple's blood-pressure and glucose features are still far off, the company is working on other upgrades that will launch sooner. That includes new women's health, sleep, fitness and medication management features.

A spokeswoman for Cupertino, California-based Apple declined to comment.

The Apple Watch has become a vital piece of Apple's hardware lineup since it launched in 2015. It's part of the company's wearables, home and accessories division. Sales in the segment increased 25% last year to $38.4 billion — more revenue than what the Mac or iPad generates.

The company is planning to add a body-temperature sensor to the watch as early as this year. The feature, which Bloomberg first reported on last year, would initially be designed to help with fertility planning. Future versions of the watch could expand the feature to determine if a user has a higher than normal body temperature, but — like with blood pressure — is unlikely to show an actual measurement.

Apple is also looking to expand its atrial fibrillation detection feature on its watch to calculate what is known as "burden," or how often a person is in a state of atrial fibrillation across a certain period. That feature could appear in watchOS 9, the next software update that will debut in June and ship with the next watches in the fall.

Read why Apple should break its Watch free from the iPhone

For watchOS 9, Apple also is planning a new low-power mode that is designed to let its smartwatch run some apps and features without using as much battery life. Currently, Apple Watches in low-power mode — known on the device as Power Reserve — can only access the time. The company is also planning to refresh many of its built-in watch faces currently shipping with the device.

An updated version of the Health app on the iPhone this year is expected to add expanded sleep tracking functionality, medicine management and new women's health features. Apple has been working on a medicine management tool that will let users scan their pill bottles into the app. The software will monitor adherence and remind users to take their medication. But the initial version of the feature for this year is unlikely to include all of the planned functionality.

Apple also is planning to add more workout types and additional metrics related to running within the Workout app on the watch.

While Apple is making headway with several health features, some employees haven't been satisfied with the company's progress. They argue that enhancements are taking too long and that the company isn't making big enough bets in the area.

Recently, the company has seen a larger than normal number of departures by physicians who work on health teams. The exits include Charles Wang, a former Johnson & Johnson executive; as well as James Kretlow, Lawrence Huan, Yaniv Kerem and Bronwyn Harris.

The Health team has about 1,000 employees but hasn't grown in at least two years. Separately, a pair of health-focused executives on the company's strategic deals team — Gregg Spivey and Maziar Brumand — left in recent months. Last year, Kevin Lynch, who has run Apple Watch and Health software, stepped back from the team to lead the company's self-driving car efforts.

Apple is planning as many as three new Apple Watches for this year, including a standard Series 8 model, a low-end SE version and an upscale model with a rugged casing that is aimed at extreme athletes.

The company is also working to eventually bring satellite connectivity to the Apple Watch, setting the stage for emergency texting and SOS response features. It's planning to release those capabilities on iPhone as early as this year. The technology would allow users to send text messages to emergency personnel over satellite networks and report incidents.

Mark Gurman reports for Bloomberg News.

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