Thank you for reaching out to us.

Ordinarily, we do not permit taking temperature over Shabbat and Yom Tov, because it is a form of measuring. So ideally, you would not use the wristband at all on Yom Tov or Shabbat.

However, if we understand correctly, once you have put on the wristband you make no adjustments to it and do not need to recharge it, you are not looking at information on a display, and you do not plan to look at the information on the app until after Yom Tov and Shabbat. Furthermore,  there is an important health-related need, tracking your fertility as accurately as possible to avoid a potentially dangerous pregnancy.

In this situation, our site's Rabbinic Supervisor, Rav Kenneth Auman, would permit putting the wristband on before Yom Tov begins. However, you should not remove the wristband on Yom Tov or Shabbat, but should wear it continuously throughout the three days, until after Shabbat. If this would present a problem, please get back to us. 

Note that you would not be able to refer to the app for contraceptive information over Yom Tov or Shabbat.

That being said, there is an additional non-halachic concern. The app, by relying solely on temperature readings and an algorithm, is likely not tracking enough fertility signs to be considered reliable. Learn more about the challenges of relying on this type of app here.

Learn more about the multi-pronged Fertility Awareness Method of contraception here.  See our discussion of choosing a contraceptive method here. Given that you have compelling medical reasons to avoid pregnancy, we recommend discussing with your healthcare provider what methods would be considered safe and effective for you.

Please let us know if we can be of further assistance.

Chag kasher vesameach!