taking basal body temperature on Shabbat

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December 18, 2006

Question

I would like to start keeping track of my basal body temperature, in order to know when I'm ovulating, so I can try and conceive. From what I've read, it's important to take your temperature every day at the same time.

I understand that electronic thermometers with a large scale (within a 1/10th of a degree) are needed in order to accurately measure the temperature.
How do you do this on Shabbat? Is there a way to take your temperature on shabbat? If you can use a mercury thermometer and not an electric one, is this accurate enough? Should I then always use the same mercury thermometer, to make sure the readings aren't affected by the thermometer ?

Thanks for all your help!

Answer

Using a mercury thermometer is permissible on Shabbat (using a digital one is not).

There is a special mercury thermometer as well meant for measuring basal body temperature, which is available in most pharmacies. This is what you should use for Shabbat. As no two thermometers are exactly the same, and accuracy is important, we suggest using both the mercury and digital thermometer for a few days at the same time, in order to see the difference between the thermometers. In this way you can calculate what the number would be on the electric thermometer had you used this on Shabbat, but actually measure with the mercury thermometer.

Furthermore, there are additional signs of ovulation that one can learn to recognize such as the changes in cervical fluid. We suggest that you consult a fertility awareness counselor for training in this. Knowing two signs increases the accuracy of the information. Checking cervical fluid is not a problem on Shabbat.

Behatzlachah!

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