Nishmat's Women’s Health and HalachaIn memory of Chaya Mirel bat R' Avraham

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How to calculate onot

13 September, 2004


My cycle usually begins 35-37 days after the last cycle started. How am I supposed to know when my onot are? What days do I do a bedika?

Thank you in advance


Your onot perishah are calculated as outlined in the articles on the site – the same Hebrew date, the interval between the last two periods, and day 30. On each of these days you observe the onah (daytime or night-time) corresponding to the beginning of your last period. Our article How to Keep a Calendar should help you get started.

Keep track for a number of months of your exact interval. If it is always more than 30 days, ask a halachic question about whether you need to continue to keep the Hebrew date and day 30.

On each onat perishah you need to abstain from relations and do at least one bedikah. Please read the articles on the site for a detailed presentation of this topic, and feel free to get back to us with additional questions.

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