Figuring Out the Period When Using Pills

Calculating a period when taking pills?How is the calculation done, is it like for a woman with a fixed cycle, or like for a woman without a fixed cycle? Some women take pills but are not sure if it will come on the specific date. I would appreciate a detailed answer for the benefit of the public. 

 Shalom uVracha,

Quoting from the Hilchos Taharah guide of our teacher, HaGaon Rav Amrom Fried shlit”a:

A woman who takes pills that delay her period while she is taking them, and then after stopping the pills her period comes shortly after:

• If she knows how long after stopping the pills her period usually comes, she should be concerned about that time. A few hours beforehand she should avoid marital relations, though hugging and kissing are still permitted until the bleeding actually starts.
• If she doesn’t know, she should be concerned from two days after stopping the pills, since that’s usually when women get their period after stopping.

Note: Some pills that are meant for use while nursing don’t cause bleeding right away, and sometimes it can take a few weeks. In such a case, there’s no need to be concerned until the period actually comes.