Shaykh Ibn ‘Uthaymeen رحمه الله was asked about a woman who fasted when she was unsure whether her period was over, then when morning came she saw that it had indeed ended. Does her fast count even though she was not certain that her period had ended?
He replied: Her fast does not count, and she has to make up that day, because the basic principle is that the menses was still there, and her starting to fast when she was not certain that her menses was over means that she started to do an act of worship when she was uncertain as to whether one of the conditions of it being valid was fulfilled or not, and this means that it does not count.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (19/107).
الشيخ ابن عثيمين رحمه الله عن امرأة صامت وهي شاكة في الطهر من الحيض ، فلما أصبحت فإذا هي طاهرة هل ينعقد صومها وهي لم تتيقن الطهر ؟
فأجاب : "صيامها غير منعقد ، ويلزمها قضاء ذلك اليوم ، وذلك لأن الأصل بقاء الحيض ودخولها في الصوم مع عدم تيقن الطهر دخول في العبادة مع الشك في شرط صحتها ، وهذا يمنع انعقادها
"مجموع فتاوى الشيخ ابن عثيمين" (19/107).