الجواب وبالله التوفيق

One should issue divorce during the state of purity of a woman (i.e. not during her menstrual cycle) as it is prohibited to issue divorce during the menstrual cycle.  However if someone issues a divorce during the menstrual period, the divorce would still take effect. Therefore, if a divorce is issued during this time, it will take effect.

The period of ‘iddah would not include the menstrual cycle during which the divorce was issued rather it will be equal to three menstrual cycles after that.

واذا طلق الرجل امراتہ فی حالۃ الحیض وقع الطلاق لان النھی عنہ لمعنی فی غیرہ وھو ماذکرنا فلا ینعدم مشروعیتہ (ھدایہ کتاب الطلاق باب طلاق السنۃ، ص 375، فتاویٰ محمودیہ ص 46، ج18

 واللہ اعلم بالصواب