Menses Resume after Sex: Is that a Sin?

First of all, it should be clear that Allah, Most High, doesn’t hold people accountable for what they do by mistake, with no intention of violating Shari`ah-based rules. In this case, couples are blameless for the period was thought to be over.
Dr. `Abdul-Fattah `Ashoor, Professor of the Exegesis of the Qur’an at Al-Azhar Univ., states:
“As a husband, you are to know that as long as you mistakenly thought that your wife’s period was over, you are blameless for having sex with her. It goes without saying that you have to abstain from having sex with your wife as long as there is a menstrual bleeding.
If bleeding continues for more than 15 days, what is above the 15th day is considered Istihadah, in which the woman has to pray and you can have sex with her. She is also recommended, in this case, to seek medical check-up.”