Doing intercourse during menstruation period is haramas it is mentioned in the holy Quran:
وَلاَ تَقْرَبُوهُنَّ حَتَّىَ يَطْهُرْنَ (الآية) [البقرة : 222]
And do not have intimacy with them until they are cleansed.
If by mistake one commits this sin he must do tauba and istighfar. Both of you (husband and wife) seek forgiveness from Allah by offering two rakah salatut tauba and give something in charity and avoid this act in future.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband