If she brings on a period is she subject to the rulings thereon?

Praise be to Allah.

A woman should not make up the prayers if she brings on her period, because when the menstrual blood is present the rulings thereon apply, just as if she takes medicine to prevent menstruation, then she has to pray and fast, and she does not make up the fasts, because she is not menstruating. The ruling is connected to the reason for it. Allaah says (interpretation of the meaning): “They ask you concerning menstruation. Say: that is an Adha (a harmful thing for a husband to have a sexual intercourse with his wife while she is having her menses)” [al-Baqarah 2:222]. When that adha is present, the rulings thereon apply, and when it is not present the rulings thereon do not apply. End quote. 

Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him).