Why is it that a woman has menses every month and not every two months or every year for instance?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Allaah predestined menses on women as the Prophet said: "This is a matter which Allaah has predestined on the daughters of Aadam (i.e. women).” [Al-Bukhari] The wisdom of this is explained by Ibn Qudaamah as he said: 'Allaah created menses as a way of becoming pregnant, and if a woman is not suitable for bearing children, then there is no wisdom in her having menses.'

As regards why is it that women have menses every month and not every two months or every year, it is only Allaah Who knows the wisdom of it, and knowing this does not imply any act of worship.

Allaah Knows best.