God made natural signs for determining fertility and infertility.
God Himself made woman in such a way that pre-ovulation estrogen causes a healthy discharge of mucus from the cervix and also causes several physical changes in the cervix. Both the mucus and the physical changes of the cervix can be detected and evaluated by informed women. Every man and woman deserve to know these aspects of God’s creation.
John F. Kippley
Sex and the Marriage Covenant
Natural Family Planning: The Complete Approach