The production of cervical mucus is stimulated by :

Cervical mucus is a glycoprotein gel that expand and contract in response to cyclic changes in the steroid hormone environment across the menstrual cycle to facilitate or inhibit the passage of sperms. Estrogen stimulates cervical mucus production, and as levels rise during the follicular phase, mucus becomes more abundant and watery, less cellular, and more easily penetrated by sperm. Progesterone inhibits cervical mucus production and renders it opaque, viscid, and impenetrable. Ref: Clinical Gynaecologic Endocrinology and Infeility; 8th Edition, Chapter 27