Periods | Hormones | Fertility on Instagram: "Once pregnancy begins, the female body activates a powerful biological “security system” to protect the growing baby. The very first change happens in the cervix — the gateway between the vagina and uterus. After fertilization, rising progesterone hormone thickens the cervical mucus into a solid, jelly-like mucus plug. This plug completely seals the cervix, acting as a biological lock that blocks sperm, bacteria, and external particles from entering