When the egg is not fertilized, the thickened uterine lining, which was prepared for implantation, breaks down and is shed along with blood and mucus through the vagina. This cyclic process is called menstruation in females.
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When the egg is not fertilized, it cannot develop into an embryo. The unfertilized egg disintegrates within the fallopian tube, and the hormone levels of estrogen and progesterone start to decline. As a result, the thickened uterine lining, which had been prepared to receive a fertilized egg, is no longer maintained. The lining breaks down and is shed from the body through the vagina in the form of menstrual bleeding. This marks the start of a new menstrual cycle.