The changes the uterus undergoes to receive the zygote:

- Endometrium thickening: The endometrium, which is the inner lining of the uterus, undergoes thickening in preparation for the implantation of the zygote. This thickening is primarily due to an increase in the number and size of the blood vessels, as well as the accumulation of nutrients and glycogen in the cells of the endometrium.

- Increased glandular activity: The glands in the endometrium become more active and secrete a nutrient-rich fluid, known as uterine milk or glycogen-rich mucus. This fluid provides nourishment to the potential embryo.

- Increased blood supply: The blood supply to the uterus increases, ensuring an adequate oxygen and nutrient supply to support the growth and development of the zygote.

- Softening and increased elasticity: The cervix, which is the lower part of the uterus, undergoes changes to become softer and more elastic. This allows for easier passage of sperm into the uterus during fertilization.

- Changes in cervical mucus: The cervical mucus becomes thinner and more slippery, facilitating the movement of sperm through the cervix and into the uterus.

If the zygote is not formed:

- Menstruation: If fertilization does not occur, the thickened endometrium is shed during menstruation. The blood vessels in the endometrium constrict, leading to a decrease in blood flow and the shedding of the uterine lining. This results in the release of blood and tissue through the vagina.

- Regeneration of endometrium: Following menstruation, the endometrium begins to regenerate and prepare for the next menstrual cycle. The thickening process starts again under the influence of hormonal changes.

- Return to normal cervical mucus: The cervical mucus returns to its usual consistency, becoming thicker and less conducive to sperm transport.

- Normalization of blood supply: The increased blood supply to the uterus decreases, returning to its regular levels.

- Resting phase: The uterus enters a resting phase, where it remains inactive until the next ovulation and potential fertilization.

Overall, the changes that occur in the uterus to receive the zygote and the subsequent changes if fertilization does not occur are part of the natural reproductive cycle of a woman. These changes are regulated by hormonal fluctuations and are crucial for the successful implantation of a fertilized egg or the shedding of the uterine lining during menstruation.