Changes occurring in the uterus during the post-menstrual phase are governed by estrogen.

Estrogen and the Menstrual Cycle:
During the menstrual cycle, the levels of hormones in a woman's body fluctuate, leading to various changes in the uterus. The menstrual cycle is divided into several phases, including the post-menstrual phase, which occurs after the completion of menstruation.

Post-Menstrual Phase:
The post-menstrual phase, also known as the proliferative phase, is the phase when the uterus prepares for a possible pregnancy. It begins immediately after menstruation and lasts until ovulation. During this phase, the endometrium, which is the lining of the uterus, starts to thicken and regenerate.

Effects of Estrogen:
Estrogen plays a crucial role in the post-menstrual phase as it is responsible for the following changes in the uterus:

1. Endometrial Regeneration: Estrogen stimulates the growth and proliferation of the endometrium. It promotes the development of new blood vessels and glands in the endometrial lining, preparing it to receive a fertilized egg.

2. Increased Uterine Blood Flow: Estrogen causes vasodilation, which leads to increased blood flow to the uterus. This ensures that the endometrium receives an adequate supply of oxygen and nutrients.

3. Thickening of the Endometrium: Estrogen stimulates the growth and thickening of the endometrial lining. This thickening provides a suitable environment for implantation of a fertilized egg.

4. Production of Cervical Mucus: Estrogen stimulates the cervical glands to produce thin and slippery mucus. This mucus facilitates the entry of sperm into the uterus and provides a favorable environment for their survival.

5. Increased Uterine Contractility: Estrogen also causes mild uterine contractions, which help to propel sperm towards the fallopian tubes where fertilization occurs.

6. Inhibition of FSH: Estrogen exerts negative feedback on the pituitary gland, inhibiting the secretion of follicle-stimulating hormone (FSH). This prevents the development of multiple follicles and ensures that only one dominant follicle matures and releases an egg during ovulation.

By exerting these effects, estrogen prepares the uterus for potential pregnancy during the post-menstrual phase. If fertilization occurs, estrogen levels will continue to rise, leading to further changes in the uterus to support pregnancy. However, if fertilization does not occur, estrogen levels will decrease, leading to the onset of the next menstrual cycle.