Which hormone is responsible for the thickening of the lining of the uterus each month?

The hormone responsible for the thickening of the lining of the uterus each month is estrogen. Here are the steps of how it works:

1. At the beginning of the menstrual cycle, the levels of estrogen start to increase.
2. Estrogen is produced by the follicles in the ovaries.
3. As the levels of estrogen rise, it stimulates the growth and thickening of the endometrium, which is the lining of the uterus.
4. This thickening prepares the uterus for potential implantation of a fertilized egg.
5. If fertilization does not occur, the levels of estrogen decrease, leading to the shedding of the endometrium in what is known as menstruation.