Final answer:
Estrogen is the hormone responsible for thickening the endometrium. It is produced by the follicles in the ovaries and stimulates the endometrium after ovulation. Progesterone also contributes to the thickening of the endometrium.
Step-by-step explanation:
The hormone responsible for the thickening of the endometrium is estrogen. Estrogen is produced by the follicles in the ovaries. After ovulation, the level of estrogen increases, which stimulates the endometrium to thicken in preparation for potential implantation of a fertilized egg. Progesterone, another hormone produced by the corpus luteum, also plays a role in maintaining the thickened endometrium.