Endometrium
The endometrium is a key component of the uterus and undergoes cyclical changes during the menstrual cycle. It is the innermost layer of the uterus and plays a crucial role in pregnancy and menstruation.

Menstrual Cycle
The menstrual cycle is a series of events that occur in the female reproductive system, preparing the body for pregnancy. It is typically a 28-day cycle but can vary from person to person. The cycle is divided into four phases: menstruation, follicular phase, ovulation, and luteal phase.

Menstruation
Menstruation marks the beginning of the menstrual cycle. During this phase, the endometrium sheds off as menstrual blood through the vagina. The shedding of the endometrium occurs when pregnancy does not occur. The menstrual flow usually lasts for 3-7 days.

Follicular Phase
The follicular phase begins after menstruation and lasts for about 10-14 days. During this phase, several follicles in the ovaries start to develop and mature. Each follicle contains an immature egg. As the follicles grow, they produce estrogen, which stimulates the thickening of the endometrium.

Ovulation
Ovulation occurs around the middle of the menstrual cycle, usually on day 14. One dominant follicle releases a mature egg into the fallopian tube. Ovulation is triggered by a surge in luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

Luteal Phase
After ovulation, the ruptured follicle transforms into a structure called the corpus luteum. The corpus luteum secretes progesterone, which prepares the endometrium for possible implantation of a fertilized egg. If fertilization does not occur, the corpus luteum breaks down, and progesterone levels decrease. This leads to the shedding of the endometrium and the start of a new menstrual cycle.

Role of Endometrium
The endometrium plays a vital role in the menstrual cycle. It thickens and prepares itself for possible implantation of a fertilized egg. If fertilization does not occur, the endometrium sheds off during menstruation. This cyclic process allows the uterus to be ready for potential pregnancy every month.

The other options mentioned in the question are not correct. The perimetrium is the outer layer of the uterus, the myometrium is the middle layer responsible for uterine contractions during labor, and the peritoneum is the lining of the abdominal cavity. None of these undergo cyclical changes like the endometrium does during the menstrual cycle.