Menstrual Cycle Overview

Introduction

• The menstrual cycle is the process that involves an egg ripening and being released, while the uterus prepares for a possible pregnancy.
• The cycle occurs repeatedly between menarche and menopause.
• Menarche marks the first menstrual period in a woman's life.
• The average age of menarche is 12-13 years.
• Menarche between 8 and 16 years is considered normal.
• Factors such as genetics, diet, and overall health can influence the timing of menarche.
• The average length of menstrual cycle is 28 days, although it can vary from 21 to 35 days.
• Menopause marks the end of the menstrual cycles, with an average age of 52.
• Menopause between 45 and 55 is considered common.
• Menopause before the age of 45 is termed premature.

Endocrine Control of Menstrual Cycle

• The hypothalamus secretes gonadotropin-releasing hormone (GnRH) which stimulates the pituitary gland to release hormones.
• The pituitary gland secretes Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH).
• The ovaries produce estrogen and progesterone.

Hypothalamic-Pituitary-Ovarian Axis

• The hypothalamus regulates the secretion of hormones from the pituitary gland.
• GnRH released from the hypothalamus stimulates the pituitary gland.
• The pituitary gland controls ovarian function through FSH and LH.
• Prolactin is another hormone released by the pituitary gland.

Follicle-stimulating hormone (FSH)

• Secreted towards the end of the previous menstrual cycle, peaking at day 5 and again at day 12.
• Responsible for follicle ripening and maturation, estrogen secretion from follicles, and ovulation alongside LH.

Luteinizing hormone (LH)

• Constant throughout the cycle with a peak around day 12-13.
• Promotes follicle ripening with FSH.
• LH surge triggers ovulation.
• Creates the corpus luteum from the ruptured follicle.
• Stimulates estrogen and progesterone production from the corpus luteum.

Prolactin

• Its release is inhibited by FSH and LH.
• Its exact role in ovulation is unknown but may prevent further ovulation after fertilization.

Physiology of Menstrual Cycle

• The normal menstrual cycle is divided into the ovarian cycle and the uterine cycle.
• The ovarian cycle consists of the follicular phase, ovulation, and luteal phase.
• The uterine cycle consists of menstruation, the proliferative phase, and the secretory phase.

The Ovarian Cycle

• The ovarian cycle represents changes in the ovary through three stages: follicular phase, ovulatory phase, and luteal phase.

Follicular Phase (day 1-13)

• Primordial follicles mature into Graafian follicles under FSH influence.
• Usually, only one follicle matures fully, while others regress.
• The empty follicle becomes the corpus luteum, which eventually atrophies into the corpus albicans.
• The Graafian follicle produces increasing amounts of estrogen, inhibiting FSH secretion.

Ovulatory Phase (day 14)

• This stage releases a mature ovum from the ovarian follicle into the fallopian tube; LH surge starts around day 12 and lasts 48 hours.
• The mature ovum measures 20 mm.
• Fimbriae sweep the ovum into the fallopian tube.
• An unfertilized ovum disintegrates in the fallopian tube after about a day.
• Fertilization takes place in the ampulla of the fallopian tube.
• A fertilized ovum begins embryo development.

Luteal Phase (day 15-28)

• This marks the final stage of the ovarian cycle.
• The corpus luteum produces estrogen and progesterone.
• If fertilization doesn't occur, the corpus luteum degenerates into the corpus albicans, decreasing estrogen and progesterone levels.
• Menstruation begins due to the hormone drop.
• If fertilization happens, the fertilized ovum releases human chorionic gonadotropin (HCG), preventing corpus luteum degeneration.
• The corpus luteum continues to produce estrogen and progesterone, preventing menstruation, indicating pregnancy.

Menstrual Cycle and Feedback Mechanism

• Positive and negative feedback loops regulate hormone levels.
• Low estrogen levels encourage the hypothalamus to release more GnRH (positive feedback).
• Increased GnRH stimulates the pituitary to release more FSH (positive feedback).
• High estrogen levels inhibit GnRH production (negative feedback).

The Uterine Cycle

• The uterine cycle consists of three phases: menstruation, proliferative phase, and secretory phase.

Menstruation

• This marks the first phase of the uterine cycle.
• It represents the shedding of the endometrium, resulting in blood loss, signaling the lack of pregnancy.
• Degeneration of the corpus luteum triggers menstruation.

Menstrual Flow

• Duration is typically 3-5 days (normal range: 2-7 days).
• Average blood loss is 50-80 ml.
• Color: dark red.
• Composition: RBCs, cervical mucus, epithelial cells, and bacteria.

Proliferative Phase

• In this phase, estrogen causes the endometrium to grow and proliferate.
• Maturing follicles release increasing oestradiol levels.
• Estrogen develops a new layer of endometrium in the uterus and stimulates cervical mucus production.

Secretory Phase

• The corpus luteum secretes progesterone during this phase, preparing the endometrium for blastocyst implantation and early pregnancy support.
• Progesterone increases blood flow, uterine secretions, and reduces uterine smooth muscle contractility.