What are the phases of the menstrual cycle?

Phases of the Menstrual Cycle

The menstrual cycle consists of two major phases - follicular and luteal - separated by ovulation, with characteristic hormonal fluctuations that affect metabolism, energy utilization, and various physiological functions throughout a woman's reproductive years. 1

Normal Menstrual Cycle Parameters

• The normal menstrual cycle typically lasts between 21-35 days in adult women, with considerable individual variation 1, 2
• Menstruation (menses) typically lasts about 5-6 days 3, 4
• The fertile phase extends from 5 days before to the day of ovulation 3

Follicular Phase

• Begins with the first day of menstrual bleeding 1
• Characterized by rising estrogen levels and low progesterone 1, 5
• FSH rises at the luteal-follicular transition, stimulating follicular growth 3, 5
• Inhibin B is secreted in the early follicular phase 3
• The dominant follicle is selected in the mid-follicular phase 3
• As the dominant follicle grows, it increasingly secretes estradiol and inhibin A for approximately a week before ovulation 3, 5
• Early follicular phase is optimal for muscle glycogen storage when combined with high carbohydrate intake 1
• The follicular phase is more variable in length than the luteal phase, with a mean length of approximately 18.5 days (median 17 days) 4, 6

Ovulation

• Occurs following a sharp increase in luteinizing hormone (LH), estrogen, and FSH levels 1, 5
• Typically happens around days 12-14 in a standard 28-day cycle, but varies with cycle length 1, 2
• For shorter cycles (e.g., 25 days), ovulation occurs earlier 1
• The LH surge causes the initiation of the process of ovulation 5
• Ovulation separates the follicular phase from the luteal phase 7

Luteal Phase

• Begins after ovulation and lasts until the start of the next menstrual period 1
• Characterized by the formation of the corpus luteum, which secretes progesterone, estradiol, and inhibin A 3, 5
• High estrogen and progesterone levels progressively increase until the mid-luteal phase, when their levels start to slowly decline 7
• The corpus luteum reaches its peak in terms of size, secretions, and vascularization 6-7 days after ovulation 3
• High estrogen levels during the luteal phase improve glucose metabolism by increasing glucose availability and glycogen storage in skeletal muscle 7, 1
• Estrogen increases the availability of free fatty acids and enhances oxidative energy metabolism 1
• Increased ventilatory drive occurs during the luteal phase 1
• The luteal phase is more consistent in length than the follicular phase, with a mean length of approximately 11.7 days (median 12 days) 4, 6
• If pregnancy does not occur, luteal regression occurs, leading to menstruation 3, 5

Hormonal Fluctuations

• Follicle-stimulating hormone (FSH), estrogen, progesterone, and luteinizing hormone (LH) show typical fluctuations throughout the cycle 7
• FSH stimulates follicular growth and inhibin B secretion in the early follicular phase 1, 5
• LH stimulates theca cells to produce androstenedione 5
• FSH stimulates the synthesis of aromatase in granulosa cells, which converts androstenedione to estradiol 5
• A critical concentration of estradiol from the dominant follicle causes positive feedback in the hypothalamus, resulting in an increase in GnRH secretion and an LH surge 5
• Progesterone prepares the endometrium for implantation of the conceptus 5

Endometrial Changes

• The endometrium undergoes cyclic changes in response to hormonal fluctuations 5, 8
• Proliferative phase (corresponds to follicular phase): Estradiol stimulates the endometrium to proliferate 5, 8
• Secretory phase (corresponds to luteal phase): Estradiol and progesterone cause the endometrium to become differentiated to a secretory epithelium 5, 8
• Menstrual phase: In the absence of pregnancy, the functional layer of the endometrium is shed 3, 8

Common Variations

• Two-thirds of women show two follicle waves and one-third show three follicle waves per cycle 3
• Women with three follicle waves have longer cycles and a later estradiol rise and LH surge 3
• Within-woman differences between the longest and shortest total cycle length >7 days are found in 43% of women 4
• Within-woman differences between the longest and shortest follicular phase >7 days are found in 41.7% of women 4
• Within-woman differences between the longest and shortest luteal phase >3 days are found in 58.8% of women 4