What are the stages of the menstrual cycle and its effects on the female body?

Stages of the Menstrual Cycle and Its Effects on the Female Body

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, 2

Menstrual Cycle Phases and Hormonal Changes

Follicular Phase

• Begins with menstruation and lasts until ovulation, typically ranging from days 1-14 in a 28-day cycle 2
• Characterized by rising estrogen levels and low progesterone 1
• Follicle-stimulating hormone (FSH) rises at the luteal-follicular transition, stimulating follicular growth 2
• A dominant follicle is selected in the mid-follicular phase, increasingly secreting estradiol and inhibin A for approximately one week before ovulation 2
• Early follicular phase is optimal for muscle glycogen storage when combined with high carbohydrate intake 1

Ovulation

• Occurs following a sharp increase in luteinizing hormone (LH), estrogen, and FSH levels 1
• Typically happens around days 12-14 in a standard 28-day cycle, but varies with cycle length 3
• For shorter cycles (25 days), ovulation occurs earlier, with fertile periods typically between days 6-11 3
• Represents the most fertile period, with fertility extending from 5 days before to the day of ovulation 2

Luteal Phase

• Spans from ovulation to the beginning of the next menstrual period (typically days 15-28 in a 28-day cycle) 1, 2
• Characterized by rising progesterone and estrogen levels that peak in the mid-luteal phase 1
• The corpus luteum reaches its peak in size, secretions, and vascularization 6-7 days after ovulation 2
• Luteal regression occurs passively if fertilization doesn't take place, leading to menstruation 2

Physiological Effects Throughout the Cycle

Metabolic Effects

• During the luteal phase, high estrogen levels improve glucose metabolism by increasing glucose availability and glycogen storage in skeletal muscle 1
• Estrogen increases the availability of free fatty acids and enhances oxidative energy metabolism 1
• Women have higher rates of lipid oxidation and lower rates of carbohydrate and protein metabolism during exercise compared to men, particularly during the luteal phase 1
• Protein requirements may be higher during the luteal phase due to increased progesterone levels causing higher rates of protein catabolism 1

Respiratory and Cardiovascular Effects

• Increased ventilatory drive occurs during the luteal phase 1
• Estrogen has complex effects on the respiratory and cardiovascular systems 1
• Despite these changes, menstrual phase generally does not significantly influence exercise performance at low altitudes 1

Temperature Regulation

• Basal body temperature rises after ovulation and remains elevated during the luteal phase, serving as a reliable clinical indicator of ovulation 4

Endometrial Changes

• The functional layer of the endometrium undergoes hormone-dependent proliferation during the follicular phase 2
• Differentiation occurs during the luteal phase in preparation for potential implantation 2
• In the absence of fertilization, menstruation is initiated by progesterone-responsive decidual cells and executed by prostaglandins, vasoconstriction, and matrix metalloprotease secretion by leukocytes 2

Common Variations and Considerations

• Normal menstrual cycles typically range from 23-38 days in length 1
• Menstrual bleeding typically lasts 3-7 days 5
• Two-thirds of women show two follicular waves per cycle, while one-third show three waves 2
• Women with three-wave patterns tend to have longer cycles and later estrogen rise and LH surge 2
• Cycle length and fertility decline with age 2
• Menopause typically begins around age 51, characterized by gradual loss of estrogen and absence of the menstrual cycle 1
• Menopausal changes affect glucose and fatty acid metabolism, impacting multiple organ systems including adipose tissue, bones, gut microbiome, and skeletal muscles 1

Clinical Significance

• Tracking menstrual cycles can help predict ovulation for conception or contraception purposes 6
• The correlation between cycle length and follicular phase length is stronger than between cycle length and luteal phase length 6
• Absence of menarche by age 15, absence of menses for three consecutive months, or menses lasting eight days or longer warrant further evaluation 5
• The menstrual cycle should be considered a "vital sign" as it provides important information about overall health 5