Normal Menstrual Cycle Characteristics
A normal menstrual cycle is characterized by a cycle length of 21-35 days, with menstrual bleeding lasting 3-7 days, and is an important indicator of overall reproductive health. 1, 2
Cycle Length and Phases
- Normal cycle length: 21-35 days (average 28-29 days) 1, 3
- Follicular phase: Average 16.9 days (range 10-30 days) 4
- Begins with menstruation
- FSH rises at luteal-follicular transition, stimulating follicular growth
- Dominant follicle is selected in mid-follicular phase
- Increasing estradiol and inhibin A secretion occurs for a week before ovulation
- Ovulatory phase: Occurs when the mature follicle releases an egg
- Fertile window extends from 5 days before to the day of ovulation 3
- Luteal phase: Average 12.4 days (range 7-17 days) 4
- Corpus luteum secretes progesterone, estradiol, and inhibin A
- Peaks in size and function 6-7 days after ovulation 3
Menstrual Flow
- Duration: 3-7 days (average 5.2 days) 5
- Flow pattern: Typically heavier in the first 1-2 days, gradually decreasing
- Volume: 30-80 mL total blood loss per cycle is considered normal
Age-Related Variations
- Cycle length decreases by approximately 0.18 days per year of age from 25 to 45 years 4
- Follicular phase shortens by about 0.19 days per year of age from 25 to 45 years 4
- Adolescents may initially experience longer cycles (21-45 days) following menarche, which typically regularize over time 6
Hormonal Patterns
- Follicular phase: Rising estrogen, low progesterone
- Ovulation: LH surge triggers release of egg
- Luteal phase: Rising progesterone and estrogen, which decline if pregnancy doesn't occur
Common Variations and Considerations
- 46% of women with self-reported "regular" cycles actually have a cycle length variation of 7 days or more 5
- 20% have cycle variations of 14 days or more 5
- BMI impacts cycle regularity: women with BMI >35 have 14% higher cycle length variation compared to women with BMI 18.5-25 4
- Two-thirds of women show two follicular waves per cycle, while one-third show three waves 3
When to Consider Abnormal
- Absence of menarche by age 15 6
- Absence of menses for three consecutive months 6
- Menstrual bleeding lasting 8 days or longer 6
- Cycle length less than 21 days or greater than 35 days 7
- Markedly heavy or light menstrual flow
Impact of Energy Balance
- Low energy availability can disrupt normal menstrual cycles, particularly in female athletes 2, 1
- Disordered eating, excessive exercise, and rapid weight loss can lead to menstrual irregularities 2
- Functional hypothalamic amenorrhea may result from energy deficiency, altering hormone levels and menstrual function 2
Understanding normal menstrual cycle characteristics helps identify potential reproductive health issues early. Significant deviations from these norms warrant further evaluation as they may indicate underlying health concerns affecting morbidity and mortality.