Normal Menstrual Cycle

Normal Menstrual Cycle
Understanding the Physiology and Regulation of Reproductive Function
Professor Mykhailo Medvediev
Evidence-Based Medicine
Overview of the Menstrual Cycle
The normal menstrual cycle is a tightly coordinated process involving stimulatory and inhibitory effects that result in the release of a single mature oocyte from hundreds of thousands of primordial oocytes. This complex process involves hormones, paracrine, and autocrine factors working in harmony.
The cycle represents a sophisticated interplay between the hypothalamus, pituitary gland, and ovaries, creating a feedback system that ensures reproductive capability.
Source: Welt CK. Normal menstrual cycle. UpToDate 2025. Available at: uptodate.com
Defining Normal Menstruation
Normal menstruation is characterized by four key parameters that help clinicians assess reproductive health and identify potential abnormalities.
Frequency
Menstrual bleeding occurs every 24 to 38 days in normal cycles
Regularity
Cycle variation of ≤7-9 days between shortest and longest cycles
Duration
Bleeding lasts up to 8 days per menstrual period
Volume
≤80 mL blood loss per cycle; should not interfere with quality of life
Source: Munro MG, Critchley HOD, Fraser IS. FIGO Menstrual Disorders Committee. Int J Gynaecol Obstet 2018;143:393
Cycle Regularity by Age
The definition of normal cycle regularity varies with age, reflecting the physiological changes in reproductive function across a woman's lifespan.
1
Ages 18-25
Cycle variation ≤9 days is considered normal
2
Ages 26-41
Cycle variation ≤7 days is considered normal
3
Ages 42-45
Cycle variation ≤9 days is considered normal
For patients under 18 and over 45 years, infrequent or unpredictable ovulation is more common, making the definition of normal regularity more challenging in these populations.
Source: Harlow SD, Lin X, Ho MJ. Analysis of menstrual diary data. J Clin Epidemiol 2000;53:722
Three Phases of the Menstrual Cycle
The menstrual cycle is divided into three distinct phases, each characterized by specific hormonal and physiological changes.
Follicular Phase
Begins with menses onset, ends day before ovulation. Follicle grows and produces estradiol
Midcycle Surge
LH surge triggers ovulation. Oocyte released approximately 36 hours after surge begins
Luteal Phase
Begins after ovulation, ends at next menses. Corpus luteum secretes progesterone
Source: Welt CK. Normal menstrual cycle. UpToDate 2025. Available at: uptodate.com
Hormonal Regulation Overview
The menstrual cycle is orchestrated by a complex interplay of hormones from the hypothalamus, pituitary gland, and ovaries, creating a sophisticated feedback system.
Key Hormones
GnRH (Gonadotropin-Releasing Hormone)
FSH (Follicle-Stimulating Hormone)
LH (Luteinizing Hormone)
Estradiol
Progesterone
Inhibin A and B
Regulatory Mechanisms
Negative feedback loops
Positive feedback at midcycle
Pulsatile hormone secretion
Paracrine and autocrine factors
Source: Hall JE, Schoenfeld DA, Martin KA, Crowley WF Jr. J Clin Endocrinol Metab 1992;74:600
Early Follicular Phase
Phase 1
The early follicular phase represents the time when the ovary is least hormonally active, with low serum estradiol and progesterone concentrations. This creates the hormonal environment necessary for follicle recruitment.
Hormonal Changes
FSH increases approximately 30% due to release from negative feedback. LH pulse frequency increases to one pulse every 90 minutes.
Ovarian Activity
Recruitment of follicle cohort begins. Small follicles of 3-8 mm diameter become visible on ultrasound.
Unique Feature
LH pulses slow or cease during sleep—a phenomenon unique to this phase of the cycle.
Source: Hall JE, Schoenfeld DA, Martin KA, Crowley WF Jr. J Clin Endocrinol Metab 1992;74:600
FSH and Follicle Recruitment
The modest increase in FSH secretion during the early follicular phase is critical for recruiting the next cohort of developing follicles. This represents a key regulatory checkpoint in the menstrual cycle.
FSH Role in Recruitment
The approximately 30% rise in FSH stimulates growth of multiple follicles from the recruitable pool. One of these will eventually become the dominant follicle that ovulates during that cycle.
Serum inhibin B, secreted by small follicles, reaches maximum levels and helps suppress the FSH rise through negative feedback, preventing excessive follicle recruitment.
Source: Gougeon A. Dynamics of follicular growth in the human. Hum Reprod 1986;1:81
Mid-Follicular Phase
Phase 2
During the mid-follicular phase, several follicles grow to the antral stage, with their granulosa cells hypertrophying and dividing to produce increasing amounts of estradiol and inhibin A.
Estradiol Rise
Multiple growing follicles produce increasing estradiol, which feeds back negatively on the hypothalamus and pituitary
Gonadotropin Suppression
Rising estradiol suppresses FSH and LH concentrations and reduces LH pulse amplitude
GnRH Pulse Frequency
Pulse generator speeds slightly to approximately one pulse per hour
Within approximately seven days from menses onset, several 9-10 mm antral follicles become visible on ovarian ultrasonography.
Source: Filicori M, Santoro N, Merriam GR, Crowley WF Jr. J Clin Endocrinol Metab 1986;62:1136
Endometrial Proliferation
Rising estradiol concentrations during the follicular phase trigger significant changes in the uterine endometrium, preparing it for potential implantation.
Proliferative Changes
Endometrium becomes thicker
Increase in number of glands
Development of "triple stripe" pattern on ultrasound
Enhanced vascularization
Cellular proliferation and growth
These changes are essential for creating a receptive environment for embryo implantation should fertilization occur.
Source: Fleischer AC, Kalemeris GC, Entman SS. Ultrasound Med Biol 1986;12:271
Late Follicular Phase: Dominant Follicle Selection
Phase 3
A critical transition occurs during the late follicular phase: one dominant follicle is selected and begins to grow more rapidly than all others in the ovary.
Selection
One follicle becomes dominant
Growth
Grows 2mm per day to 20-26mm
Hormone Production
Daily increase in estradiol and inhibin A
LH Receptors
FSH induces LH receptors in granulosa cells
Source: Welt CK, Martin KA, Taylor AE, et al. J Clin Endocrinol Metab 1997;82:2645
Follicular Atresia
As the dominant follicle is selected, the remaining follicles in the growing cohort undergo a process called atresia—programmed cell death that prevents multiple ovulations.
Why Atresia Occurs
The dominant follicle's increasing estradiol and inhibin A production suppresses FSH levels. Other follicles, which require FSH support for continued growth, cannot survive in this low-FSH environment.
This mechanism ensures that typically only one oocyte is released per cycle, preventing multiple pregnancies and optimizing reproductive success.
Source: Richards JS. Hormonal control of gene expression in the ovary. Endocr Rev 1994;15:725
Cervical Mucus Changes
Rising estradiol concentrations produce characteristic changes in cervical mucus that facilitate sperm transport and can be used as a fertility indicator.
Increased Volume
Mucus production increases significantly in response to estradiol
Spinnbarkeit
Mucus becomes clear, stretchy, and slippery—optimal for sperm passage
MUC5B Peak
Late follicular phase peak in mucin protein MUC5B aids sperm transit
Source: Gipson IK, Moccia R, Spurr-Michaud S, et al. J Clin Endocrinol Metab 2001;86:594
The Midcycle LH Surge
The LH surge represents a dramatic neuroendocrine switch from negative to positive feedback control—a unique phenomenon critical for ovulation.
10X
LH Increase
Serum LH concentrations increase 10-fold during the surge
36
Hours to Ovulation
Ovulation occurs approximately 36 hours after surge begins
1
Pulse Frequency
LH pulse frequency remains at approximately one per hour
The surge cannot be recreated simply by administering estrogen and progestin, indicating that other ovarian factors contribute to this critical event.
Source: Adams JM, Taylor AE, Schoenfeld DA, et al. J Clin Endocrinol Metab 1994;79:858
Ovulation: The Release of the Oocyte
The LH surge initiates a cascade of events culminating in the release of a mature, fertilizable oocyte from the dominant follicle.
01
Meiotic Division
Oocyte completes first meiotic division, becoming fertilizable
02
Enzyme Activation
Plasminogen activator and cytokines increase for follicle rupture
03
Follicle Rupture
Oocyte released from follicle at ovarian surface
04
Fallopian Tube Entry
Oocyte enters distal fallopian tube opening
05
Luteinization Begins
Granulosa cells begin producing progesterone
Source: Tsafriri A, Chun SY, Reich R. Follicular rupture and ovulation. In: The Ovary, 1993
Corpus Luteum Formation
After the oocyte is released, the remaining follicular cells undergo a remarkable transformation into the corpus luteum—a temporary endocrine structure essential for early pregnancy support.
Structural Changes
The fluid-filled follicle collapses and its cells reorganize. Granulosa and theca cells luteinize, becoming large, yellow cells filled with lipid droplets.
The corpus luteum develops extensive vascularization, becoming one of the most highly perfused tissues in the body relative to its size.
This structure secretes large amounts of progesterone and estradiol, creating the hormonal environment necessary for endometrial preparation and early pregnancy maintenance.
Source: Stocco C, Telleria C, Gibori G. Endocr Rev 2007;28:117
Luteal Phase Hormonal Profile
Luteal Phase
The luteal phase is characterized by high progesterone secretion from the corpus luteum, with peak levels occurring in the mid-luteal phase approximately 7 days after ovulation.
14
Days Duration
Luteal phase typically lasts 14 days
6-25
Progesterone (ng/mL)
Normal mid-luteal progesterone range
4
Hours Between Pulses
LH pulse frequency slows to one every 4 hours
Progesterone pulses occur soon after LH pulses, resulting in significant excursions in serum progesterone concentrations during the luteal phase.
Source: Filicori M, Butler JP, Crowley WF Jr. J Clin Invest 1984;73:1638
Endometrial Transformation
Progesterone from the corpus luteum induces profound changes in the endometrium, transforming it from a proliferative to a secretory state optimized for embryo implantation.
1
Glandular Changes
Endometrial glands become tortuous and begin secreting glycogen-rich fluid. Mitoses cease as cells differentiate.
2
Stromal Decidualization
Stromal cells enlarge and accumulate glycogen and lipids, preparing for potential implantation.
3
Vascular Modifications
Spiral arteries develop and endometrial blood supply increases significantly.
4
Ultrasound Appearance
The "triple stripe" pattern is lost; endometrium becomes uniformly bright and echogenic.
Source: Noyes RW, Hertig AT, Rock J. Fertil Steril 1950;1:3
Implantation Window
The mid-luteal phase, approximately 6-10 days after ovulation, represents the "window of implantation"—a brief period when the endometrium is receptive to embryo attachment.
Molecular Markers
Pinopodes appear on endometrial surface
Adhesion molecules are expressed
Cytokines and growth factors peak
Immune cells modulate local environment
This precisely timed window ensures that implantation occurs only when the endometrium is optimally prepared, maximizing the chances of successful pregnancy.
Source: Noyes RW, Hertig AT, Rock J. Dating the endometrial biopsy. Fertil Steril 1950;1:3
Corpus Luteum Regression
In the absence of pregnancy and human chorionic gonadotropin (hCG) rescue, the corpus luteum undergoes programmed regression, leading to menstruation.
Day 10-11 Post-Ovulation
Without hCG signal, corpus luteum begins functional decline
Progesterone Decline
Progesterone and estradiol production decrease rapidly
Structural Luteolysis
Cells undergo apoptosis; corpus luteum becomes corpus albicans
FSH Rise
Decreased inhibin A allows FSH to rise, recruiting next follicle cohort
Source: Welt CK, Pagan YL, Smith PC, et al. J Clin Endocrinol Metab 2003;88:1766
Menstruation: Endometrial Shedding
The decline in progesterone and estradiol from the regressing corpus luteum triggers a cascade of events leading to menstrual bleeding.
Vasoconstriction
Spiral arteries constrict, reducing endometrial blood flow and causing ischemia
Tissue Breakdown
Ischemic endometrium releases prostaglandins and matrix metalloproteinases
Shedding
Functional endometrial layer sloughs off, leaving basal layer intact
Menstruation typically begins approximately 14 days after ovulation, though there is considerable interindividual variability in timing relative to hormone decline.
Source: Hall JE, Schoenfeld DA, Martin KA, Crowley WF Jr. J Clin Endocrinol Metab 1992;74:600
Clinical Assessment of Ovulation
Several methods are available to assess whether ovulation is occurring, each with different costs, timeframes, and clinical applications.
1
Menstrual Cycle Charting
Simple, free method. Cycles between 25-35 days generally indicate ovulation. Note molimina symptoms.
2
Basal Body Temperature
0.5°F rise in luteal phase confirms ovulation retrospectively. Requires special thermometer and daily measurement.
3
Serum Progesterone
Mid-luteal level of 6-25 ng/mL confirms ovulation. Single measurement sufficient if elevated.
4
Ultrasound Monitoring
Visualizes follicle growth (20-26mm) and corpus luteum formation. Most accurate but expensive.
Source: Welt CK. Evaluation of the menstrual cycle and timing of ovulation. UpToDate 2025
Predicting Ovulation for Conception
For couples trying to conceive, the highest probability of pregnancy occurs with intercourse 1-2 days before ovulation, making accurate prediction valuable.
Fertility Window
The fertile period spans approximately 6 days: the 5 days before ovulation and the day of ovulation itself.
Sperm can survive in the female reproductive tract for up to 5 days, while the oocyte remains viable for approximately 12-24 hours after ovulation.
76%
1-Month Pregnancy Rate
With timed intercourse using fertility awareness
50%
3-Month Rate
Without timed intercourse in fertile couples
Source: Stanford JB, White GL, Hatasaka H. Obstet Gynecol 2002;100:1333
Urinary LH Detection Kits
Home ovulation predictor kits detect the LH surge in urine, providing advance notice of ovulation and helping couples time intercourse optimally.
How They Work
Detect LH surge 12 hours after it appears in serum. Ovulation occurs ~36 hours after surge begins.
When to Test
Begin testing 1-2 days before expected surge to observe increase over baseline levels.
Accuracy Considerations
False positives occur in PCOS, POI, menopause. User error causes 7% false-positive interpretation.
Digital monitors that detect both estradiol rise and LH surge identify more fertile days and may improve accuracy.
Source: McGovern PG, Myers ER, Silva S, et al. Fertil Steril 2004;82:1273
Emerging Ovulation Detection Technologies
New technologies offer additional options for ovulation detection, though evidence for their effectiveness varies.
Salivary Ferning
Saliva forms fern-like pattern at ovulation. Requires microscope and slides. Lower accuracy than other methods.
Temperature Sensors
Vaginal or underarm sensors take continuous measurements. Data analyzed by algorithm to predict ovulation.
Smartphone Apps
Combine multiple data sources (temperature, LH, cervical mucus). Some incorporate ultrasound technology.
Source: Welt CK. Evaluation of the menstrual cycle and timing of ovulation. UpToDate 2025
Ovarian Reserve Assessment
Evaluation of ovarian reserve is important for older women and those with infertility, helping predict reproductive potential and guide treatment decisions.
1
Early Follicular FSH
Measured on cycle day 2-3. Elevated levels (>10 IU/L) suggest diminished reserve. Must be paired with estradiol.
2
Anti-Müllerian Hormone
Secreted by small antral follicles. Correlates with total follicle count. Minimal cycle variability.
3
Antral Follicle Count
Ultrasound count of 2-10mm follicles. Performed in early follicular phase. Direct assessment of follicle pool.
4
Inhibin B
Secreted by small follicles. Decreases with age. Less commonly used than AMH.
Source: Sherman BM, West JH, Korenman SG. J Clin Endocrinol Metab 1976;42:629
Age-Related Changes in Ovarian Function
As women age, gradual changes occur in the hypothalamic-pituitary-ovarian axis, affecting cycle characteristics and fertility potential.
1
Reproductive Prime (20s-Early 30s)
Regular cycles, optimal fertility, normal FSH levels, abundant follicle pool
2
Subtle Changes (Mid-30s)
Follicular phase shortens, early follicular FSH rises, AMH begins declining
3
Diminished Reserve (Late 30s-40s)
Cycle irregularity increases, FSH elevation more pronounced, fertility declines significantly
4
Perimenopause (40s-50s)
Irregular cycles, anovulation common, FSH markedly elevated, approaching menopause
Source: Welt CK, McNicholl DJ, Taylor AE, Hall JE. J Clin Endocrinol Metab 1999;84:105
Clinical Applications: Infertility Evaluation
Understanding menstrual cycle physiology is essential for evaluating and managing female infertility, guiding both diagnosis and treatment strategies.
Initial Assessment
Menstrual history and cycle regularity
Ovulation documentation
Ovarian reserve testing
Hormonal evaluation
Ultrasound assessment
Common Findings
Anovulation (25-30% of infertility)
Diminished ovarian reserve
Luteal phase defects
PCOS (most common cause)
Premature ovarian insufficiency
Proper evaluation of the menstrual cycle forms the foundation for diagnosing ovulatory disorders and guiding appropriate fertility treatments.
Source: Welt CK. Female infertility: Evaluation. UpToDate 2025. Available at: uptodate.com
Clinical Applications: Abnormal Uterine Bleeding
Understanding normal menstrual physiology is crucial for identifying and managing abnormal uterine bleeding patterns in reproductive-age women.
Irregular Bleeding
Cycle variation >7-9 days suggests anovulation. Requires hormonal evaluation and possible treatment.
Prolonged Bleeding
Duration >8 days may indicate anovulation, structural abnormalities, or coagulation disorders.
Heavy Bleeding
Volume interfering with quality of life requires evaluation for structural and hormonal causes.
Infrequent Bleeding
Cycles >38 days apart often indicate anovulation, requiring endocrine evaluation.
Source: Munro MG, Critchley HOD, Fraser IS. FIGO Menstrual Disorders Committee. Int J Gynaecol Obstet 2018;143:393
Summary: Key Clinical Pearls
Essential Knowledge
Normal Cycle Parameters
Frequency 24-38 days, regularity ≤7-9 days variation, duration ≤8 days, volume ≤80mL or not interfering with quality of life
Three Distinct Phases
Follicular phase (follicle growth, estradiol production), midcycle surge (LH peak, ovulation), luteal phase (corpus luteum, progesterone production)
Ovulation Confirmation
Mid-luteal progesterone 6-25 ng/mL, basal body temperature rise 0.5°F, regular 25-35 day cycles with molimina
Fertility Timing
Highest conception probability 1-2 days before ovulation. LH surge predicts ovulation ~36 hours in advance
Ovarian Reserve
Early follicular FSH, AMH, and antral follicle count assess reproductive potential, especially important with advancing age
Primary Sources: Welt CK. Normal menstrual cycle. UpToDate 2025; Munro MG, et al. FIGO systems for normal and abnormal uterine bleeding. Int J Gynaecol Obstet 2018;143:393