The Menstrual Cycle

Last updated: April 10, 2026Bookmark

Table Of Contents

The Menstrual Cycle
The Ovaries
Menstruation
Steroidogenesis across the lifespan

The menstrual cycle is a monthly hormonal and physiological process that prepares the body for potential pregnancy. It involves a coordinated interplay of hormones and negative and positive feedback loops involving the ovaries and uterus, regulated by the hypothalamic-pituitary-ovarian axis.

By convention, the first day of menstrual bleeding is designated as day 1 of the menstrual cycle. The cycle usually lasts 28 days (a range of 21 to 35 days), and is divided into 2 phases – the follicular and luteal phase.

Components of the HPO axis

Component	Hormones
Hypothalamus	Gonadotropin Releasing Hormone (GnRH)
Anterior pituitary	Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH)
Ovaries	Estradiol (E2) and Progesterone (P4)
Uterus	Responds to E2 and P4

	Follicle/ovary	Endometrium	Dominant hormone
Day 1-14 (variable)	Follicular phase	Proliferative phases	Estrogen (Estradiol)
Day 15-28	Luteal phase	Secretory phase	Progesterone

Phases of the menstrual cycle

Phase	Duration
Follicular phase	14 +/- 7 days
Luteal phase	14 days
Menstrual flow	4 +/- 2 days
Blood flow	20 – 80 ml of blood

Quantity of menstrual blood flow

Category	Quantity
Normal flow	20 – 80 ml of blood
Super absorbent pad	Holds about 10 ml of blood
Tampon	Holds about 10 ml of blood

The Ovaries

Cell types of the ovaries

Cell type	Description
Germ cells	Oogonia (fetal mitotic cells that disappear before birth), primary oocyte (arrested in meiosis I until ovulation), secondary oocyte (haploid, arrested in metaphase II until fertilization)
Follicular cells (somatic support cells)	Granulosa and Theca cells. Granulosa cells produce estradiol and anti-Mullerian hormone, while theca cells produce androgens.
Stromal cells	Fibroblasts and hilar cells. Hilar cells secrete androgens
Corpus luteum cells	Formed after ovulation from granulosa + theca cells. Secretes progesterone to maintain pregnancy
Dominant follicles	Measures 1 – 2 cm in diameter
Follicular cysts	Measure > 3 cm

Functions of the ovaries
- Generation of mature oocysts (folliculogenesis)
- Production of steroid and peptide hormones, including oestrogen and progesterone

Folliculogenesis

Each ovary contains 300,000 – 400,000 primordial follicles, which respond to FSH to produce a dominant follicle. Dominant follicles produce the oocyte and estradiol (ET). The process of recruiting follicles usually takes 2-3 months and is driven by the number of FSH receptors on the follicle.

Folliculogenesis

Stage	Size	Key Events	Hormonal Control
Primordial follicle	0.03 mm	Dormant, formed in the fetus	Independent of FSH
Primary follicle	0.1 mm	Granulosa cells proliferate	Early FSH influence
Secondary follicle	0.2–0.4 mm	Theca cells appear	FSH + LH
Tertiary follicle	5–10 mm	Antrum forms, estradiol peaks	FSH dominance
Graafian follicle	18–28 mm	LH surge → ovulation	LH surge
Follicular cysts	> 3 cm

Corpus Luteum

In response to a surge in luteinizing hormone around day 14, the dominant follicle ruptures, resulting in ovulation. This follicular remnant closes, becomes vascularized, and is now the corpus luteum (CL) cyst. The CL cyst produces progesterone, which maintains the endometrial lining.

The corpus luteum lasts 14 days unless supported by hCG and is usually 2 – 4 cm in diameter. They contain clear fluid without solid components (similar to follicular cysts).

Menstruation

If there is no implantation by day 24 (in a 28-day cycle), the corpus luteum begins to involute and stops secreting progesterone. Progesterone withdrawal causes constriction of spiral arteries → endometrial necrosis and shedding.

Prostaglandins released from necrosis cause pain and contraction of the uterus. The decidua functionalis (superficial endometrial lining) separates from the basalis layer and is shed through the vagina.

Steroidogenesis across the lifespan

Childhood
- LH and FSH levels are high after birth but fall within a few months of life
- FSH > LH
Puberty
- Gonadotropin levels gradually rise
- Sleep-associated LH increase is one of the first signs of puberty
- Estradiol rises to give secondary sex characteristics, and menarche
- LH > FSH
Menopause
- Follicles decrease, amoung ot estrogen and inhibins, and LH and FSH increase
  - High incidence of twinning
  - Short follicular phase (metorrhagia)
- FSH > LH
- LH stimulates thecal androstenedione production → estrone

Reference Intervals ›

Biochemistry

ACTH	P: <80 ng/L
ALT	P: 5–35 U/L
Albumin	P: 35–50 g/L
Aldosterone	P: 100–500 pmol/L
Alk. phosphatase	P: 30–130 U/L
α-Amylase	P: 0–180 IU/dL
α-Fetoprotein	S: <10 kU/L
Angiotensin II	P: 5–35 pmol/L
ADH	P: 0.9–4.6 pmol/L
AST	P: 5–35 U/L
Bicarbonate	P: 24–30 mmol/L
Bilirubin	P: 3–17 μmol/L
BNP	P: <50 ng/L
CRP	P: <10 mg/L
Calcitonin	P: <0.1 mcg/L
Calcium (ionized)	P: 1.0–1.25 mmol/L
Calcium (total)	P: 2.12–2.60 mmol/L
Chloride	P: 95–105 mmol/L
Cholesterol	P: <5.0 mmol/L
VLDL	P: 0.128–0.645 mmol/L
LDL	P: <2.0 mmol/L
HDL	P: 0.9–1.93 mmol/L
Cortisol AM	P: 450–700 nmol/L
Cortisol Midnight	P: 80–280 nmol/L
CK ♂	P: 25–195 U/L
CK ♀	P: 25–170 U/L
Creatinine	P: 70–100 μmol/L
Ferritin	P: 12–200 mcg/L
Folate	S: 2.1 mcg/L
FSH	P: 2–8 U/L ♂; >25 menopause
GGT ♂	P: 11–51 U/L
GGT ♀	P: 7–33 U/L
Glucose (fasting)	P: 3.5–5.5 mmol/L
Growth hormone	P: <20 mu/L
HbA1C (DCCT)	B: 4–6%
HbA1C (IFCC)	B: 20–42 mmol/mol
Iron ♂	S: 14–31 μmol/L
Iron ♀	S: 11–30 μmol/L
Lactate (venous)	P: 0.6–2.4 mmol/L
Lactate (arterial)	P: 0.6–1.8 mmol/L
LDH	P: 70–250 U/L
LH	P: 3–16 U/L
Magnesium	P: 0.75–1.05 mmol/L
Osmolality	P: 278–305 mosmol/kg
PTH	P: 0.8–8.5 pmol/L
Potassium	P: 3.5–5.3 mmol/L
Prolactin ♂	P: <450 U/L
Prolactin ♀	P: <600 U/L
PSA	P: 0–4 mcg/mL
Protein (total)	P: 60–80 g/L
Red cell folate	B: 0.36–1.44 μmol/L
Renin (erect)	P: 2.8–4.5 pmol/mL/h
Renin (recumbent)	P: 1.1–2.7 pmol/mL/h
Sodium	P: 135–145 mmol/L
TBG	P: 7–17 mg/L
TSH	P: 0.5–4.2 mU/L
T4	P: 70–140 nmol/L
Free T4	P: 9–22 pmol/L
TIBC	S: 54–75 μmol/L
Triglycerides	P: 0.50–2.3 mmol/L
T3	P: 1.2–3.0 nmol/L
Troponin T	P: <0.1 mcg/L
Urate ♂	P: 210–480 μmol/L
Urate ♀	P: 150–390 μmol/L
Urea	P: 2.5–6.7 mmol/L
Vitamin B12	S: 0.13–0.68 nmol/L
Vitamin D	S: 50 nmol/L

Arterial Blood Gases

pH	7.35–7.45
PaCO₂	4.7–6.0 kPa
PaO₂	>10.6 kPa
Base excess	±2 mmol/L

Urine

Cortisol (free)	<280 nmol/24h
Hydroxyindole acetic acid	16–73 μmol/24h
Hydroxymethylmandelic acid	16–48 μmol/24h
Metanephrines	0.03–0.69 μmol/mmol cr.
Osmolality	350–1000 mosmol/kg
17-Oxogenic steroids ♂	28–30 μmol/24h
17-Oxogenic steroids ♀	21–66 μmol/24h
17-Oxosteroids ♂	17–76 μmol/24h
17-Oxosteroids ♀	14–59 μmol/24h
Phosphate (inorganic)	15–50 mmol/24h
Potassium	14–120 mmol/24h
Protein	<150 mg/24h
Protein/creatinine ratio	<3 mg/mmol
Sodium	100–250 mmol/24h

Haematology

WCC	4.0–11.0 ×10⁹/L
RBC ♂	4.5–6.5 ×10¹²/L
RBC ♀	3.9–5.6 ×10¹²/L
Hb ♂	130–180 g/L
Hb ♀	115–160 g/L
PCV ♂	0.4–0.54 L/L
PCV ♀	0.37–0.47 L/L
MCV	76–96 fL
MCH	27–32 pg
MCHC	300–360 g/L
RDW	11.6–14.6%
Neutrophils	2.0–7.5 ×10⁹/L (40–75%)
Lymphocytes	1.0–4.5 ×10⁹/L (20–45%)
Eosinophils	0.04–0.44 ×10⁹/L (1–6%)
Basophils	0–0.10 ×10⁹/L (0–1%)
Monocytes	0.2–0.8 ×10⁹/L (2–10%)
Platelets	150–400 ×10⁹/L
Reticulocytes	0.8–2.0% / 25–100 ×10⁹/L
Prothrombin time	10–14 s
APTT	35–45 s

Paediatric

Pulse Rate (bpm)
Neonate	140–160
Infant <1yr	120–140
1–5 years	110–130
5–12 years	80–120
>12 years	70–100
Respiratory Rate (tachypnoea)
0–2 months	≥60/min
2–12 months	≥50/min
1–5 years	≥40/min
>5 years	≥30/min
Blood Pressure (mmHg)
Term	65/45
1 year	75/50
4 years	85/60
8 years	95/65
10 years	100/70
Weight Formulas
3–12 months	(a + 9)/2 kg
1–6 years	2a + 8 kg
>6 years	(7a − 5)/2 kg
Haemoglobin (g/dL)
Term newborn	13–20
1 month	11–18
2 months	10–15
1–2 years	10–13
>2 years	11–14
MUAC (6 months–5 years)
Obese	>17.5 cm
Normal	13.5–17.4 cm
At risk	12.5–13.4 cm
Moderate malnutrition	11.5–12.4 cm
Severe malnutrition	<11.5 cm
Developmental Milestones
Social smile	1.5 months
Head control	4 months
Sits unsupported	7 months
Crawls	10 months
Stands unsupported	10–12 months
Walks	12–13 months
Talks	18 months
CSF WBC (/mm³)
Term newborn	0–25
>2 weeks	0–5

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