Menstrual Cycle Explained: Phases, Hormones, and What's Normal

The Menstrual Cycle: A Hormonal Symphony

Menstruation = shedding of the uterine lining (endometrium) when pregnancy does not occur. Average cycle: 28 days (range 21–35).

Phases

1. Menstrual (Days 1–5): Endometrium shed; estrogen/progesterone low. FSH rises → follicular recruitment.
2. Follicular (Days 1–13): Dominant follicle grows; estradiol ↑ → proliferative endometrium. LH surge triggered at peak estradiol.
3. Ovulation (Day ~14): LH surge → oocyte release. Fertile window: 5 days before + day of ovulation.
4. Luteal (Days 15–28): Corpus luteum secretes progesterone (+estradiol) → secretory endometrium. If no implantation, corpus luteum regresses → hormone drop → menstruation.

Key Hormones

• GnRH (hypothalamus) → FSH/LH (pituitary) → Estradiol/Progesterone (ovary).
• Feedback loops: Negative (early follicular), Positive (LH surge), Negative (luteal).

Normal Variations

• Cycle length: 21–35 days.
• Bleeding duration: 2–7 days; volume 30–80 mL.
• Mild cramps, breast tenderness, mood changes common.

When to Seek Evaluation

• Cycles <21 or >35 days consistently.
• Amenorrhea >3 months (not pregnant).
• Heavy bleeding (>80 mL or >7 days).
• Severe pain disrupting daily life.
• Bleeding between periods or after sex.