What Is a Menstrual Cycle?
The menstrual cycle is a monthly series of hormonal changes that prepares a woman's body for the possibility of pregnancy. It begins on the first day of menstrual bleeding and ends the day before the next period begins. Understanding your cycle is fundamental to reproductive health, fertility planning, and overall wellbeing.
The Four Phases of the Menstrual Cycle
Phase 1: Menstrual Phase (Days 1–5)
The menstrual phase begins on Day 1 of your period. During this phase, the uterine lining (endometrium) that built up in the previous cycle sheds because fertilization did not occur. This shedding is what we call menstrual bleeding.
- Duration: 2–7 days (average 5 days)
- Hormones: Estrogen and progesterone are at their lowest
- Common symptoms: Cramps, fatigue, lower back pain, bloating
- Energy: Typically lower; rest and self-care are important
Phase 2: Follicular Phase (Days 1–13)
Overlapping with menstruation, the follicular phase begins as the brain's pituitary gland releases follicle-stimulating hormone (FSH). FSH signals the ovaries to develop follicles—small fluid-filled sacs each containing an immature egg. Usually, one dominant follicle emerges and produces increasing amounts of estrogen.
- Duration: ~13 days in a 28-day cycle
- Hormones: Rising estrogen, FSH active
- Physical changes: Uterine lining rebuilds and thickens
- Energy: Rising energy, improved mood, mental clarity
Phase 3: Ovulation (Around Day 14)
Ovulation is triggered by a surge in luteinizing hormone (LH). The dominant follicle releases a mature egg (ovum) into the fallopian tube, where it can be fertilized by sperm. The egg survives only 12–24 hours after release, but sperm can live up to 5 days—making the 5 days before ovulation to 1 day after your fertile window.
- Duration: Egg lives 12–24 hours
- Signs: Egg-white cervical mucus, mild pelvic pain (mittelschmerz), slight temperature rise
- Hormones: LH surge, peak estrogen, rising progesterone
- Energy: Peak energy, sociability, and libido
Phase 4: Luteal Phase (Days 15–28)
After ovulation, the follicle transforms into the corpus luteum, which produces progesterone. Progesterone maintains the thickened uterine lining to support potential implantation. If no fertilization occurs, the corpus luteum breaks down, progesterone drops, and the cycle restarts with menstruation.
- Duration: ~14 days (relatively constant)
- Hormones: High progesterone, estrogen rises then falls
- Common symptoms: PMS — bloating, breast tenderness, mood changes, food cravings
- Energy: Declining toward end of phase
Cycle Length: What's Normal?
| Measurement | Normal Range | Average |
|---|---|---|
| Cycle length | 21–35 days | 28 days |
| Period duration | 2–7 days | 5 days |
| Blood loss per period | 20–80 mL | 35 mL |
| Luteal phase length | 12–16 days | 14 days |
Hormones That Drive Your Cycle
Four key hormones orchestrate the menstrual cycle:
- Estrogen — Builds the uterine lining; peaks just before ovulation; drives energy and libido
- Progesterone — Maintains lining after ovulation; supports pregnancy; can cause PMS when it drops
- FSH (Follicle-Stimulating Hormone) — Signals ovaries to develop follicles
- LH (Luteinizing Hormone) — Triggers the release of the egg (ovulation)
Signs Your Cycle Is Healthy
- Period arrives every 21–35 days with less than 7-day variation cycle to cycle
- Bleeding lasts 2–7 days
- Flow ranges from light to moderate (not soaking more than one pad/tampon per hour)
- Manageable cramps in the first 1–2 days
- Regular energy and mood fluctuations that correlate with cycle phases
When to See a Doctor
Consult a healthcare provider if you experience:
- Cycles shorter than 21 days or longer than 35 days
- Periods lasting more than 7 days
- Soaking through a pad or tampon every hour for several consecutive hours
- Severe pain that disrupts daily activities (could indicate endometriosis)
- No period for 3+ months (amenorrhea) without pregnancy
- Spotting between periods or after sex
Frequently Asked Questions
Is a 28-day cycle the only normal cycle?
No. The 28-day cycle is a statistical average, not a universal standard. Any cycle between 21 and 35 days that is consistent for you is considered normal.
Why does my cycle length change from month to month?
Cycle length naturally varies due to stress, illness, travel, significant weight changes, intense exercise, or hormonal fluctuations. Minor variations of ±2–4 days are normal. Larger, persistent variations may warrant medical evaluation.
Can the luteal phase be shorter than 12 days?
A luteal phase shorter than 10 days is called "luteal phase defect" and may affect fertility, as the uterine lining may not have adequate time to prepare for implantation. If you suspect this, consult a reproductive endocrinologist.