Educational content, not medical advice. Clinical review of this page is in progress.
A menstrual cycle isn’t just ‘your period’ — it’s a roughly month-long hormonal cycle made up of four distinct phases, each with a specific job.
1. Menstrual Phase (Days 1–5)
The uterine lining sheds if no pregnancy occurred, marking day 1 of the new cycle. Estrogen and progesterone are at their lowest.
2. Follicular Phase (Days 1–13, overlapping)
The pituitary gland releases FSH, prompting several ovarian follicles to develop; one becomes dominant. Estrogen rises as the uterine lining rebuilds.
3. Ovulatory Phase (Around Day 14)
A surge in LH triggers release of the mature egg. This is the fertile window — see our signs of ovulation guide.
4. Luteal Phase (Days 15–28)
The ruptured follicle becomes the corpus luteum, producing progesterone to prepare the uterine lining for possible implantation. If no pregnancy occurs, hormone levels drop and the cycle restarts.
Hormones at Work in Each Phase
Estrogen rises through the follicular phase and peaks just before ovulation. A surge in luteinizing hormone (LH) triggers the release of an egg. During the luteal phase, progesterone rises to prepare the uterine lining; if there is no pregnancy, hormone levels fall and the lining sheds, starting a new cycle.
Which Phase Is Variable
The luteal phase is fairly consistent, usually 12 to 14 days. The follicular phase is what makes cycles shorter or longer, so a person with a 35-day cycle typically ovulates later than day 14. Use the Ovulation Calculator to estimate your own timing.
How Cycle Awareness Helps
Knowing where you are in your cycle helps you time intercourse, interpret symptoms and spot changes worth discussing with a provider, such as very heavy bleeding or cycles shorter than 21 or longer than 35 days. See irregular cycles for more, and NHS: Fertility in the menstrual cycle for an overview.
Related Reading
- Fertility Guide — a full overview of getting pregnant
- Factors That Affect Ovulation — stress, weight, health and more
- Ovulation Calculator — estimate your fertile window
- Signs of Ovulation — recognize your fertile days
Also see our calculator FAQs, pregnancy FAQs and pregnancy terms glossary.
Frequently Asked Questions
Which phase of the menstrual cycle is most fertile?
The days leading up to and including ovulation, at the end of the follicular phase, are the most fertile. Pregnancy is very unlikely once the luteal phase is well underway.
How long is a normal menstrual cycle?
In adults, cycles of about 21 to 35 days are considered typical, counted from the first day of one period to the first day of the next.
Why does cycle length vary from person to person?
Most of the variation comes from the follicular phase, which can be shorter or longer depending on when ovulation occurs. The luteal phase is usually more consistent, at about 12 to 14 days.
Related Resources
- Ovulation 101: The Complete Guide
- Signs and Symptoms of Ovulation
- How to Track Ovulation: 4 Proven Methods
- Ovulation Calculator
- The Complete Fertility Guide
Sources and Further Reading
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN or midwife about your own pregnancy.