The menstrual cycle is a monthly series of changes that prepares the body for a possible pregnancy. Understanding its phases helps you recognise your fertile days and notice changes that may need attention.
How long is a menstrual cycle?
A cycle is counted from the first day of one period to the day before the next. A normal cycle lasts between about 21 and 35 days, with 28 days being an average rather than a rule. See are your cycles regular?
The four phases of the menstrual cycle
1. Menstrual phase
The cycle begins with the period, when the lining of the uterus is shed because pregnancy has not occurred. Bleeding usually lasts two to seven days.
2. Follicular phase
This phase starts on the first day of the period and lasts until ovulation. The hormone FSH stimulates several follicles in the ovaries, and usually one becomes dominant. Rising oestrogen thickens the lining of the uterus. The length of this phase varies most between women and explains most differences in cycle length.
3. Ovulation
A surge of LH causes the dominant follicle to release an egg, usually about 14 days before the next period. The egg survives for about 12 to 24 hours.
4. Luteal phase
After ovulation, the empty follicle becomes the corpus luteum and produces progesterone, which prepares the lining for implantation. If pregnancy does not occur, hormone levels fall and the next period begins, usually about 14 days after ovulation.
Your fertile window
Because sperm can survive for up to five days, the fertile window is about six days long: the five days before ovulation and the day of ovulation. Regular intercourse every two to three days covers this window without exact timing. Ovulation predictor kits and cycle tracking can also help. See what is normal fertility?
Hormones that control the cycle
Hormones from the brain (FSH and LH) and the ovaries (oestrogen and progesterone) work together to control the cycle. Thyroid hormones and prolactin also affect it. See hormonal assays.
When to see a doctor
- Cycles shorter than 21 days or longer than 35 days
- Periods that stop for three months or more
- Very heavy or very painful periods. See painful periods and infertility
- Bleeding between periods or after intercourse
- Difficulty conceiving
Frequently asked questions
When am I most fertile?
In the five days before ovulation and on the day of ovulation. In a 28-day cycle, ovulation is usually around day 14.
Can I get pregnant during my period?
It is unlikely but possible, especially with short cycles, because sperm can survive for several days.
Why is my cycle length different each month?
Small variations are normal. Stress, illness, weight changes and conditions such as PCOS can cause larger changes.
If you have questions about your own situation, you can meet our doctors at our Basavanagudi, Koramangala and Ballari centres, or contact us to book an appointment.
This article is for general information and does not replace medical advice. Treatment decisions should be made with a qualified doctor who knows your history.