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Follicular phase: what happens and why its length varies

The follicular phase is the first part of the menstrual cycle, from the first day of a period to ovulation, when follicles in an ovary grow, one egg matures and estrogen rises. It is also the phase whose length varies most, which is why one cycle can be 24 days and the next 32.

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Medically reviewed by Dr Mary Davidson, Reproductive endocrinologist, checked on

A woman in sage activewear doing a side stretch on a yoga mat at home during the follicular phase of her cycle

The follicular phase is the first of the four menstrual cycle phases to begin and the last to end before ovulation. It is the stretch of the cycle in which the body prepares an egg, and because it can be short in one cycle and long in the next, it explains most of the difference between one cycle length and another.

What is the follicular phase?

The follicular phase is named after the follicles, the small sacs in the ovary that each hold an egg (NICHD, 2026). In each cycle, hormones make several follicles grow until one matures and releases its egg at ovulation (NICHD, 2026). So what does the follicular phase mean? The follicular phase meaning, in short, is the part of the cycle in which an egg gets ready to be released.

When is the follicular phase?

The follicular phase begins on the first day of the period and ends when you ovulate (Cleveland Clinic, 2025). It therefore overlaps the menstrual phase: the days of bleeding are its first days. In an average 28-day cycle the egg is released around day 12 to 14, so the follicular phase covers roughly the first two weeks, but ovulation can come anywhere between day 10 and day 21 of the cycle, and the follicular phase ends with it (NICHD, 2026).

What happens in the follicular phase

  1. The period starts. The first day of bleeding is day 1 of the cycle and of the follicular phase (Cleveland Clinic, 2025; NICHD, 2026).
  2. FSH starts the follicles growing. Follicle-stimulating hormone (FSH), made by the pituitary gland at the base of the brain, triggers the growth of eggs in the ovaries and gets them ready for ovulation (MedlinePlus, 2023).
  3. One follicle takes the lead. One follicle grows faster than the others, and the egg inside it begins to mature (Cleveland Clinic, 2025).
  4. Estrogen rises and the lining thickens. After bleeding ends, usually around day 5, estrogen rises and the lining of the womb thickens to prepare for a fertilized egg (NICHD, 2026).
  5. The other follicles stop growing. As estrogen rises, the body makes less FSH, the smaller follicles stop growing, and the egg in the leading follicle finishes maturing (Cleveland Clinic, 2025).
  6. LH surges and ovulation follows. Luteinizing hormone (LH) begins to surge about 36 hours before ovulation and peaks about 12 hours before it (Office on Women's Health, "Your menstrual cycle", 2025). The release of the egg ends the follicular phase.

The LH surge is what urine ovulation tests detect; the guide to ovulation predictor kits covers how they work.

How long is the follicular phase?

The sources disagree more about this phase than any other, because it really does vary:

Follicular phase length: what the sources report
SourceWhat it reports
612,613 cycles from a fertility app (2019)16.9 days on average; 95 in 100 between 10 and 30 days; 15.4 days in cycles of exactly 28 days
NICHDOvulation anywhere between day 10 and day 21 of the cycle
US Office on Women's Health13 to 20 days before ovulation, different between women and from month to month
Cleveland Clinic10 to 14 days on average, shorter or longer in some cycles
Study of 221 women (BMJ, 2000)Ovulation as early as day 8 and as late as day 60

The app figures come from 612,613 ovulatory cycles logged by 124,648 users, who estimated ovulation from temperature and urine LH tests (Bull et al., 2019). NICHD gives the range for ovulation (NICHD, 2026), the US Office on Women's Health gives 13 to 20 days (Office on Women's Health, "Trying to conceive", 2025), and Cleveland Clinic gives 10 to 14 days on average (Cleveland Clinic, 2025). The widest range comes from a study of 221 women in which ovulation came as early as day 8 and as late as day 60 (Wilcox et al., 2000).

The reason these figures matter is that the follicular phase decides cycle length. In a study of 141 women with regular cycles who logged 1,060 cycles, 42.5 in 100 had cycles that varied by more than 7 days, and the authors found that the follicular phase contributed most of that variation (Fehring et al., 2006). The luteal phase that follows is steadier. The follicular phase also shortens with age: in the app data it fell by 0.19 days for each year from age 25 to 45, almost exactly the 0.18 days a year by which the whole cycle shortened (Bull et al., 2019).

This is why an ovulation date worked out from cycle length is an estimate. The ovulation calculator shows a range for that reason.

Follicular phase symptoms

The follicular phase starts with the period, which usually lasts about 2 to 7 days (NHS, 2023). After that, the clearest of the symptoms of the follicular phase that medical sources describe is a change in cervical mucus. The symptoms of premenstrual syndrome belong to the other half of the cycle: they start 14 or more days after the first day of the period and usually go 1 to 2 days after the next period starts (MedlinePlus, 2026).

Follicular phase discharge

Right after a period there are usually a few days with no mucus, sometimes called dry days; then, as the egg starts to mature, mucus increases (Office on Women's Health, "Trying to conceive", 2025). Just before ovulation the cervix makes noticeably more mucus, and it becomes thin and slippery (ACOG, 2019). The NHS compares secretions around ovulation to raw egg white: thinner and stretchy (NHS, 2023). This change is one of the main signs that the follicular phase is ending. The guide to cervical mucus covers it in detail.

Follicular phase mood

The medical sources cited here do not describe a mood pattern specific to the follicular phase. Claims that it brings more energy or confidence, or that diet and exercise should follow the phase, are not supported by them.

Can you get pregnant in the follicular phase?

Yes, at its end. ACOG says you can become pregnant from sex from 5 days before ovulation until 1 day after it (ACOG, 2019), so most of the fertile window falls in the last days of the follicular phase. In a study of 221 women, the chance of conception rose from about 10 in 100 for sex five days before ovulation to about 33 in 100 on the day of ovulation itself (Wilcox et al., 1995). The guide to the most fertile days has the day-by-day figures.

Early in the phase the chance is low but not zero. In the same group of women, about 2 in 100 were already in their fertile window by day 4 of the cycle and 17 in 100 by day 7, and women with short cycles reached it sooner (Wilcox et al., 2000). The guide to can you get pregnant on your period explains why.

Follicular vs luteal phase

Luteal vs follicular phase: how the two halves compare
Follicular phaseLuteal phase
Runs fromFirst day of the period to ovulationOvulation to the day before the next period
Leading hormonesFSH, then estrogenProgesterone
Average length in app data16.9 days12.4 days
How much it variesMost of the variation in cycle lengthThe most consistent part of the cycle
Fertile daysThe last 5 days or soThe first day or so

The averages are from the app data (Bull et al., 2019), and ACOG describes the days between ovulation and the next period as the most consistent part of the cycle (ACOG, 2019). The menstrual cycle phases guide sets all four phases side by side. If your cycles are often shorter than 21 days or longer than 35, or vary by more than 7 to 9 days, ACOG counts that as abnormal (ACOG, 2021); the guide to irregular periods covers what can cause it.

Sources

  1. NHS. Periods and fertility in the menstrual cycle. nhs.uk, 2023.
  2. Eunice Kennedy Shriver National Institute of Child Health and Human Development. Menstruation and Menstrual Problems. NICHD, 2026.
  3. Cleveland Clinic. Follicular Phase. my.clevelandclinic.org, 2025.
  4. National Library of Medicine, MedlinePlus. Follicle-Stimulating Hormone (FSH) Levels Test. MedlinePlus, 2023.
  5. Office on Women's Health, US Department of Health and Human Services. Your menstrual cycle. womenshealth.gov, 2025.
  6. Office on Women's Health, US Department of Health and Human Services. Trying to conceive. womenshealth.gov, 2025.
  7. Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine 2: 83, 2019.
  8. Fehring RJ, Schneider M, Raviele K. Variability in the phases of the menstrual cycle. Journal of Obstetric, Gynecologic and Neonatal Nursing 35(3): 376-384, 2006.
  9. Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ 321(7271): 1259-1262, 2000.
  10. Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. New England Journal of Medicine 333(23): 1517-1521, 1995.
  11. American College of Obstetricians and Gynecologists. Fertility Awareness-Based Methods of Family Planning (FAQ024). ACOG, 2019.
  12. American College of Obstetricians and Gynecologists. Abnormal Uterine Bleeding (FAQ095). ACOG, 2021.
  13. National Library of Medicine, MedlinePlus. Premenstrual syndrome. MedlinePlus Medical Encyclopedia, 2026.

Frequently asked questions

What is the follicular phase?

It is the part of the menstrual cycle from the first day of a period to ovulation. Follicle-stimulating hormone triggers the growth of eggs in the ovaries (MedlinePlus, 2023), estrogen rises and thickens the lining of the womb, and one follicle matures until it releases its egg (NICHD, 2026).

How long is the follicular phase?

It varies more than any other phase. In more than 600,000 cycles logged in a fertility app it averaged 16.9 days, and 95 in 100 lasted between 10 and 30 days (Bull et al., 2019). NICHD says ovulation, which ends the phase, can come anywhere between day 10 and day 21 of the cycle (NICHD, 2026).

Do you still bleed during the follicular phase?

Yes, at the start of it. The follicular phase begins on the first day of the period (Cleveland Clinic, 2025), so the days of bleeding are part of it. The period usually lasts about 2 to 7 days (NHS, 2023).

Can I get pregnant in my follicular phase?

Yes, in its last days. You can become pregnant from sex from 5 days before ovulation until 1 day after it (ACOG, 2019), and most of those days fall at the end of the follicular phase. Early in the phase the chance is low but not zero, especially in short cycles (Wilcox et al., 2000).

Why is my follicular phase long?

The follicular phase is the part of the cycle that varies most, even in people with regular cycles (Fehring et al., 2006), so a longer phase now and then is common. Ovulation came as late as day 60 in one study (Wilcox et al., 2000). Cycles that are often longer than 35 days, or very unpredictable, are worth raising with a doctor (ACOG, 2021).