The Follicular Phase: What Happens and Why Its Length Varies

What is the follicular phase?

The follicular phase is the first half of your menstrual cycle. It starts on day 1, the first day of your period, and ends at ovulation, when an egg is released. Your period is part of it: while the womb lining is shed, the ovary is already growing follicles, the structures in which eggs mature.

This page is for anyone trying to understand their cycle, especially if you want to know when ovulation is likely. For all four phases, see menstrual cycle phases.

What happens during the follicular phase?

A follicle is chosen, grows and releases an egg. In order:

1. Your period. Oestrogen and progesterone have fallen, so the womb lining is shed. The NHS says periods last around 2 to 7 days.
2. Follicles start to grow. Follicle-stimulating hormone (FSH), made by the pituitary gland, makes several follicles grow at once.
3. One follicle takes the lead. The growing follicles make oestrogen, which damps FSH down. The follicle most sensitive to the lower level keeps growing while the others stop. This is the dominant follicle.
4. Oestrogen rises. The dominant follicle keeps producing oestrogen, and the NHS says the womb lining starts to thicken. ACOG, the American College of Obstetricians and Gynecologists, says cervical mucus noticeably increases just before ovulation and becomes thin and slippery.
5. The LH surge. Sustained high oestrogen triggers a sudden release of luteinising hormone (LH). A 2017 review by Su and colleagues puts the start of the surge 35 to 44 hours before ovulation and its peak 10 to 12 hours before.
6. Ovulation ends the phase. The follicle releases its egg and becomes the corpus luteum, which starts the luteal phase.

How long is the follicular phase?

On average a little over two weeks, but it is the part of the cycle that varies most. Across 612,613 ovulatory cycles logged in the Natural Cycles app, it averaged 16.9 days against 12.4 for the luteal phase. Split by cycle length:

Source: Bull and colleagues, npj Digital Medicine, 2019, funded by Natural Cycles; several authors work for or founded the company, and only cycles with detected ovulation were included.

A smaller academic study points the same way. Richard Fehring and colleagues at Marquette University followed 141 women through 1,060 cycles with an electronic fertility monitor. Cycles averaged 28.9 days, 42.5% of the women had more than 7 days of variability in their cycles, and the follicular phase contributed most to that variability.

Even in cycles of exactly 28 days, 13% of the 2019 total, the follicular phase averaged 15.4 days. The authors concluded that the belief that ovulation consistently happens on day 14 is not correct.

Why does the follicular phase change from cycle to cycle?

It only ends when a follicle is ready and the LH surge comes, so anything that slows that process lengthens the phase and the whole cycle. The factors with evidence behind them:

• Age. In the 2019 data the follicular phase shortened by about 0.19 days per year of age between 25 and 45, from an average of 18.0 days at 18 to 24 to 14.8 days at 40 to 45. The luteal phase barely changed.
• Weight. Users with a BMI over 35 had a follicular phase 0.9 days longer on average than those with a BMI of 18.5 to 25.
• Life stage and health. The NHS lists puberty, the start of menopause, losing or gaining a lot of weight, stress and anxiety, exercising too much, some hormonal contraception, and conditions such as polycystic ovary syndrome (PCOS), which it now calls PMOS, or thyroid problems among the causes of irregular periods. It adds that with irregular periods you might not release an egg regularly.

Follicular phase symptoms, mood and discharge

Most of what you can notice in this phase is your period at the start and changing discharge towards the end. Su's review describes cervical mucus outside the days around ovulation as thick, scant and viscous. As oestrogen rises it increases, and the NHS says that around ovulation it becomes thinner and stretchy, a bit like raw egg white. More on mucus: understanding cervical mucus.

On mood, ACOG's definition of premenstrual syndrome (PMS) requires symptoms to end within 4 days of a period starting, so low mood that persists through this phase is not premenstrual.

Follicular phase vs luteal phase

What the follicular phase means for ovulation timing

Because the variable part of the cycle comes before ovulation, counting forward from your period is unreliable. In a study of 696 cycles from 221 women planning a pregnancy, Allen Wilcox and colleagues found the fertile window fell entirely within days 10 to 17, the days clinical guidelines pointed to, in only about 30% of women. The authors advised that the timing of the fertile window "can be highly unpredictable, even if their cycles are usually regular".

The NHS counts back instead: in most women, ovulation happens around 10 to 16 days before the next period. Fertile mucus and a positive LH test are the signs that the phase is ending, and for some people so is ovulation pain. How to track ovulation compares the methods, the ovulation and fertile window calculator gives a first estimate, and sex, timing and fertility myths covers how often to have sex.

When to see a GP

The NHS says to see a GP if:

• your periods are irregular, meaning a gap of less than 21 days or more than 35 days between them
• your periods last longer than 7 days
• you have bleeding between periods
• you have irregular periods with symptoms such as weight gain, tiredness, hair growth on your face, or dry or oily skin
• your periods are irregular and you are struggling to get pregnant

If you are trying for a baby with irregular periods, the NHS says sex every 2 or 3 days throughout your cycle can help. See irregular periods and trying to conceive.

Frequently asked questions

What are the symptoms of the follicular phase?

Your period comes first, lasting around 2 to 7 days according to the NHS. After it, cervical mucus gradually increases and, as ovulation approaches, turns thin, slippery and stretchy. Your resting temperature stays lower than in the second half of the cycle. Premenstrual symptoms, by ACOG's definition, end within 4 days of your period starting.

What discharge is normal in the follicular phase?

Thick and scant early on, then more as oestrogen rises. ACOG says the amount noticeably increases just before ovulation and becomes thin and slippery; the NHS describes it as thinner and stretchy, a bit like raw egg white. The NHS says to see a GP if you are concerned about a change in your vaginal discharge.

How many days is the follicular phase?

It varies more than any other part of the cycle. In 612,613 app-tracked cycles it averaged 16.9 days: 12.4 days in 21- to 24-day cycles, 15.2 in 25- to 30-day cycles and 26.8 in 36- to 50-day cycles. It also tends to shorten with age, by about 0.19 days a year between 25 and 45.

How does the follicular phase affect mood?

The sources on this page do not describe a mood pattern specific to the follicular phase. What they describe is premenstrual symptoms ending: ACOG's criteria require them to stop within 4 days of a period starting. If low mood continues through this phase, raise it with a GP, because ACOG notes depression and anxiety often overlap with PMS.