The menstrual cycle is a repeating set of changes that prepares the body for a possible pregnancy. Day 1 is the first day of period bleeding, and the cycle ends the day before the next period starts. Hormones guide each phase, but the timing and symptoms can vary from one person and one cycle to the next.

What is the menstrual cycle?

The menstrual cycle involves the ovaries and the womb (uterus). The ovaries hold and release eggs. The uterus builds a soft inner lining that could support a pregnancy.

Hormones are chemical messengers that help coordinate these changes. The main hormones involved include oestrogen, progesterone, follicle-stimulating hormone, and luteinising hormone. You do not need to measure these hormones to track a cycle.

The NHS overview and ACOG cycle guide describe the same basic sequence: bleeding begins, an egg develops, ovulation may occur, and the uterine lining either supports a pregnancy or sheds as the next period.

What are the phases of the menstrual cycle?

The menstrual phase

The menstrual phase is your period. If pregnancy has not occurred, hormone levels fall and the uterine lining leaves the body through the vagina as blood and tissue.

Day 1 of full bleeding is Day 1 of a new cycle. Light spotting before full bleeding is usually recorded separately so your cycle count stays consistent.

The follicular phase

The follicular phase starts on Day 1 and continues until ovulation. A follicle is a small fluid-filled sac in an ovary that contains an immature egg. Hormone signals help one follicle develop, while oestrogen helps rebuild the uterine lining.

This phase can change in length from cycle to cycle. That is one reason a future period or ovulation date cannot be known with certainty from a calendar alone.

Ovulation

Ovulation means an ovary releases an egg. It does not always happen on Day 14. The NHS notes that exact timing can be difficult to pinpoint and depends on the length of the cycle.

Calendar-based fertile dates are therefore estimates. These dates support conception planning and awareness. They are estimates, not a reliable way to avoid pregnancy.

The luteal phase

The luteal phase begins after ovulation. The empty follicle makes progesterone, which helps maintain the uterine lining. If pregnancy does not occur, progesterone and oestrogen fall and the next period begins.

Some people notice breast tenderness, bloating, headaches, skin changes, tiredness, or mood changes during this part of the cycle. A repeating pattern may be useful to record, but the pattern alone cannot confirm a health condition.

How do you count cycle days?

Count the first day of full period bleeding as Day 1. Continue counting each day until the day before the next period begins. The number of days between those two start dates is your cycle length.

For example, if one period starts on 1 May and the next starts on 29 May, the cycle is 28 days. You can use Femio's cycle-length calculator to work out an average from several cycles.

An average can be useful, but it does not mean every cycle will match it. Illness, stress, travel, life-stage changes, and some medicines can coincide with a different cycle. A clinician can help assess a change that persists or worries you.

How much can a cycle vary?

There is no single cycle length that fits everyone. ACOG describes 21 to 35 days as a usual adult range, while cycles can be wider and less regular in the first years after periods begin. The pattern that is usual for you also matters.

Look beyond one number. Notice whether the time between periods, the number of bleeding days, the amount of bleeding, pain, or other symptoms have changed. A sudden or lasting shift is more useful to discuss than whether you match a textbook average.

If your dates vary, prediction windows should become wider. Femio does not label a period as “late” when the logged pattern is irregular.

How can cycles change across life stages?

Cycles often change over a lifetime. In the first years after periods begin, the gap between periods may vary. A steadier pattern may develop with time.

Pregnancy and breastfeeding can pause or change bleeding. Hormonal medicines can also change flow or timing. Later, perimenopause can make cycles less regular again.

None of these stages has one fixed pattern. Keep track of what is new for you. Tell a clinician about a change that lasts, causes pain, or affects daily life. If a medicine may be involved, do not stop it on your own. Ask the prescriber what to expect.

What is useful to track?

You may want to record:

• The first and last day of bleeding.
• Flow as light, medium, or heavy in your own terms.
• Spotting between periods.
• Pain and how it affects sleep, school, work, or movement.
• Mood, headaches, bowel changes, skin changes, or tiredness.
• Medicines, illness, major stress, or other context you want to remember.

You do not have to track everything. Choose details that answer a question or help you prepare for care. If bleeding is affecting daily life, read the guide to heavy periods. If pain is the main concern, see painful periods.

When to see a doctor

Arrange a routine appointment if your periods stop without a known reason, happen much more or less often than before, last longer than usual, or regularly disrupt daily life. Also speak with a doctor about bleeding between periods, bleeding after sex, or new pelvic pain. ACOG lists these as patterns that clinicians may need to assess.

Bring your dates and notes if you have them. The doctor may ask about pregnancy possibility, medicines, health history, bleeding, pain, and other symptoms. Your log supports the conversation; it does not supply the answer by itself.

Seek care now: Get urgent medical help for very heavy bleeding with fainting, dizziness, chest pain, trouble breathing, or severe weakness. Sudden severe pelvic pain also needs urgent assessment. If you are unsure how urgent it is, contact local urgent care or emergency services.

How Femio can help you track your cycle

Femio lets you log period dates, flow, symptoms, and notes in one calendar. It shows future dates as ranges and adjusts confidence to the data you enter. You can also make a printable summary for a doctor visit.

Use the practical guide to tracking your menstrual cycle for a focused record. If mood or body symptoms repeat before periods, see how to track PMS and PMDD symptoms. Many different things can cause cycle changes, so tracking cannot tell you which one applies.

Ask a clinician to explain any health term that is unclear.

Frequently asked questions

Is Day 1 spotting or full bleeding?

For consistent cycle tracking, Day 1 usually means the first day of full period bleeding. Record earlier spotting as spotting if your tracker allows it.

Does ovulation always happen halfway through a cycle?

No. Ovulation timing can vary, and a calendar cannot confirm the exact day. A midpoint estimate may be less useful when cycle lengths change.

Can one unusual cycle mean something is wrong?

One different cycle does not explain its cause. Record it and look at what happens next. Speak with a doctor sooner if the change comes with severe pain, very heavy bleeding, pregnancy concerns, or other symptoms that worry you.