The Complete Guide to Your Menstrual Cycle — Phases, Hormones and What’s Normal

What is the menstrual cycle?

The menstrual cycle is your body’s monthly process of preparing for a potential pregnancy. It begins on the first day of your period and ends the day before your next period starts. While the average cycle is 28 days, a normal cycle can range anywhere from 21 to 35 days — and what matters most is what is consistent for you personally.

I spent years thinking my cycle was broken. As someone who has lived with PCOS since my teens, my cycle was never the textbook 28 days that every tracker seemed to assume. It was only when I started genuinely understanding what my cycle was doing — and why — that I stopped feeling like something was wrong with me. That is exactly why I built Cyluna’s free period calculator — to give women tools that actually reflect how cycles work in real life, not just in textbooks.

The menstrual cycle is controlled by a complex interplay of hormones produced by the brain and the ovaries. These hormones rise and fall in a predictable pattern each cycle, driving the development and release of an egg, the thickening of the uterine lining, and — if no pregnancy occurs — the shedding of that lining as your period.

The four phases of the menstrual cycle

Your menstrual cycle has four distinct phases. Understanding each one helps you make sense of why you feel the way you do at different points in your cycle — and gives you far more useful information than just knowing when your next period is due.

It is worth noting that the day numbers above are based on a 28-day average cycle. If your cycle is longer or shorter, each phase adjusts accordingly. Women with longer cycles typically have a longer follicular phase — ovulation simply happens later. The luteal phase, however, is usually fairly consistent at 12–16 days regardless of total cycle length.

You can use our free period calculator to see exactly which phase you are in today, along with your energy and mood forecast for the current phase.

The hormones that drive your cycle

Four key hormones orchestrate your menstrual cycle. Understanding what each one does makes the whole cycle far easier to understand — and helps explain why you feel the way you do at different points each month.

How hormones affect your mood and energy

The hormonal fluctuations throughout your cycle do not just affect your reproductive system — they have a direct and significant impact on your brain chemistry, mood, energy levels, appetite, libido and even cognitive function.

Rising oestrogen in the follicular phase tends to lift mood, increase energy and sharpen focus — many women feel their most productive and social during this phase. The ovulatory phase, with its LH surge, often brings a short peak in confidence and physical energy. Then, as progesterone rises in the luteal phase and oestrogen drops after its second smaller peak, many women experience the classic PMS symptoms: irritability, fatigue, bloating, food cravings and low mood. These are not weaknesses — they are direct hormonal effects, and recognising them as such can make them much easier to manage.

What is a normal menstrual cycle?

One of the most common questions I get asked — and one I spent years asking myself — is whether a cycle is “normal.” The honest answer is that normal covers a wide range, and what matters most is what is consistent for you.

Normal cycle length

A cycle between 21 and 35 days is considered within the normal range according to the NHS and ACOG. Most women fall between 24 and 32 days. The 28-day average you see everywhere is exactly that — an average, not a standard everyone should expect to match.

Normal period duration

A period typically lasts between 2 and 7 days. Bleeding that consistently lasts longer than 7 days is worth discussing with a doctor.

Normal blood loss

Most women lose between 30 and 80ml of blood during a period — roughly 2 to 6 tablespoons. Losing more than 80ml (heavy menstrual bleeding, or menorrhagia) is worth investigating as it can cause iron deficiency anaemia.

Normal variation between cycles

It is completely normal for your cycle length to vary by a few days from month to month. Stress, illness, travel, significant weight changes and disrupted sleep can all shift your cycle. A variation of up to 7 days from your average is generally considered normal.

How to track your menstrual cycle

Tracking your cycle is one of the most useful things you can do for your health. It helps you predict your period, understand your fertile window, recognise patterns in your mood and energy, and spot changes that might warrant a doctor visit.

What to track

• Period start date — the first day of full bleeding (not spotting)
• Period end date — when bleeding stops
• Flow heaviness — light, medium, heavy, very heavy
• Pain levels — 1-10 scale, location of cramps
• Mood and energy — even brief daily notes reveal patterns over time
• Cervical mucus — changes in consistency indicate where you are in your cycle
• Any unusual symptoms — spotting, breast tenderness, headaches

Using a period calculator

The quickest way to understand your cycle is to use our free period calculator. Enter the date your last period started and your average cycle length, and it will tell you your current cycle phase, next period date, ovulation date, fertile window, PMS window, safe days and a full 3-month visual calendar — all in one place, with no account required and nothing stored.

If your cycle is irregular, our irregular period calculator is specifically built for unpredictable cycles. Rather than assuming a fixed cycle length, it takes your shortest and longest cycle and gives you a probability window rather than a single date.

When cycles become irregular

Irregular periods are extremely common — affecting between 14% and 25% of women according to research. An irregular cycle is one that varies significantly in length from month to month, or that falls consistently outside the 21–35 day range.

Common causes of irregular periods

• PCOS (polycystic ovary syndrome) — one of the most common causes of irregular cycles, affecting around 1 in 10 women. PCOS disrupts the hormonal signals that trigger ovulation, causing cycles that can be very long, very irregular or absent altogether.
• Thyroid disorders — both an underactive and overactive thyroid can disrupt the menstrual cycle.
• Stress — chronic or acute stress affects the hypothalamus, which controls the reproductive hormones.
• Significant weight changes — both weight gain and weight loss can disrupt hormone levels.
• Perimenopause — in the years before menopause, cycles often become increasingly irregular.
• Excessive exercise — particularly relevant in elite athletes or those who exercise very heavily.
• Certain medications — including some antidepressants, antipsychotics and hormonal contraceptives.

If you have irregular cycles, particularly related to PCOS, tracking your cycle with a tool designed for irregular periods makes a significant difference. Standard calculators that assume a fixed 28-day cycle will consistently give you wrong predictions. Our irregular period calculator handles variable cycle lengths properly, giving you a realistic prediction window instead of a single date that is likely to be wrong.

Mood and energy through your cycle — what to expect

One of the most practically useful things you can understand about your cycle is how it affects your mental and emotional state. Many women notice significant differences in their mood, energy, social drive and even cognitive performance at different cycle phases — and these are driven directly by hormones, not imagination.

Follicular phase (days 1–13)

As oestrogen rises through the follicular phase, most women notice gradually improving energy, mood and mental clarity. This is often the best phase for tackling demanding work, starting new projects, socialising and exercise that requires high output. Many women report feeling most like themselves during this phase.

Ovulatory phase (around day 14)

The LH surge and oestrogen peak around ovulation often brings a short window of peak energy, confidence and social drive. This is frequently when women feel most attractive, communicative and motivated. It typically lasts only 2–3 days.

Luteal phase (days 15–28)

As progesterone rises after ovulation, many women notice a gradual shift toward lower energy, reduced social drive and increased need for rest and comfort. In the week before your period, falling progesterone and oestrogen trigger classic PMS symptoms for many women — irritability, anxiety, fatigue, bloating, cravings and low mood. These are real hormonal effects, not character flaws.

Menstrual phase (days 1–5)

During your period, both oestrogen and progesterone are at their lowest. Many women find this a naturally introspective, lower-energy time. Honouring that rather than fighting it — reducing commitments where possible, prioritising sleep and rest — tends to make periods more manageable.

When to see a doctor about your cycle

While most cycle variations are normal, some symptoms warrant medical attention. As a general rule, any significant change from your own normal pattern is worth discussing with a healthcare provider, even if it falls within the broader normal range.

Seek medical advice if you experience:

• Periods that suddenly become much heavier or much lighter than usual
• Severe period pain that does not respond to over-the-counter pain relief
• Periods that stop for 3 or more months when you are not pregnant (amenorrhoea)
• Bleeding between periods or after sex
• Periods that become suddenly much more irregular after previously being regular
• Symptoms of PCOS: irregular periods alongside acne, excess hair growth, weight gain or difficulty losing weight
• Symptoms of endometriosis: severe pain during periods, during sex or when using the toilet

This article is for informational purposes only. Always consult a qualified healthcare provider for advice about your specific situation. See our full medical disclaimer.