Your Menstrual Cycle,
Explained Simply

The menstrual cycle is one of the body's most elegant systems — a monthly sequence of hormonal signals that prepares for a possible pregnancy and, when there isn't one, resets and starts again. Knowing how it works helps you read your own body: predict your period, find your fertile days, make sense of mood and energy shifts, and spot when something is off.

How the Cycle Is Counted

Your cycle begins on Day 1 — the first day of proper menstrual bleeding — and ends on the day before your next period starts. The number of days in between is your cycle length. So a "28-day cycle" means your period arrives every 28 days. That famous number is just an average; anything from 21 to 35 days is considered normal, and your own length can wobble by a few days each month.

The Four Phases

One cycle moves through four phases, driven by the rise and fall of four hormones — oestrogen, progesterone, FSH and LH.

One detail is worth remembering: the luteal phase is the constant part. Whether your cycle is 24 days or 34, the gap from ovulation to your next period stays around 14 days. The follicular phase is what stretches or shrinks. This is exactly why ovulation is calculated by counting back 14 days from your next period rather than forward from your last one.

What Counts as "Normal"

• Cycle length: 21–35 days. Teens and women near menopause naturally run longer and less regular.
• Period length: 2–7 days of bleeding.
• Variation: a swing of a few days month to month is fine. Perfect 28-day clockwork is the exception, not the rule.
• Flow: changes through the period — heavier at the start, lighter toward the end.

To know your normal, track a few cycles. Once you have an average length, the Period Calculator can project your next several periods, plus your ovulation and fertile window for each.

Why Periods Become Irregular

An occasional late or early period is almost always nothing. But if irregularity becomes the pattern, there is usually a reason:

• Stress — high cortisol can delay or skip ovulation.
• Weight change — rapid loss or gain disrupts the hormones that drive the cycle.
• PCOS (polycystic ovary syndrome) — a very common cause of irregular or absent periods in India.
• Thyroid imbalance — both under- and over-active thyroid affect cycles.
• Intense exercise — heavy training can pause periods entirely.
• Life stages — the first years after menarche, breastfeeding, and perimenopause are all naturally irregular.
• Pregnancy — the most common reason for a sudden missed period.

Understanding PMS

In the luteal phase, after ovulation, the dip in oestrogen and progesterone can trigger premenstrual syndrome (PMS) — bloating, cramps, sore breasts, irritability, low mood, food cravings and fatigue. It's real and hormonal, not "in your head." Most people find symptoms ease as soon as bleeding starts. Regular exercise, reducing salt and caffeine, good sleep and stress management all help; if PMS is severe enough to disrupt your life, a doctor can offer treatment.

How Your Cycle Changes Through Life

• Teenage years: cycles can be long and erratic for the first two to three years — this is normal as the system matures.
• 20s and 30s: usually the most regular and predictable phase.
• Perimenopause (40s–early 50s): cycles often shorten, then lengthen and skip, before periods stop altogether at menopause.

Because the pattern shifts over the years, revisit your average cycle length now and then so your predictions stay accurate.

Tracking your cycle isn't only about pregnancy — it gives you a baseline for your own health. When you know your normal, you notice changes early. Start with the Period Calculator to map the months ahead.

This article is general information, not medical advice. For personal guidance about your periods or any health condition, consult a qualified doctor or gynaecologist.