Understanding the menstrual cycle and fertility

What the menstrual cycle is

The menstrual cycle is a series of natural changes, controlled by hormones, that prepares the womb for a possible pregnancy each month. It runs from the first day of one period to the day before the next. A cycle length of around 28 days is often quoted as typical, but anything roughly between 21 and 35 days in adults is common and normal, and cycles can vary from month to month. The main hormones involved — including oestrogen and progesterone from the ovaries, and signals from the brain — rise and fall in a coordinated pattern. This hormonal orchestra drives the release of an egg and the build-up and shedding of the womb lining.

The phases of the cycle

The cycle has several overlapping phases. It begins with the period itself, when the womb lining is shed as menstrual bleeding, typically lasting a few days. In the days that follow, rising oestrogen thickens the womb lining again and an egg matures in the ovary. Around the middle of the cycle, ovulation occurs: a mature egg is released from the ovary and travels towards the womb. After ovulation, the ovary produces progesterone, which maintains the womb lining in case a fertilised egg implants. If pregnancy does not happen, hormone levels fall, the lining is shed, and a new cycle begins. The timing of ovulation, not the period, is the key to fertility.

Ovulation and the fertile window

Fertility centres on ovulation and the days around it. An egg survives for only about a day after it is released, but sperm can live inside the body for several days, so the fertile window spans roughly the five days before ovulation and the day of ovulation itself. In a typical cycle ovulation happens around the middle, but this varies widely between people and even between cycles, which is why predicting it exactly can be tricky. Some people notice signs of ovulation such as a change in vaginal mucus to a clearer, stretchier consistency, a slight rise in body temperature afterwards, or mild one-sided lower tummy discomfort. These signs can help identify the fertile time.

Tracking your cycle

Tracking the cycle can help whether you are trying to conceive, trying to avoid pregnancy, or simply understanding your body. The simplest method is recording the dates of your periods to learn your usual cycle length and spot changes. People trying to conceive sometimes also watch for fertile signs, use ovulation predictor kits that detect a hormone surge before ovulation, or track body temperature. Apps can make recording easier, but they only estimate the fertile window and are not reliable enough to be used alone as contraception. For preventing pregnancy, established contraceptive methods are far more dependable, and a clinician or pharmacist can explain the options.

When to seek help

It is worth speaking to a GP if your periods change noticeably or cause problems: periods that become very heavy, very painful, irregular, unusually frequent or that stop for several months without pregnancy all deserve assessment, as do bleeding between periods or after sex, and any bleeding after the menopause, which should always be checked. For fertility, general advice is to see a GP if you have been having regular unprotected sex for a year without conceiving, or sooner — around six months — if the woman is over 35 or there are known reproductive health concerns. Many causes are treatable, and early assessment opens up more options.