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Back to Bloody Basics: What is a Period?

By Dr Sarah Simons


Illustration by Linda Brinkhaus


The Oxford English dictionary defines period as “a length or portion of time, the interval of time between successive occurrences of the same state.”


But for roughly half of the population, the word period is inextricably linked with menstruation and specifically the five days of bleeding that occur once a month in a biological female of childbearing age.





Despite approximately 50% of the population having periods, the technicalities of what a period actually entails is rarely discussed. To get your head around periods and the menstrual cycle, it’s helpful to have an idea of your own anatomy and your uterus (womb) in order to understand the continuous interactions and changes kicking off within your reproductive system.

DAY 1-5: So what is a period and what actually is that blood?


Let’s consider the first day of bleeding as Day 1. From Day 1 to Day 5 your body is shedding the inner layer of your womb (uterus) containing small blood vessels in the form of menstrual fluid. This uterus lining has built up over the previous 4 weeks in order to support the possible products of conception - if an egg and a sperm fertilise, they become implanted in this max-strength uterus lining in order to become a growing foetus and 9 months later, a baby.


If there is no fertilisation and no implantation, the body recognises that it doesn’t need this womb lining and so sheds it in the form of menstrual fluid and thus your period via the vagina. Most menstruators lose about 50-80ml of fluid (roughly 3-5 tablespoons) whilst on their period, though this varies from person to person.


Some people also experience cramps just before their period and whilst they are bleeding. This is usually because the muscles in your womb and your abdomen are doing their best to contract and get rid of the menstrual fluid.




DAY 1-13: Behind the scenes and prepping your body for the next month


Whilst your womb is busy expelling the stuff you didn’t need, from day 1 to day 13 your brain is also busy releasing hormones to give your body the best chance of conceiving a baby next time.


These hormones act on the ovaries, helping the resident egg cells grow and mature. It takes roughly 13 days for an egg cell to become mature enough to be able to fertilise with a sperm. These hormones also cause the uterus lining to build up, encouraging blood vessels to form and the muscular layer in your uterus to get stronger.


Contraceptive pills like the COC (combined oral contraceptive tablet) or the POP (progesterone only pill) work by interfering with the normal release of these hormones and so prevent conception this way.





DAY 14: Halfway house and the Mexican wave of ovulation


By day 14, the middle of the menstrual cycle, your body has chosen an egg that it thinks is its best bet for fertilisation. This egg cell is released from an ovary in a process called ovulation and the egg is pushed down the Fallopian tube by tiny brush-like cells in a process a little like a reproductive system version of a Mexican wave.


Approximately 20% of people experience ovulation pains around this time and sometimes just on the one side where the egg is released. German medics helpfully coined the term “Mittelschmerz” for this symptom, which literally means “middle pain”.






DAY 15-28: The window of fertilisation opportunity


After ovulation, your uterus lining is thick, strong and ready to go. Once the egg cell has been released, there’s only a short window of opportunity for the egg cell to meet a sperm, fertilise and implant in this ideal uterine lining. The body releases different hormones at this stage to encourage conception over the 14 days in this second half of the menstrual cycle.


There are some contraceptive methods which rely on recognising this stage of someone’s menstrual cycle, but they tend to be pretty high risk as the length of time of this window varies between person to person and there is very limited scientific evidence to support this as an effective and safe method of contraception.


If a fertilised cell implants in the uterus, the body releases a specific chemical called hCG – the presence of this chemical is what pregnancy tests look for. If fertilisation occurs, hormone levels continue to stay high and support the cells as they develop into a pregnancy.


In most cases, when no fertilisation occurs, the hormone levels drop – this is the pre-menstrual stage where many people experience a change in mood and physical symptoms or discomfort. This hormonal dip triggers menstruation as the uterus lining breaks down and leaves the body as menstrual fluid in the form of a period. The body resets itself for another month and the whole cycle begins again.



Ultimately, just like people, periods come in all shapes and sizes. Different people experience different flows, cycle length, regularity and symptoms. And whilst this is often just natural variation, it’s important know your body and seek medical attention if you think something’s changed or just not right.

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