What do humans, some primates, some bats,
the spiny mouse, and the elephant shrew have in common?
They all get periods.
I'm Gabriella Kountourides,
and I'm studying for my doctorate at the University of Oxford.
And I am fascinated about menstrual cycles.
Menstruation is a natural process that affects
half of the world's population during their lives,
but it's little understood.
So here's five things you probably didn't know about periods.
First, some myth-busting.
Moon syncing is not a thing.
It's such a compelling idea,
the idea that menstrual cycles sync up with the cyclical waxing
and waning of the moon, much like the ocean tides.
The moon's cycle is 29.5 days,
but in 2019, an analysis of 7.5 million menstrual cycles
from 1.5 million women around the world
found no relationship at all.
Another commonly-held view is that when people live together,
their periods sync up. Humans love to find patterns,
and a certain amount of overlap is to be expected.
But actual syncing of cycles is not backed by data.
The textbook idea of a 28-day cycle is a bit of a myth too.
A normal cycle can be anything up to seven days
longer or shorter than this. Everyone is unique.
Hormones can be powerful... in a good way.
OK. Let's talk about oestrogen.
It's an incredible hormone
that we're only just starting to understand.
There's a specific type of oestrogen that's
made in the ovaries called estradiol.
This is released throughout the menstrual cycle,
and it peaks just around ovulation.
Research suggests oestrogen can have some powerful effects.
In 2021, neuroscientists
at the University of California, Santa Barbara,
for the first time tracked a woman's hormones throughout the cycle
and looked at whether these had any impact on the brain.
They found an increase in connectivity
throughout the brain at times when oestrogen was high.
One theory is that oestrogen causes our neurons
to form new connections to other neurons,
which helps with learning and memory.
98% of mammals don't have periods.
Lots of animals do have reproductive cycles,
like cats and elephants, but they reabsorb
the lining of the womb rather than shedding it as a period.
So why do humans menstruate?
The truth is, we don't know. One theory,
put forward by evolutionary geneticist Deena Emera,
is that it's a tug-of-war of energy between the mother and baby.
Human embryos bury deep into the lining of the womb,
which gives them more access to nutrients from the mother's body.
So the lining of the womb builds up nice and thick each month
to create a shield to protect the mother.
This thick lining also makes it easier to shed, so she doesn't
expend a huge amount of energy if a foetus won't ultimately make it.
The ancient Greeks thought the womb wandered around the body, literally.
And as it floated around,
they believed this wandering womb caused all sorts of diseases.
Bizarre as it sounds, this idea stuck around for centuries,
and so did the idea of hysteria -
a kind of catch-all pseudo-medical term
which was used to undermine women.
In 1931, a new term was coined - PMT, or premenstrual tension.
This was later renamed premenstrual syndrome or PMS,
and refers to any symptoms experienced
in the week before the start of a period.
Some argue the term "syndrome" is not useful
and it would be more accurate to refer to premenstrual symptoms.
PMS might be linked to inflammation.
Why do women experience premenstrual symptoms?
It could be down to inflammation.
Inflammation is the body's way of protecting itself from infection,
and just as our hormone levels change during a menstrual cycle,
so do levels of inflammation.
At ovulation, part of our immune system
is temporarily suppressed a little.
That's so a fertilised egg, if there is one,
can implant itself without getting rejected.
While inflammation is a sign of a strong immune system,
too much inflammation is bad.
Symptoms include headaches, tiredness,
changes in appetite - sounds a bit like PMS, doesn't it?
Some have suggested it's this rise in inflammation
in the second half of the cycle
that's actually behind what we call PMS.
So that's a quick rundown of a few facts,
myths and theories about periods.
And still, there's so much unknown about something that happens
to half of the world's population at some point in their life.
But that's exciting, because it means
there is so much more to discover.