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Why do women have periods?

Join us as we take an in-depth look at the physiology behind the menstrual cycle and answer the question ‘why do women have periods?’ We’ll look at the length and phases of the menstrual cycle and also cover the average amount of bleeding that happens during menstruation. Also, if you want to know what animals have periods, we’ve got a surprise in store! Read on to find out more….

Why does a period happen?

Humans are unique in many ways and when it comes to our reproductive health it’s no exception. In fact, menstruating is extremely rare in the animal kingdom, it’s only primates, bats, spiny mice and elephant shrews who menstruate! Other mammals such as dogs may bleed when they’re fertile, but our physiology means for the most part our fertility indicators are hidden.

When it comes to the menstrual cycle, there is a lot going on. While the cycle may feel pretty mystical or a downright mystery, once you know what you’re looking for it becomes a bit easier to understand what’s going on, and when to expect our periods.

Periods happen to be the most recognizable part of our cycles, because, let’s face it, bleeding from your vagina is hard to miss! However, the unseen happenings going on behind the scenes have a direct impact on our bodies and are the very reason why periods happen… We’re talking about hormones.

The menstrual cycle is split into two key phases, the follicular phase, and the luteal phase. Your period happens in the follicular phase and marks the very start of your cycle. At this point hormone levels are relatively low. As the days go on, levels of the hormone estrogen begin to rise, this peaks right before your ovary releases an egg cell (this is called ovulation). After this estrogen levels start to decline. After ovulation happens you enter the luteal phase - at this point, the hormone progesterone is the main sex hormone at play.

Progesterone is the hormone responsible for those premenstrual syndrome symptoms, such as tiredness and breast tenderness. However, this hormone is also really important during early pregnancy. Progesterone levels are higher during the second part of the cycle. What’s happening at this phase in your cycle is your body is getting ready to grow a baby, and progesterone works to prepare the uterus for this.

If an egg cell is fertilized after ovulation, progesterone helps the fertilized egg attach to the lining of your uterus, also known as the uterine wall, where it will develop into a fetus.

However, if this implantation doesn’t happen (and there is no fertilized egg cell) the levels of progesterone drop. This is the cue for the lining of your uterus to shed. The bloody tissue exits through the vagina and hey presto, your period starts! And so the cycle begins all over again.

What’s the average age to get your first period?

Most girls get their first periods between 10-15 years of age, with 12 years old being the average age. First periods can be lighter or more irregular than the ones we get when we’re older.

Like all things relating to our reproductive health, there is a lot of variety and you can begin menstruation when you’re older or younger and still be perfectly healthy. However, it’s always a good idea to talk to a professional if you have questions about your reproductive health. 

How long is a typical menstrual cycle?

Until recently the textbook menstrual cycle was always thought to be 28 days in length. Our recent research into cycle length showed that in fact, on average, most menstrual cycles are actually 29 days long, and only 13% of cycles follow the 28-day rule. Cycle length varies significantly between individuals, some of us are like clockwork and others will experience different lengths and irregularities in our bleeding. 

Your age, diet, whether or not you’re on birth control, plus so many other factors can impact the cycle. Stress has also been shown to mix things up, it can delay ovulation and thus cause cycles to be longer than normal, causing your next period to be late! Life phases such as going through puberty or menopause can also impact regularity. 

What’s a normal amount of bleeding during my period?

Again, the average amount of bleeding varies between individuals - when it comes to a normal menstrual cycle, what really matters is what’s a normal cycle for us. Some people will bleed for just two or three days, while others may bleed for around a week or longer! 

It’s thought that on average a woman will only lose around two or three tablespoons' worth of menstrual blood during her period, but many of us can testify to having experienced heavier bleeding. It’s also common to pass blood clots during your period

A heavier menstrual flow can be caused by many factors such as using the copper IUD or experiencing hormonal imbalance. Chronically heavy bleeding or longer periods is called menorrhagia. If you find you're bleeding so heavily that you need to change a pad or tampon nearly every hour, or are experiencing severe pain during your period, you should talk to your doctor who will be able to offer advice and treatment. 

Other types of bleeding

Shorter, lighter menstrual bleeding is called spotting. This can happen throughout the cycle and isn’t necessarily linked to your period. For example, some people will experience spotting around ovulation or may even experience light bleeding in early pregnancy, this is known as implantation bleeding. 

Those on hormonal birth control such as birth control pills may also experience a type of bleeding known as withdrawal bleeding. This isn’t technically a period but happens due to the change in hormone levels that happen when you take placebo birth control pills or are on a break between pill packets. 

Withdrawal bleeding can also happen when you discontinue hormonal contraception and is the first bleed many will experience before their period returns. Keep in mind it can take some time for regular periods to resume when you stop using some types of birth control, such as the birth control shot. However, it's important you use another type of contraception as soon as you stop using a hormonal method if you want to avoid getting pregnant.

How do I know if I'm ovulating?

While tracking your period can give you an indication of where you are at one specific point in your cycle, you can also track when you’re ovulating if you know what to look out for. Finding and predicting ovulation is a really useful tool if you’re looking to either prevent pregnancy naturally or are trying to conceive, as the day of ovulation, and the five days prior is the only point when we’re actually fertile.

Tracking cervical mucus can give you an indication of the menstrual phase you’re in, as the amount and consistency of this discharge changes throughout the cycle. Mucus is most abundant during the fertile window and may not be present at all at the very end and the beginning of your cycle. This is an extremely useful fertility indicator, but can be hard to interpret.

You can also take ovulation tests. These measure the level of luteinizing hormone present in your urine. This is a key hormone in regulating the menstrual cycle and is highest around 24-48 hours before ovulation. 

While LH or ovulation tests can be a useful indication that ovulation may be approaching they do not confirm ovulation and it’s possible to get a positive result and then not ovulate. On the other hand, if you measure basal body temperature, you can see a definite spike that happens after ovulation, this is also caused by (yes you guessed it) hormones. Temperature levels stay slightly elevated during the second half of the cycle and drop again when a new cycle begins. 

Get to know your reproductive health

Until recently the only way to know your fertility was to manually chart your cycle using the above fertility indicators to uncover the hidden changes happening. While this is a good way to get to know your body better, it can be time-consuming and open to human error. At Natural Cycles we’ve developed the world’s first birth control app that can help you plan or prevent pregnancy naturally. 

The app uses body temperature data, paired with an algorithm that works out where you are in your cycle and predicts your fertile window. You also have the option to give the algorithm a boost by logging ovulation tests. Natural Cycles is designed to help you get to know your body while removing some of the labor of traditional fertility awareness-based methods.

Going hormone-free with NC°

Thanks for reading up on the ins and outs of menstruation. Interested in finding your fertile window while learning lots more about your body? Join more than 3 million women who’ve already switched, and check out how Natural Cycles can help you plan or prevent pregnancy naturally today.

Did you enjoy reading this article?

Are you ready to go hormone-free with Natural Cycles?

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Written By

Jennifer Gray

Jennifer Gray is an award-winning writer with more than five years’ experience covering reproductive topics ranging from birth control to planning pregnancy. She is passionate about providing women with accurate information grounded in science they can use to take charge of their own health - while also dispelling myths that exist within the field of women’s health. She holds a Master of Science from the University of Edinburgh and currently lives in Ireland.

Jack in a suit and tie holding a microphone and giving a presentation.

Scientifically Reviewed

Jack Pearson

Dr. Jack Pearson is a previously HCPC registered Embryologist with a PhD in reproductive medicine. Prior to joining Natural Cycles leading Medical Affairs, he worked for more than 10 years in a clinical setting working at some of the busiest fertility clinics in the UK. Today he spends most of his time working with experts at the world’s leading institutions to carry out important research with the vision to further the field of female health. He earned his PhD from the University of Sheffield specializing in Sperm Metabolism and currently lives in London.

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