Know your cycle – the 101 guide

Jessie Day, Senior Editor  |   3 Mar 2023

Phases, length, symptoms & more – the menstrual cycle, explained

The menstrual cycle. Regardless of our fertility goals, so much of our whole-body health is rooted in this amazing process, for at least 30 years on average (and up to 39.3 years, according to latest research) of our lifetime.

Knowing exactly how the menstrual cycle works gives you a fantastic baseline understanding of your health, subtle changes to watch out for and, crucially – what’s normal for you. It’s also a solid place to start when diagnosing conditions like PCOS and endometriosis, and managing symptoms. Plus, it’s a turn-key for understanding our fertility.

Whatever your goals for health – from management of cramps and hormonal headaches to trying for a baby, or preventing pregnancy, knowing where you are in your cycle is empowering. And for this, our go-to right now is Natural Cycles.


Here’s the nutshell

Natural Cycles is a temperature-based app that finds your unique fertile window, and helps you understand the best days to conceive. It also uses your cycle information and data, to give you an in-depth picture of your menstrual cycle.

Before you read down, flick over to Natural Cycles for an amazing 20% off their app subscription, which includes a free thermometer, with code TRB20.

Editor’s note – I’ve used Natural Cycles for years, and right across my fertility journey (before I joined TRB, in fact!) According to my app I have 56 cycles clocked up, to date – and I’ve relied on it for recalibrating after miscarriage, planning two pregnancies and as my hormone-free birth control, in-between. It’s my go-to for all-things menstrual cycle, periods, TTC and prevention.

There are so many fertility apps to choose from. But for us, understanding our menstrual cycle is all about the intel. We’ve partnered with Natural Cycles because of their incredible capability to use data from our unique cycle, to predict and confirm ovulation. As a result, Natural Cycles is a regulated medical device, it is CE marked and FDA Cleared.

If you have a condition such as endometriosis or PCOS, you can still use Natural Cycles to plan – or prevent – pregnancy and track your cycle. It just might take a little longer to find your fertile window.

There’s no better team to tap into for the 101 on menstrual cycle phases, symptoms, length and baseline info. Here’s our complete guide to the menstrual cycle, proudly in partnership with Natural Cycles.

What is the menstrual cycle, and how does it work?

Also known as the uterine cycle, our menstrual cycle is an essential part of reproduction. It’s also deeply interwoven with our whole-body health. You don’t need to be trying for a baby, to have an idea of the menstrual cycle’s influence on how you’re feeling, week-to-week.

The menstrual cycle involves a fine balance of hormones, released at key points to stimulate distinct menstrual cycle phases. For many women, this process results in a period, and/or symptoms like menstrual cramps, or hormonal acne and changes in mood, and energy. It’s easy to only think about periods, when we look at the menstrual cycle, but there’s actually far more involved.

Menstrual cycle length – how much does it vary?

You’ve guessed it – your menstrual cycle length is a unique aspect. It varies from person to person, and month to month, but on average, a regular cycle is pinned as lasting between 21 and 35 days. 28 days is the standard you’ll see on official content from the NHS or CDC, but in reality only 13% of us have 28-day cycles.

Medical conditions like PCOS and thyroid conditions are some of the most common causes for fluctuating cycle lengths – you may find for example, that your cycle lasts 33 days one month, and 27 the next. But variation can also be due to other factors, from stress to viral illness.

The key to being confident on menstrual cycle length is understanding what’s ‘normal’ for you. And for this, instead of your own calendar and memory, we recommend using an app like Natural Cycles (it stores and remembers your data, so you don’t have to).

Menstrual phases (they’re so important)

We like to think about our menstrual phases as the blueprint of our month. For Natural Cycles, these cover two distinct periods of time – follicular and luteal – punctuated by ovulation (when we release an egg from an ovary) and our period (when our uterine lining breaks down and leaves the body, as a period).

The follicular and luteal phases are essential to each other, and involve an intricate balance of hormones and triggers to move our cycle along.

We asked Natural Cycles for the essential info on how our follicular and luteal phases work.


Natural Cycles say – This is the first part of your menstrual cycle, beginning on the first day of your period, and ending with ovulation. The follicular phase is around 16 days long, but varies from person to person, and month to month.

The hormones you release during this time trigger follicle growth inside the ovaries, which gives this phase its name. There’s also a surge in the main female sex hormone, estrogen, which stops follicle production, allowing the existing ones to mature. The follicle which becomes dominant will finish off the process by releasing an egg cell – this is ovulation.

Natural Cycle users – Cyclers – will notice they have a lower basal body temperature during the follicular phase. And if you’re using an LH urine test, you’ll probably also see an increase in LH (luteinizing hormone) just before you ovulate. We use all of this information and data to help Cyclers confirm exactly where they are in their cycle.


Natural Cycles say – On to part two! This second stage is known as the ‘luteal phase’ and occurs after ovulation, building up to your period. The dominant follicle (corpus luteum) starts to produce the hormone progesterone, which causes your basal body temperature to rise. Progesterone also causes the uterine lining to soften, before shedding.

If your egg isn’t fertilized, the uterine lining breaks down, and shows up as your period. It’s a pretty amazing process.


Menstrual cycle symptoms  – when & why

Also known as PMS (premenstrual syndrome), your menstrual symptoms are unique to you and your cycle. These are the most commonly reported ones on the Natural Cycles app (be sure to DM us on Insta if you’d like more in-depth info on each!)

  • rapid mood changes (often referred to as ‘mood swings’)
  • feelings of anxiety, low mood or irritability
  • changes to sex drive and libido (this could be an increase or decrease)
  • insomnia or tiredness
  • appetite changes
  • bloating, stomach pain
  • indigestion and/or constipation
  • cramping and/or lower backache
  • sore or tender breasts
  • headache
  • changes to skin and hair (for example, hormonal acne)

Alongside planning or preventing pregnancy, as Cyclers with Natural Cycles, we’re able to use the app and its powerful algorithm to work out ovulation, predict key points in our cycles, and plan month-to-month, week-to-week, day-to-day around how we’re feeling, and our symptoms.

From rescheduling that HIIT session to planning for a few earlier nights – and maybe some carb-loading – it’s empowering information making a difference to every cycle, and our daily lives. We’re huge fans.

Our readers get 20% off a Natural Cycles subscription, 15 ovulation tests, and a free thermometer, with code TRB20. Start your journey today and let us know how you get on. 

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