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    What is the menstrual cycle? Why it’s so much more than your period

    Published 27 February 2024
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    Medically reviewed by Dr. Jennifer Boyle, Obstetrician and gynecologist, Massachusetts General Hospital, Massachusetts, US
    Written by Ella Braidwood
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    Understanding the phases of your menstrual cycle can help you understand your body better. Here’s the lowdown — from how long your cycle might be to what can make it irregular.

    You might have heard the terms menstrual cycle and menstruation used interchangeably, but did you know they’re not the same thing? While they sound similar, your period is actually just one part of your cycle

    Confused? You’re not the only one. In a 2023 study, eight in 10 US teens said they were taught more about the biology of frogs than of the human female body in school — and many said there is still a serious lack of communication and education surrounding menstruation. More research on 125 women in the US found that 49.6% didn’t know the average number of days in a regular menstrual cycle, and 47.2% said they weren’t sure what ovulation is. 

    Understanding the different menstrual cycle phases can help you get to know your body better — from when you’re most fertile to why you might feel a bit off in the days leading up to your period. So, let’s break it down.

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    Key takeaways 

    What is the menstrual cycle?

    Simply put, menstrual cycle is the term used to describe the process your body goes through each month in preparation for a potential pregnancy. A new cycle starts on the first day of a new period and lasts until the day before your next period. 

    While your period is a big signifier that a new cycle has started, there are lots of other processes and hormonal changes that happen during your menstrual cycle, too. 

    How long does it usually last?

    Your menstrual cycle is unique to you. While the average length of a cycle is 28 days, yours may last from 21 to 35 days and still be considered normal. This means that you may have a period every 21 to 35 days. The bleeding can last between a couple of days and a week. 

    However, it can be difficult to talk about “normal” menstrual cycles as they can change throughout your life

    Hormonal birth control, puberty, the weeks and months following pregnancy, and perimenopause can all impact the length and regularity of your cycle. This is because all of these can impact your hormone levels — the chemicals that keep your cycle going. 

    What are the different menstrual cycle phases?

    Your menstrual cycle is driven by the rise and fall of your sex hormones (progesterone, estrogen, luteinizing hormone, and follicle-stimulating hormone). These natural chemicals are released at different points in your cycle to trigger different processes. They’re fundamental in supporting you as you have your period and potentially prepare for pregnancy. You can think of your cycle as two phases or parts.

    Follicular phase

    The first part of your cycle is known as your follicular phase. It starts on the first day of a new period and runs up until the day that you ovulate. If you have a 28-day cycle, this means the follicular phase could last around 14 days. While the length of your period is unique to you, it’s considered normal if your period lasts between two and seven days.

    In the week following your period, your levels of a hormone called follicle-stimulating hormone (FSH) begin to increase. As FSH increases, it stimulates the growth of the follicles in your ovaries, and your estrogen levels rise, leading to the lining of your uterus thickening. You might be wondering what follicles are. Simply, they’re little sacs of fluid in your ovaries where eggs develop and get ready for ovulation. 


    Your cycle can be split into two, but it can be helpful to think of ovulation as its own event. It’s a pretty big deal, after all. Ovulation describes the moment when one of your ovaries releases an egg. Your body produces a hormone called luteinizing hormone (LH), which triggers this process. If your cycle is around 28 days, then this happens around the midpoint or day 14.

    Luteal phase

    After you’ve ovulated, you enter your luteal phase. This is the second phase of your cycle and runs through to the day before your next period. During this time, your progesterone levels rise, which causes the glands in the lining of your uterus to thicken. You might wonder why this is so crucial. Well, if the egg you released during ovulation was fertilized by a sperm, then it may travel down your uterine tubes and implant into the lining of the wall of your uterus. It helps if the lining of the walls of your uterus are thick and ready to provide nutrients so your fertilized egg can attach firmly and grow. 

    If you don’t conceive in the week before your next period, your estrogen and progesterone levels begin to decrease, causing the lining of your uterus to break down, shed, and leave your body in the form of your period. 

    The thought of losing the lining of your uterus might sound alarming, but most people only lose around 60 ml, or one and a half shot glasses full, of blood during their period. If you notice that your period is particularly heavy (you’re soaking through a tampon or menstrual pad every hour), you should speak to your health care provider. 

    Your period marks the start of a new cycle, and your body goes through this whole process all over again. 

    Does your menstrual cycle change over time?

    As you move from your teens to your 20s, 30s, 40s, and beyond, you change as a person, and so does your cycle. Dr. Sara Twogood, an obstetrician and gynecologist at Cedars-Sinai Medical Group, US, explains that normal lifestyle habits can impact your cycle. “The menstrual cycle can fluctuate from month to month and can be influenced by factors such as weight, exercise, stress, and travel,” she says. Similarly, you may see changes in your cycle as you age. 

    Generally, when your period first starts, triggering your first menstrual cycle, it’s more common for your cycle to be longer and your period to vary in how heavy it is. It can be hard to predict when you’ll have your first period. Most people start their period around the age of 12, but it is normal for it to start earlier or later. It can take a few months or cycles before your period works into a regular pattern for you. Then, in your 20s and 30s, your cycle is likely to be more regular, lasting a similar length of time between periods. 

    However, as you start to transition toward menopause, your cycle can change again. You reach menopause when you haven’t had a period for 12 months, but your cycle doesn’t just stop overnight. The years leading up to menopause are known as perimenopause and can begin anytime between your late 30s and 50s. It most often happens in your 40s, and the average age to start perimenopause is around 47 years old. 

    Perimenopause can last around four to eight years, and it’s a time of major hormonal changes. The levels of estrogen produced by your ovaries start to fluctuate, which means that your periods might become irregular, your cycles may become longer, and you may start to experience some of the symptoms associated with perimenopause, such as hot flashes and night sweats. 

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    You also go through serious hormonal changes during pregnancy and in the time after you’ve given birth. During pregnancy, your menstrual cycle pauses. Your ovaries don’t release eggs or ovulate, and you don’t have a period. After you’ve given birth, your estrogen and progesterone levels fall. How soon your cycle returns after giving birth depends on whether you are breastfeeding or not. It is possible, however, for your cycle to return quickly after giving birth, and you may even have a period within five to six weeks

    Trying to figure out what your cycle will be like after giving birth is tough because everybody is different. If you’re exclusively breastfeeding your baby, then it may take longer for your cycle to return. This is because of the hormone prolactin. It’s important for milk production during pregnancy and after birth, and it can delay the return of your period. This is not dangerous or unhealthy. If you’re not sure if what you’re experiencing is normal after giving birth, chat with your health care provider.

    Is your menstrual cycle irregular? 

    As your cycle is so unique to you, it can be hard to define what an irregular or abnormal cycle is. A typical menstrual cycle lasts 21 to 35 days, and a normal period lasts about seven days. It would be thought of as being irregular if you had a period every 20 days or less or didn’t have one for more than 35 days. Similarly, if your cycle varies in length by more than seven to nine days, it could be considered irregular. 

    If you don’t have a period for two months in a row and you aren’t pregnant, you experience bleeding between periods, or your period is much heavier, lighter, or longer than usual, then this may be a sign that something isn’t quite right. If your periods become more painful or your PMS symptoms are more severe, then you should schedule an appointment with your doctor. 

    It’s totally normal to have slight fluctuations in your cycle length. If you track your period using an app like Flo, you’ll be able to see what’s normal for you. “An irregular period every once in a while is very common — it is typically recommended to be evaluated if periods are consistently irregular or irregular more than two cycles a year,” advises Dr. Twogood.

    Changes to your cycle can be concerning, so if you ever have any questions or are unsure of what’s going on, then it’s best to reach out to your health care provider. There’s no such thing as a silly cycle question.  

    What could cause an irregular cycle?

    Between lifestyle factors and underlying health conditions, there are lots of reasons why you might see a change in your cycle. They include: 

    This is by no means an exhaustive list, and as Dr. Twogood highlighted, slight changes in your cycle are common and often nothing to worry about. However, if you do notice something different for you, then the best thing to do is speak to your doctor. 

    When to see a doctor

    The idea of talking to a doctor about your cycle might feel a bit awkward. But, if something feels off, then know that they’re trained to support you. When it comes to your health, there’s nothing to be embarrassed about.

    Speak to your health care provider if: 

    Dr. Twogood says that while it’s really important to get the help you need when you notice changes, it’s also really helpful to keep your health care provider in the loop with what’s normal for you. “I encourage people to see a gynecologist for any concern with their cycle,” she says. “Sometimes some education and reassurance can be very helpful.”

    More FAQs

    Why do periods change dates?

    It’s normal for your period to vary by a couple of days each month because your cycle would be considered to be normal if it lasts between 21 and 35 days. There can be lots of reasons why your period might change dates, including how stressed you feel and the exercise you’re doing. It’s a good idea to track your period using an app like Flo so you can monitor if your cycle length changes from month to month. 

    How late can a period be?

    Annoyingly, there’s no “right” answer to how late a period can be because everyone’s cycle length is different. Generally speaking, your cycle can vary in length by seven days and still be considered normal. Your period is technically only considered to be late if it’s eight days since you expected your period. At this point, it’s worth taking a pregnancy test if there’s a possibility that you could be pregnant.

    Can your period skip a month?

    “It’s not uncommon for people to skip a period every once in a while,” says Dr. Twogood. “Usually, this is nothing to be concerned about if it happens just once or twice a year, but the rest of the periods are normal.” Using an app like Flo to track your period and premenstrual symptoms can help you spot when you’ve missed a period.


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