Calculating your period and menstrual cycle calendar: how do I do it?

Written by Marion Goilav

Relecture professionnelle

It's not uncommon for women to wonder when their period will arrive. This may be for a variety of reasons, such as organizing your activities around your cycle, planning a baby, or buying disposable or reusable sanitary protection.... So how can you reliably calculate the arrival of your period?

How do you calculate the date of your period?

Knowing the day of your period is both trivial and requires knowledge of the different phases of the menstrual cycle! To anticipate the onset of menstrual bleeding, you first need to establish the average length of your cycle.

Determining the length of your menstrual cycle

A menstrual cycle starts on the 1st day of menstruation and ends the day before the next period.

To do this, simply subtract the date of the first day of your last period from the date of the day before your last period. It's best to repeat the operation over your last 3 cycles to obtain the average length of your menstrual cycles.

Define your period and pregnancy calendar

Keeping track of your menstrual periods - and remembering all those dates on the calendar - can be tricky! Every woman is different, and can have short, long, regular or irregular cycles... So it's a good idea to note down the dates of your menstrual bleeding, ovulation or any other changes each month in a small notebook or paper menstrual calendar, if you want to keep track of your monthly bleeding! Your general practitioner, gynecologist, midwife or other healthcare professional can also use an obstetrical roulette to calculate this information for pregnancy!

Use a menstruation app

A very practical and smartphone-friendly method is to use a period app to automatically track your period calendar and the day of your upcoming uterine bleeding, and to better understand your symptoms throughout your cycle. There are plenty of menstrual apps on the market, so it's up to you to choose the one that suits you best, or read our comparison! They all have their own specific features, and some even go beyond simple period tracking, offering functions such as pill reminder,ovulation date, body temperature, ovulation calendar or pregnancy calendar... However, they all work on the same principle: all you have to do is answer a few questions about yourself and your previous cycles.

Is it possible to calculate your period with an irregular cycle?

As we've seen, the menstrual cycle is calculated by subtracting the date of the day before your last period from the start of your last period. But how do you calculate your menstrual calendar when your cycle is particularly irregular and unpredictable?

To do this, we need to understand how the female cycle works. It's divided into three stages:

  • The follicular phase (before the oocyte is released), lasts around 13 days.
  • The ovulatory phase (release of the egg), lasts around 24 hours.
  • The luteal phase (after release), lasts approximately 14 days.

In a conventional 28-day cycle, ovulation occurs on the 14th day, dividing the menstrual cycle in two. But in the case of a longer, shorter or irregular cycle, it's actually the follicular phase that lasts longer or shorter, while the luteal phase remains unchanged, lasting an average of 14 days (although it can normally last between 11 and 16 days).

So, to know the day of your next menstrual flow when you have an irregular cycle, it can be useful to know how to spot the signs of ovulation. All you then need to do is add an average of 14 days (perhaps a little more or a little less, depending on your previous luteal phases) to this date to find out your next menstrual flow.

The following signs indicate that you are approaching ovulation:

  • Increased body temperature
  • Increased production of egg-white cervical mucus. Vaginal discharge becomes heavier the closer you get to ovulation. Usually, when you reach the peak, you may even feel an oily sensation when wiping your vulva on the toilet. Then, from one day to the next, your secretions suddenly diminish, and you may even feel dry. This is a sign that you've just ovulated.
  • Stimulatedsexual desire
  • Sorebreasts or lower abdomen

All you have to do is pay attention to these symptoms and listen to your body to find your place in the cycle, or even use an ovulation test to confirm if you're trying to get pregnant! In the meantime, to avoid unpleasant surprises, you can wear menstrual panties as the fateful date approaches, to be prepared for any eventuality.

When does ovulation take place during the menstrual cycle?

As we saw earlier, the female rhythm is made up of 3 phases, with ovulation occurring on average around the 14th day before menstruation. This period corresponds to the moment when the ovary expels an oocyte, and lasts between 16 and 32 hours. Once expelled into the fallopian tubes, theegg is called theovum. The egg is therefore the gamete ready to be potentially fertilized by a spermatozoon. In the absence of fertilization, the uterus eliminates this mucous membrane, resulting in monthly blood loss.

For women who do not have a 28-day cycle, it is possible to determine their ovulation period a posteriori by subtracting 14 days (or the value of your usual luteal phase) from the date of their last period. If you had unprotected intercourse during this period, there's a possibility of pregnancy (it's not 100%): if your period is late, you can take a pregnancy test. You can then contact your health professional for advice.

Is it possible to ovulate twice during the same cycle?

Although very rare, it is possible for both ovaries to produce an oocyte each at the same time during the ovulation period. This is what gives rise to fraternal twins!
However, this double ovulation is simultaneous and only occurs during the same window of opportunity, at the time of the ovulation period, which lasts around 16 to 32 hours. It is therefore not possible to have an ovulation on the 14th day of the cycle and then a second ovulation several days later. While double ovulation can occur, there is only oneovulatory phase per cycle!

Is it possible to get pregnant outside the ovulation period?

To put it simply: Yes, you have to differentiate between the ovulation phase and your fertile period! Ovulation lasts only 16 to 32 hours, but theoocyte survives 12 to 48 hours after expulsion. Spermatozoa, on the other hand, can survive up to 5 days in the body! It is therefore possible to have fertilizing intercourse a few days before or after the actual ovulation period, thus affecting the window of fertility. Fertilization will then take place when the egg and sperm meet!

How does pregnancy affect the calculation of menstrual periods?

Pregnancy can affect the calculation of menstrual periods in two ways.

At the start of pregnancy

You've just become pregnant, in which case your periods are delayed. Pregnancy suppresses menstrual flow, as the endometrium becomes a soft nest in which the embryo implants. There is therefore no evacuation of the uterine lining, and consequently no menstruation. A delay in menstruation of more than a few days should therefore alert you to the fact that a potential pregnancy has begun, especially if your cycle is regular!

After childbirth

You've just given birth, in which case your period won't return until you're back from childbirth. Immediately after giving birth, you expel blood debris and possibly placental remnants linked to the pregnancy, the lochia, which can last 4 to 6 weeks. Then, the first post-delivery menstrual period can take place at the earliest 6-7 weeks after delivery, and more depending on whether or not you've been breast-feeding for a long time. These first periods are called the return of childbirth. The cycle will then regularize and return to your original natural rhythm.

FAQ for calculating your period

How to calculate the arrival of your period?

To calculate the arrival of your menstrual periods, you need to know how long your average menstrual cycle lasts: simply subtract the date of the day before your last period from the first day of your last period. You can then add this figure to the day of your last menstrual period to find out when your next periodis due!

How can I find out the date of my next period?

You can calculate the date of your next period by hand or via an app. If you listen carefully to your body, premenstrual syndrome symptoms (hormonal acne, chest pain, mood swings, lower abdominal cramps...) can give you an idea of when your period is due.

Do my periods always arrive on the same date?

It's very variable, and depends on each woman's cycle! Some women have particularly regular periods, while others have irregular cycles. Stress, contraception and other factors can also influence the onset of flow.

What are the dangerous days of the menstrual cycle?

If you don't want to have children at the moment, the fertile phase may be a dangerous one for you. Cycle observation methods such as symptothermia and the Billings method are effective ways of knowing your fertile period, and are therefore an effective means of contraception. These methods require support and training over several months before you can rely on them alone. It's not advisable to rely on the calendar to know your fertile period, as you can never be sure of a change in your cycle and a shift in ovulation. That's why effective natural methods are observational. Basically, they allow you to detect the onset of ovulation. Your fertile window extends from 5 days before ovulation to 48 hours afterwards.