Why do periods change dates?

Aug 24, 2023

‘Why does my period date change every month?’ is a question many of us have found ourselves asking. The ever-changing nature of some of our menstrual cycles can be puzzling, and sometimes frustrating, but it’s not uncommon to notice that the dates of your menstrual cycle seem to fluctuate from month to month. With the help of obstetrics and gynaecology doctor, Dr Emily, we'll delve into how you can calculate your next period date and suggest five period calculator apps that can help you track your cycle effectively. 

In this article we’ll be covering: 

  • Why Does My Period Date Change Every Month?
  • How to Calculate Your Next Period Date
  • 5 apps to help you track your period

Why Does My Period Date Change Every Month?

On average, a menstrual cycle lasts between 28 to 32 days, although variations are entirely normal. The cycle begins on the first day of your period and ends on the day before your next period starts. Dr. Emily, a health expert, explains that the menstrual cycle begins on the first day of your period in the follicular phase. During this phase, hormones are released to stimulate follicles on the ovary and the lining of the womb begins to thicken. 

Ovulation, when an egg is released, happens at the end of the follicular phase, usually 14 days before your next period. Following this, the luteal phase further thickens the lining of the womb in preparation for pregnancy. If no pregnancy occurs, the hormone levels drop and cause the lining of the womb to shed away during menstruation. Whilst this is the average pattern of a menstrual cycle, several factors can influence why your period date changes from month to month, including:

Hormonal changes: The dates of your period can change due to various factors, as Dr Emily points out, one of the key factors is hormonal changes, such as imbalances or fluctuations which can affect the length and regularity of your menstrual cycle. Stress, changes in weight, illness, or certain medications can all influence your hormones and consequently, your period dates. Additionally, ovulation timing plays a crucial role in determining the timing of your period. Any shift in ovulation due to stress, illness, or lifestyle changes can cause your period date to change. 

Ovulation timing: Ovulation, the release of an egg from the ovaries, plays a crucial role in determining the timing of your period. The timing of ovulation can also vary due to factors like stress, illness, or changes in lifestyle. Since the length of the menstrual cycle is counted from the first day of your period to the day before your next period, any shift in ovulation can cause your period date to change.

External influences: Environmental factors such as changes in sleep patterns, travel, or significant life events, can disrupt your hormonal balance and, consequently, affect the regularity of your menstrual cycle. Your body's response to these external influences can cause your period to arrive earlier or later than expected.


How to Calculate Your Next Period Date

While each person's menstrual cycle is unique, tracking your periods can help you understand your body better. To manage these fluctuations, Dr. Emily suggests tracking your periods either manually or using an app. By recording the start date of your period for a few months, you can establish an average cycle length, which will help you predict your next period date more accurately. Although each person's menstrual cycle is unique and variations can occur, tracking can increase overall awareness of your health. Here's a simple method to calculate your next period date: 

Record your menstrual cycle: Keep track of the start date of your period for at least a few months. Note down the day your period begins and your last period date. This will help you establish an average cycle length.

Determine your average cycle length: Add up the number of days between the start of your period and the start of the next period for each month you recorded. Divide the total by the number of months to find your average cycle length.

Predict your next period: Once you have your average cycle length, count forward from the first day of your last period to determine the approximate due date of your next period. Keep in mind that this method provides an estimate, and individual variations may occur.

To make tracking your menstrual cycle even more convenient, several period date calculator apps are available to help you stay on top of your cycle. Here are five popular options:


Flo is a popular period tracking app that offers cycle predictions, symptom tracking, and a community of users to connect with. It also provides educational content related to menstrual health.


Clue is a period and ovulation tracker app that utilises scientific methods to predict and track your menstrual cycle. It provides personalised insights and reminders based on the data you input.

Moody Month

Moody month is great for those looking for daily advice on how to optimise their general wellbeing.


Wild caters to those who are active and want to know how their period can be added to their workouts, offering AI which curates your individual pattern of symptoms to understand what it means for your training.


Eve by Glow is a period and sex app offering a period tracker, sex and health log, fertility calendar, health log, sex quizzes, PMS symptom and mood tracking, ovulation calendar and community forums.

The seemingly unpredictable nature of periods can add unnecessary stress to life, Fluus is both sustainable and convenient, offering a way to alleviate anxiety and uncertainty around your period. Fluus fully-flushable period pads are delivered straight to your door and easily disposed of in a flush.