Understanding your menstrual cycle can help you learn more about your reproductive health, recognise changes in your body and identify when you may be most fertile.
While every person’s menstrual cycle is different, it generally follows four key phases: menstruation, the follicular phase, ovulation and the luteal phase. Knowing what happens during each stage can make it easier to understand when ovulation occurs and when your fertile window is likely to be.
At Manchester Fertility, we know that understanding your cycle can be an important part of your fertility journey. In this guide, we explore the different stages of the menstrual cycle and explain how the fertile window fits into the picture.
What is the menstrual cycle?
The menstrual cycle is a series of hormonal and physical changes that prepare the body for a possible pregnancy.
The cycle is usually measured from the first day of your period to the day before your next period begins. Although a 28-day cycle is often used as an example, cycle length can vary considerably between individuals and from one cycle to the next.
Hormones including oestrogen, progesterone, follicle-stimulating hormone (FSH) and luteinising hormone (LH) play important roles in controlling the cycle.
The four phases of the menstrual cycle
1. Menstruation
Menstruation, commonly known as your period, marks the beginning of a new menstrual cycle.
During the previous cycle, the lining of the womb (endometrium) thickens in preparation for a potential pregnancy. If pregnancy does not occur, hormone levels fall and this lining is shed through the vagina.
The first day of menstrual bleeding is considered day one of the menstrual cycle.
The length and characteristics of a period can vary. Some people experience heavier bleeding, cramps or other symptoms, while others have lighter or shorter periods.
2. The follicular phase
The follicular phase begins on the first day of menstruation and continues until ovulation.
During this stage, FSH stimulates follicles in the ovaries to develop. Each follicle contains an immature egg. Usually, one follicle becomes dominant and continues developing towards ovulation.
As the follicle develops, it produces increasing levels of oestrogen. This hormone helps the lining of the womb thicken, preparing it for a potential pregnancy.
The follicular phase can vary in length, which is one reason why the timing of ovulation may differ from cycle to cycle.
3. Ovulation
Ovulation occurs when an ovary releases a mature egg.
A rise in luteinising hormone, known as the LH surge, triggers ovulation. In a typical cycle, ovulation occurs around 12 to 16 days before the next period begins, although the exact timing can vary.
The egg can generally be fertilised for around 12–24 hours after it is released. However, sperm can survive in the female reproductive tract for several days under favourable conditions.
This is why pregnancy can occur from sex in the days before ovulation as well as around the time of ovulation.
4. The luteal phase
Following ovulation, the body enters the luteal phase.
The follicle that released the egg develops into a structure called the corpus luteum, which produces progesterone. Progesterone helps maintain the lining of the womb in preparation for a fertilised egg to implant.
If pregnancy does not occur, hormone levels eventually fall. The lining of the womb then breaks down, resulting in menstruation and the start of a new cycle.
If pregnancy occurs, hormonal changes help maintain the womb lining and support the early stages of pregnancy.
What is the fertile window?
The fertile window refers to the days during the menstrual cycle when there is a possibility of becoming pregnant following unprotected sex.
Although the egg only remains viable for a relatively short time after ovulation, sperm can survive for several days in the reproductive tract. As a result, the fertile window generally includes the five days before ovulation, the day of ovulation and, potentially, the day after ovulation.
The most fertile days are usually the days immediately before ovulation and around ovulation itself.
However, predicting the fertile window isn't always straightforward. Ovulation does not necessarily happen on the same day of every cycle, particularly if periods are irregular.
How can you identify ovulation?
There are several ways to track changes associated with ovulation. These can include:
Tracking your menstrual cycle
Keeping a record of when your periods begin and how long your cycles last can help you understand your individual pattern.
However, calendar-based predictions are not always accurate, particularly if your cycles vary in length.
Monitoring cervical mucus
Changes in cervical mucus can occur around ovulation. Some people notice that it becomes clearer, wetter and more slippery around their most fertile days.
Ovulation tests
Ovulation predictor kits can detect the rise in luteinising hormone that typically occurs before ovulation. A positive result indicates that ovulation may be approaching, although it cannot guarantee that ovulation will occur.
Tracking basal body temperature
Basal body temperature can rise slightly after ovulation due to increased progesterone. Tracking your temperature over several cycles may help identify a pattern, although this method generally confirms that ovulation has occurred rather than predicting it in advance.
For people trying to conceive, combining different methods may provide more information than relying on one method alone.
Can you get pregnant outside your fertile window?
Pregnancy is most likely when sex takes place during the fertile window, but predicting exactly when ovulation occurs can be difficult.
Cycle length can change, ovulation may occur earlier or later than expected, and sperm can survive for several days. For this reason, simply counting days on a calendar cannot reliably determine when pregnancy is or isn't possible.
What if your periods are irregular?
Irregular periods can make it more difficult to predict ovulation and identify your fertile window.
There are many possible reasons for irregular cycles, including hormonal changes, stress, and changes in weight.
If your periods are consistently irregular, very heavy, unusually painful or have changed significantly, it may be worth discussing your symptoms with a GP or fertility specialist.
Irregular periods can sometimes indicate that ovulation isn't occurring regularly, which may affect your chances of conceiving.
Understanding your cycle when trying to conceive
If you're trying for a baby, understanding your menstrual cycle can help you recognise when you may be approaching ovulation.
However, fertility involves much more than timing intercourse around ovulation. Factors affecting fertility can include age, ovarian reserve, sperm quality, ovulation, the fallopian tubes, the womb and other aspects of reproductive health.
If you have been trying to conceive without success, a fertility assessment can help identify whether there may be an underlying factor affecting your chances of pregnancy.
At Manchester Fertility, our fertility specialists can assess your individual circumstances and discuss appropriate investigations and treatment options with you.
When should you seek fertility advice?
You may wish to speak to a fertility specialist if:
- You have been trying to conceive for a year without becoming pregnant.
- You are aged 36 or over and have been trying to conceive for six months.
- Your periods are very irregular or absent.
- You have concerns about ovulation.
- You have a known condition that may affect fertility.
- You or your partner have concerns about your fertility.
- You have experienced recurrent pregnancy loss.
You don't necessarily need to wait before seeking advice if you have a specific fertility concern. Your GP or a fertility specialist can advise you based on your individual circumstances.
Understanding your menstrual cycle can be an important first step
The menstrual cycle involves a complex interaction between hormones, the ovaries and the womb. Understanding the four main phases, menstruation, the follicular phase, ovulation and the luteal phase, can help you understand what is happening in your body and when you may be most fertile.
If you're trying to conceive, remember that the timing of ovulation can vary, so your fertile window may not fall on the same days each month.
If you have questions about your menstrual cycle, ovulation or fertility, the team at Manchester Fertility can provide personalised advice and support.
Find out more about fertility assessments and treatments at Manchester Fertility.
Last updated: 21st October 2026
