The Fertile Window

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The fertile window is the time in your cycle in which you are most fertile, it covers the 5 days leading up to ovulation and a day after (six fertile days). Ensuring you are having sex around and within this window will maximise your chances of getting pregnant.

Before you do anything else, you need to work out the length of your cycle as this will be different for everyone and does not always follow the standard 28 days. In fact, only about 30% of women are in the fertile window identified by guidelines (between days 10 and 17). Many women will reach this window much earlier and others much later.

Even for those who have regular 28 day cycles, there will be some natural variation and the timing of the fertile window can be unpredictable. We are often so reliant on apps, gadgets and gizmos for cycle tracking however, it is important that we get to know our cycle to help us identify when we are most fertile.

To establish your pattern, chart your periods for six months, or longer if you have just come off the pill (it can take up to nine months for regular cycles to return, although you may very well be fertile during this time). Any variation in cycle length within seven days is considered normal.

Day 1 of your menstrual cycle is the first day of bleeding, the length of your cycle will vary which will change the day of ovulation. If your cycle is 28 days you will ovulate on around day 14, but if your cycle is shorter you will ovulate earlier in the month and if its longer you will ovulate later in the month. The more regular your cycle, the better chances you will have each month of getting pregnant.

There are many factors that can affect your window including hormonal imbalances, alcohol, stress, sleep and nutritional status. Diet and supplements can help an important role during this window.

Calculating your cycles

Allowing for cycle variations (that is, differences from month to month), you can calculate that your six fertile days occur between:

  • Your shortest cycle length minus 20 days
  • And your the longest cycle length minus 10.

For example, if over the last six months your cycles have been 28, 26, 31, 25, 32 and 31 days, the shortest cycle is 25 days and the longest cycle is 32 days.

25 – 20 = 5 (earliest fertile day)

32 – 10 = 22 (lastest possible fertile day)

This means you would potentially be fertile between day 5 and 22.

Only a blood test will be able to tell you exactly when you’ve ovulated however, other factors like cervical secretions or body temperature can also indicate you are ovulating. During ovulation, you may notice your cervical mucus is wetter, clearer and more slippery. The change in cervical mucus is to help build an environment in which the sperm can be nourished and swim to the Fallopian tube ready for the egg to be released.  There is also a small rise in body temperature after ovulation takes place which you may be able to detect with a thermometer (body temperature can also be affected by illness, lack of sleep or alcohol).

Key takeaways

Very few women will follow a 28 day cycle so everyone’s fertile window will be different. Tracking your cycles, cervical mucus and body temperature can indicate when you might be fertile however, ovulation is a random variation and all women will experience natural variations.

An egg will last for 12-24 hours and sperm can last up to 3-5 days in the Fallopian tube, so we would recommend more sex in the lead-up to ovulation and after.  

Further reading

  • Fertility Tests Explained


    Fertility Tests Explained

  • IVF After 40


    IVF After 40

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