Methods to identify your most fertile days

Identify your fertile days and act on them

Due to the variability in women’s menstrual cycles, it can be difficult to know when YOUR fertile days are, but we’ve seen that this knowledge is key if you are planning to get pregnant. It can be tiring and stressful to see the excitement of trying for a baby disappear into a monthly roller-coaster of emotions and baby-making sex – identifying your fertile days and having sex then (as well as at other times of the month if you fancy!) can help avoid this happening to you.

What is an accurate method to identify MY fertile days?

Your fertility is controlled by natural hormones, so an accurate way to identify YOUR personal fertile days is to detect the changes in your key fertility hormones (luteinising hormone (LH) and estrogen) using simple home testing products like the Clearblue Ovulation Tests and the Clearblue Fertility Monitor.

Hormones detected
Typical number of fertile days identified
Clearblue Ovulation Tests Luteinising hormone (LH) 2

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Other methods you may hear about

Other commonly used methods to estimate when your most fertile days are for example, calendar methods, basal body temperature or saliva, are less accurate than hormone monitoring and are more likely to be affected by external factors such as illness or medication.

Which method is more accurate?

Ovulation Tests are at least 99% accurate in detecting the LH surge that happens 24-36 hours before ovulation, so they are more accurate than calendar and temperature methods at predicting the most fertile days. They are also easier and more convenient to carry out.

  • Using a calendar

    In recent studies the calendar method has been found to predict the correct fertile days in only 1/3rd of cycles.2

    If you have a regular menstrual cycle, you can try to predict ovulation by carefully recording the length of your cycle. You’ll need to record the length of your cycle for at least a couple of months to start to build up a picture. If you have a very regular menstrual cycle, you can assume that you will ovulate between 12 and 16 days before your next period starts. However, the day you actually ovulate can still vary by several days between cycles, even if you have a regular menstrual cycle.

  • Basal body temperature recording

    Studies have shown BBT prediction of the fertile days (to within a day) is less than 70%1

    This method uses the fact that your basal body temperature rises by 0.4-1.0°C AFTER ovulation. To use this method accurately, you have to take your BBT first thing every morning before moving out of bed, and it will take several menstrual cycles before you start to build up a picture of your fertility window.

    Because the temperature rise happens after ovulation it can only be used to predict ovulation in future menstrual cycles. That means you have to have a very regular menstrual cycle to be able to work out exactly when you will ovulate in the next menstrual cycle to make the most of your most fertile days.

    It’s also worth bearing in mind that your body temperature can rise for a number of other reasons (including illness, drinking alcohol, hot drinks, movement and restless sleep), which can confuse matters.


  • Saliva observation

    Some people think that your saliva changes according to the amount of the hormone estrogen in your body, and there are kits available that can help you to monitor these changes. They usually consist of a tiny microscope with a glass surface – you place some spit on the glass surface, wait for it to dry and then look through the microscope for a ‘ferning’ pattern that is caused by salt crystals in the saliva when your estrogen level is high. However, changes in saliva have been shown to be an unreliable method of ovulation predictionvi. Saliva can also be affected by smoking, drinking alcohol and eating.

  • Cervical mucus observation

    During your menstrual cycle, the type and amount of cervical mucus — a secretion made by glands in your cervix — changes. It can be sticky or stretchy, white or cloudy. By making a note of these changes, you can predict when you will ovulate so you can have sex on your most fertile days.

    The main advantage of this method is that it gives you a better understanding of your body, making you more aware of the changes your body goes through during a menstrual cycle. However, it can take time to learn how to ‘read’ the changes, and your cervical mucus can be influenced by other factors, including infections, sex and some medication.

  • Ovulation Pain – Mittelschmerz

    Mittelschmerz is one-sided, lower abdominal pain that occurs in women at or around the time of an egg is released from the ovaries (ovulation). About 1 in 5 women have pain associated with ovulation. The pain may occur just before, during, or after ovulation.

    There are several explanations for the cause of this ovulation pain. Just prior to ovulation, follicle growth may stretch the surface of the ovary, causing pain. At the time of ovulation, fluid or blood is released from the ruptured egg follicle and may cause irritation of the abdominal lining.

    Mittelschmerz may be felt on one side one month, then switch to the opposite side the next month, or it may be felt on the same side for several months in succession.

    Ovulation symptoms include lower-abdominal pain that is:

    • One-sided
    • Typically lasting minutes to a few hours, possibly as long as 24-48 hours
    • Can be sharp, cramping, distinctive pain
    • Severe (rare)

  • 1 Brezina PR., et al. Fertil & Steril (2011); 95(6): 1867-78
  • 2 Ellis JE., et al. Human Reproduction (2011) 26: i76

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