How to identify your most fertile days?

Dr Sarah Johnson

Reviewed by Dr Sarah Johnson on May 17, 2017
Written by Julie Macken.

There are only a few days (typically six) each cycle when you can get pregnant, the days leading up to and the day of ovulation. So, knowing when these days are in your cycle is key if you are trying for a baby. There are various signs and symptoms of ovulation and most methods of detecting ovulation rely on these to help you identify your fertile days. In this article, we look at these signs and symptoms and show how they can be used to help you:

Signs and symptoms of ovulation

  • Many women experience no ovulation symptoms. Women who experience symptoms find these can vary from cycle to cycle.
  • Some of the signs only occur after ovulation has happened which makes them of little help in predicting a woman’s fertile days in that cycle
  • The LH surge (detected by ovulation tests) happens 24 – 36 hours before ovulation

Ovulation is the name of the process that happens usually once in every menstrual cycle when hormone changes trigger an ovary to release an egg. This typically happens 12 to 16 days before the next period starts. These are the common signs and symptoms of ovulation.

LH surge

As a woman approaches ovulation, her body produces increasing amounts of a hormone called estrogen, which causes the lining of her uterus to thicken and helps create a sperm friendly environment. These high estrogen levels trigger a sudden increase in another hormone called luteinising hormone (LH). This LH surge causes the release of the mature egg from her ovary - this is ovulation. Ovulation normally occurs 24 to 48 hours after the LH surge, which is why the LH surge is a good predictor of peak fertility.

Temperature rise

A woman’s basal (resting) body temperature rises by 0.4-1.0°C after ovulation. This rise is normally too small for a woman to notice, but can be detected by an accurate thermometer.

Ovulation pain – Mittelschmerz

About one in five 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. It typically lasts for only a few minutes but it can last for longer, possibly as long as 24-48 hours, and can be a sharp, cramping, distinctive pain. It is rarely described as severe pain.

Changes in saliva

It has been found that a woman’s saliva changes according to the amount of the hormone estrogen in her body. During a woman’s monthly cycle, there is a large rise in estrogen a few days before ovulation and another smaller rise a few days before her period is due.

Changes in cervical mucus

During a woman’s menstrual cycle, the type and amount of cervical mucus (a secretion made by glands in the cervix) changes. It can be sticky or stretchy, white or cloudy. In the days leading up to ovulation it changes to being clear and stretchy (and is often compared to egg white – leading to the name ‘egg white cervical mucus’). This mucus is supportive to sperm and helps them to reach the egg.

Using these signs and symptoms to detect your most fertile days

  • Some methods can only identify when ovulation has already occurred, therefore they are of little value in helping a woman to conceive during that cycle.
  • Ovulation tests detect the LH surge which happens 24-36 hours before ovulation.
  • Clearblue Ovulation Tests are at least 99% accurate in detecting the LH surge.
  • The calendar method has been found to predict the correct fertile days in only a third of cycles1.

To maximise your chances of becoming pregnant, it’s important to time intercourse for the days leading up to and the day of ovulation. These are ways in which you can use the signs and symptoms of ovulation to help you identify these fertile few days each cycle.

Ovulation tests

Clearblue Ovulation Tests and the Clearblue Advanced Fertility Monitor are easy to use and detect the key fertility hormones from a urine sample. They are at least 99% accurate in pinpointing the LH surge and can identify up to six fertile days during each cycle when you are most likely to become pregnant.

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The calendar method

If you have a regular menstrual cycle, you can try to predict ovulation by carefully recording the length of each cycle. You will need to record the length of your cycle for at least a couple of months to start to build up a picture. For a very regular menstrual cycle, ovulation normally happens 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 with a regular menstrual cycle.

In recent studies the calendar method has been found to predict the correct fertile days in only a third of cycles2

Basal Body Temperature (BBT) recording

This method uses the fact that your basal body temperature rises by 0.4-1.0°C after ovulation by which time it is too late to optimize the chances of conception in that menstrual cycle2. To use this method accurately, you have to take your basal body temperature 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. There are also many factors, like sleep cycles changes, travel, and alcohol consumption that can change your BBT. Studies have shown that using a woman’s basal body temperature to predict her fertile days (to within a day) is less than 70%2 successful.

Ovulation pain – Mittelschmerz

Approximately one in five women have pain associated with ovulation. However, because the pain may occur during, or even after, ovulation this is often not that helpful to identify the days prior to ovulation and the day of ovulation itself when you are most fertile.

Saliva observations

It has been found 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 prediction and are hard to interpret2. Saliva can also be affected by smoking, drinking alcohol and eating.

Cervical mucus observations

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. In the days leading to ovulation it becomes clear and stretchy and is often likened to egg white. 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

What to read next?

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  1. Ellis JE., et al. Human Reproduction (2011) 26: i76
  2. Brezina PR., et al. Fertil & Steril (2011); 95(6): 1867-78
  3. Sarah Johnson, Lorrae Marriott & Michael Zinaman (2018): Can apps and calendar methods predict ovulation with accuracy?, Current Medical Research and Opinion 34:9, 1587-1594
  4. Godbert S, Miro F, Shreeves C, et al. Comparison between the different methods developed for determining the onset of the LH surge in urine during the human menstrual cycle. Arch Gynecol Obstet 2015;292:1153-61
  5. Institute for Human Data Science. Patient adoption of mHealth: use, evidence and remaining barriers to mainstream acceptance. Parsippany, NJ: IMS Institute for Healthcare Informatics: 2015 [cited May 17th 2017]. Available from: pdfs/institute-reports/patient-adoption-of-mhealth.pdf
  6. Setton R, Tierney C, Tsai T. The accuracy of web sites and cellular phone applications in predicting the fertile window. Obstet Gynecol 2016;128:58-63
  7. Howards PP, Schisterman EF, Wactawski-Wende J, et al. Timing clinic visits to phases of the menstrual cycle by using a fertility monitor: the BioCycle Study. Am J Epidemiol 2009;169:105-12
  8. Creinin MD, Keverline S, Meyn LA. How regular is regular? An analysis of menstrual cycle regularity. Contraception 2004;70:289-92

Learn more about fertility and mobile apps with Professor Joyce Harper

positive results

Do you want to find out if apps can predict ovulation with accuracy?3 Join Joyce Harper, Professor at the Institute for Women's Health at UCL, talking apps and fertility today on Twitter. There is a growing number of fertility applications (apps), although few of them disclose the algorithms that they use4,5. Most predictions of the fertile window given by fertility apps are generated from user data, such as date of last menstrual period and cycle length6, or the assumption of a 28-day cycle, with ovulation on day 147. Owing to the variability of menstrual cycle length, doubt has been cast over the reliability of calendar-based fertility apps8.

Find out more with Professor Joyce Harper


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