Birthsong Childbirth Education & Support Services

Fertility & Menstruation

Natural Family Planning (Fertility Awareness)
Reviewed by Dr Philip Owen, consultant obstetrician and gynaecologist

What is fertility awareness?

This form of natural family planning helps identify the phases of a woman's menstrual cycle when she is most fertile as well as those phases when she is less fertile and, therefore, less likely to fall pregnant.

How reliable is natural family planning?

It is not reliable as a form of contraception when used on its own, but it can be helpful both for couples wishing to avoid pregnancy and for those actively trying to have a baby.

If a woman's cycle is irregular then natural family planning will be less reliable.

You should also bear in mind that none of the methods that come under the heading of natural family planning will protect you from sexually transmitted diseases.

How does natural family planning work?

Natural family planning is based upon certain facts about a woman's eggs and a man's sperm:

  • a woman usually ovulates 14 days before her next period.
  • for example, if a woman has a regular 28 day menstrual cycle, counting the first day of her period as day one, then she will usually ovulate on day 14. If her cycle lasts 32 days, then she usually ovulates on day 18.
  • once ovulation has occurred, the sperm has only 24 hours within which to fertilize the egg in the woman's Fallopian tubes.
  • sperm can survive for up to seven days following intercourse.

With this knowledge it now becomes possible to identify three phases of fertility during each menstrual cycle. However, if a woman's cycle is irregular then clearly it is more difficult to predict when ovulation is likely.

The first phase is the first infertile phase, or the first safe period when the woman is unlikely to fall pregnant. This starts on the first day of the woman's period and ends on the earliest date from which sperm could survive long enough to fertilize the egg. This first safe period is short because sperm can survive for up to seven days after intercourse and a woman may ovulate early. Therefore, unprotected intercourse during this first phase may result in pregnancy.

The fertile phase is the time when a woman is most likely to fall pregnant. Couples not wishing to become pregnant, should avoid intercourse during this time or use other methods of contraception such as condoms. If pregnancy is desired then this is the time when a woman is most likely to conceive. The fertile phase lasts from the end of the first phase until 24 hours after ovulation.

The second infertile phase or safe period when a woman is less likely to fall pregnant is more predictable than the first phase. This phase lasts from the end of the fertile phase until the beginning of the woman's next period.

Accurately identifying the time of ovulation is the cornerstone of natural family planning. The three principal methods of calculating when ovulation is likely to occur are:

  • the calendar method
  • the temperature method
  • the mucus test
  • The calendar method

What is the calendar method?

The calendar method makes use of our knowledge of when a woman is likely to ovulate. This enables many women to calculate their fertile and infertile phases described above. The fertile period is 12-16 days before the period starts.

How do I prepare for using the calendar method?

Before using the calendar method, a woman has to make a detailed note of her periods for six months. Each month, you should note the number of days between starting one period and the next. Then work out the longest and the shortest interval between your periods. Now you are ready to start calculating.

It can be hard to do it correctly and you will need a pen and paper. From the shortest interval you always subtract 18 days. If for example, the shortest time between starting one period and the next during the last six months was 27 days, by subtracting 18, you arrive at the 9th day after the start of your period.

From the longest interval you always subtract 11 days. For instance if the longest space between starting periods was 31 days, subtract 11 to arrive at the 20th day after the start of your period. Using the figures in this example, the first safe phase would be from day one to day eight, the phase when conception is most likely to occur would be between day 9 and day 20 and the second safe phase would be from day 21 to the start of your next period. Please note that these figures are an example only and you will need to do your own calculations to work out your own fertile and less fertile phases.

If your periods are more irregular, the unsafe (fertile) periods will be longer.

What demands does the calendar method make?

This method of calculating ovulation demands an accurate recording of your period intervals. If you do not wish to become pregnant you will need to be able to tolerate long periods without sex unless other forms of contraception are used. Alternatively, if a couple are trying to have a baby, it can help them identify the dates when conception is most likely to occur.

The temperature method (also known as the basal body temperature or BBT method)

What is the temperature method?

As we have described above, the chance of falling pregnant is much greater around the time of ovulation. The temperature method helps women find out when they are ovulating by taking their temperature every morning. The day after ovulation takes place a woman's temperature will go up by about half a degree Celsius under the influence of the female hormone progesterone.

How to use the temperature method

Measure your so-called 'base' temperature every morning after waking from at least five hours of undisturbed rest, and before getting out of bed. It is important that your temperature is measured correctly using the same, accurate thermometer every day. If possible, it should also be taken at approximately the same time every day. Keep a record of your daily temperature. As soon as three successively higher-than-average temperatures have been noted (based on six previous measurements that month), ovulation has taken place. This is called the 'three over six rule'.

How long does this fertile period last?

From the third day after ovulation - the third day of increased temperature - it is almost certain that the egg will not be fertilised and that the woman's body has entered the second infertile phase.

What are the drawbacks or limitatations of the temperature method?

Considerable motivation is required to measure and record the temperature in the correct manner each morning. This technique cannot be used to identify the first infertile phase, ie before ovulation. The temperature recordings may become confused if the woman has an infection such as a cold.

The mucous test

How does the mucous test work?

Over the course of every month, the mucus produced by a woman's vagina and cervix changes its characteristics because of the fluctuating levels of the female hormones oestrogen and progesterone. This will show how close or far from ovulation she is.

A few days before ovulation (when the egg is released) the mucus becomes transparent, watery and yellowish. On the day of ovulation itself, what comes out is thicker, wet and thread-like (like egg-white) and might also produce a moist feeling in the vagina. Within a day of ovulation there is less mucus and it is thicker and sticky.

How is the mucous test carried out?

Although the test is simple, it is important to practice and to record the findings in a diary.

Every morning (or every time you use the toilet) check to see what is coming out of your vagina and remove a little with your fingertip. To work out whether the secretion is at the stringy, ovulation stage mentioned above, press the sample against your thumb, then carefully separate your fingers. If the mucus draws itself out into a long 'thread' before breaking, you are probably ovulating. The second infertile phase starts four days after the day of peak mucus production (ovulation).

What are the drawbacks or limitations of the mucous test?

It becomes difficult to assess the nature of the mucus if you have recently had intercourse. This is because the presence of semen will make the mucus appear different and sexual arousal in a woman also makes the mucus more stringy.

The mucous test isn't a guaranteed form of family planning, so don't rely on it as a way to avoid getting pregnant. However, it is good to combine the test with other forms of contraception, eg condoms or diaphragms and, in cases where contraception isn't available, certainly better than nothing at all. If you wish to become pregnant then identifying when you ovulate will help you to identify your fertile phase.

Where can I find out more about natural family planning techniques?

It is possible to obtain instruction and advice in the use of natural family planning techniques (fertility awareness) from appropriately trained doctors and specialist family planning nurses. This is strongly recommended if a woman wishes to reduce her chances of falling pregnant and also gives her the opportunity to discuss alternative methods of contraception.

The opposite is also true; if a woman is having difficulty conceiving then awareness of when she is ovulating will enable her to maximise her chances of falling pregnant.

Based on a text by Dr Erik Fangel Poulsen, specialist


More Information about Fertility & Menstruation:


Aisle: Sustainable Period Products for Every Body

Supporting women's menstrual health through practical information and healthy products

Fertility Awareness Handbook

Fertility Awareness Handbook: The Natural Guide to Avoiding or Achieving Pregnancy

by Barbara Kass-Annese, Hal C. Danzer


Your Fertility Signals
Your Fertility Signals: Using Them to Achieve or Avoid Pregnancy Naturally

by Merryl Winstein


· Printable Blank Basal Body Temperature Chart (PDF)

· Instructions and Sample of How to Use Basal Body Temperature Chart (PDF)

Digital Basal Thermometer

Digital Basal Body Thermometer


Ovulation Predictor Test Strips

Ovulation Predictor Test Strips


Pregnancy Test Strips

Pregnancy Test Strips

THINX: For People With Periods

Period underwear!

The DivaCup: A Better Period Experience

Reusable menstrual cup.