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.

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