Salient. Victoria University Student Newspaper. Volume 36, Number 7. 11th April 1973
The rhythm method is based on the fact that ordinarily only once a month does a woman produce a ripe egg which can be fertilised by the male sperm and then develop into a baby. There is a period of no more than 24 hours during which the ripe egg remains alive and can be fertilised. If the fertilisation does not take place in the 24-hour period, the egg breaks apart and disappears.
This method must also take into account the fact that the male sperm can live for about 4 days after intercourse and still be able to fertilise an egg. Therefore, there is a period of between 5 and 11 days in the middle of the menstrual cycle when women cannot have intercourse. This length of time depends on how regular the woman's periods are.
To operate this method successfully, both the man and the woman must act responsibly and a lot of checking and research must be done before they embark on love-making. The main disadvantage of this method, however, is that it is by no means 100% effective. Out of 100 women who use this method for a full year, 24 will become pregnant anyway. This is a higher failure rate than any other method except the douche.