A chapter in 'Fertility' gives further details. It is true that barrier methods do not need to be used on every day of the cycle. A knowledge of fertility awareness can help couples to identify the days when they are fertile, thus reducing the use of barriers to a minimum while ensuring an effective means of avoiding pregnancy. There are special considerations, however. The use of spermicides during the pre-ovulatory phase can make the recognition of the mucus symptom difficult.
Some women rely more heavily on changes in the cervix.
It is important that couples understand the failure rate associated with barrier use. This will vary dependent on the chosen barrier - male or female condom or diaphragm and spermicide. As women recognise their limits of fertility and time of maximum fertility, many will choose to avoid intercourse completely during the days of peak fertility. The theoretical or method effectiveness is the maximum effectiveness of the method when used without error or omission, that is when used according to the instructions. In theory the effectiveness rate for the sympto-thermal method, for example is around 98 per cent.