Knowing when you are fertile
There are several different methods of determining the fertile period, with varying degrees of accuracy.

Take your temperature
One to two days follow ovulation, the temperature of a woman's body generally increases by several tenths of a degree Celsius.

This basal temperature method can also be supported by the use of a 'cycle computer' - also used for contraception. Some cycle computers even can determine the time of ovulation, based on the hormone levels in your urine.

Check your cervical mucus
The consistency of the cervical mucus changes several times over the course of a cycle. Following menstruation, there is little or no mucus, it then becomes more viscous and shortly before ovulation, clear and fluid, to facilitate the conveyance of sperm via the cervix to the egg cell.

Keeping simultaneous records of body temperature and the consistency of the cervical mucus increases the reliability of your calculations.