The egg can be fertilized for about 24 hours after ovulation. Spermatozoa can live within the female reproductive tract for three to four days. Therefore, fertilization is possible within those five days. In order to prevent unwanted pregnancy, intercourse must be avoided during this time span.
The concept of natural birth control methods is to determine the fertile phase by observing one's body. There are several variations of natural birth control. [1]

Calendar rhythm method
This very simple method relies on calculating a woman's fertile days on the calendar by initially writing down the first day of menstruation and then determining the moment of ovulation which mostly takes place a fortnight before menstruation.
Pearl Index*: between 12 - 15, that is, 15 out of 100 women, who use this method during one year, become pregnant. [2][3]

Temperature method
Ovulation causes an increase of 0.2 degree Celsius in body temperature. Once written down in a calendar, the woman can determine her fertile days indicated by the rise in temperature.

Cervical mucus examination
Since the consistency of the cervical mucus changes during the cycle it's important to check the mucus every day. The use of a calender may also help to keep a record.

Sympto-thermal approach
This approach combines the temperature method with the cervical mucus examination by means of a calender.
Pearl Index*: between 1,8 and 2,7; that is, if used correctly, this method is equally safe to pill or copper IUD. [1] [2] [4]

LH Ovulation Test
The detection of the luteinizing hormone (LH) is one of the most secure natural birth control methods. By means of test strips which are placed vertically into the urine cup the LH can be measured and the time of ovulation detected. A computer then calculates fertile and infertile days.
Pearl Index*: 5-6 [5]

On the one hand, the disadvantages of this method are a missing protection against venereal diseases and a high degree of discipline in conduct. On the other hand, the advantages are the lack of drugs, foreign objects or other material such as latex. This is why women feel a new appreciating body feeling and harmony with nature.

* The Pearl Index (PI) is used as a statistical estimation of the number of unintended pregnancies in 100 woman-years of exposure. A PI of 85 or 90 corresponds to no birth control methods used. A lower Pearl index represents a lower chance of getting pregnant. The Pearl Index can change depending on studies and observations. [3]