The rhythm (or calendar) method is one of the natural forms of birth control. A couple attempts to avoid sexual intercourse just before, during and just after ovulation.
The effectiveness of the rhythm method depends on how meticulously it is practiced. It also depends on the predictability of periods and consequently the ovulation period. In ideal circumstances the rhythm method could be 92% effective. This requires training and adherence to the guidelines of the rhythm method, which does not always occur. Therefore, for typical users the rhythm method is less than 87% effective.
Please see your doctor or family planning center if you wish to begin using the rhythm method. Ovulation occurs about 14 days before the start of a woman's period. Since sperm can live inside the vagina for up to 3 days, and an egg can stay fertile in the fallopian tubes and vagina for up to 3 days, intercourse must be avoided for more than just the day of ovulation. To be safer, a woman should abstain from sex (or use a barrier method) 5 days before ovulation, and 5 days after.
The rhythm method does not protect against STDs and is by no means a flawless method of birth control. A woman must have regular periods for the rhythm method to work, and many women do not. Another major limitation is that even if periods seem regular, a one-time change in the timing of ovulation can alter the expected days of infertility. If that is so, a woman may be having unprotected sex during a time when she can become pregnant.