Which partner uses the contraceptive method, and which method is chosen, depends on a complicated mixture of social, cultural, and psychological influences. Today for the first time in history, men and women have reliable methods to enable them to make that choice freely and relatively easily.
The principle of choice is important, as it includes not only the choice of using family planning, but the choice of the birth control method most suited to the particular circumstances of the couple. Neither the man nor the woman can make a real choice until each has the basic knowledge of the different methods available, their efficiency in protecting against pregnancy, and their advantages and disadvantages. A measure of contraceptive efficiency used by many people is the pregnancy index which is calculated in the following way:
The number of pregnancies x 1200
Total months of exposure to pregnancy
The result is expressed as the number of pregnancies per 1200 months of exposure, or preferably as the number of pregnancies per hundred woman-years. This shows how many of every 100 women making use of the particular method chosen will become pregnant if the method is used for one year.
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