Final answer:
The amenorrhea method relies on the natural postpartum infertility caused by breastfeeding to prevent pregnancy. However, this is a behavioral method with potentially high failure rates and must be used carefully according to certain conditions for it to be effective.
Step-by-step explanation:
The method of avoiding pregnancies based on the postpartum infertility that many women experience when they are breast-feeding is known as the amenorrhea method. The amenorrhea effect refers to the absence of menses during the postpartum period due to lactation. Breastfeeding can serve as a natural contraceptive method due to the hormonal changes it initiates in the woman's body. These hormonal changes suppress the release of FSH and LH hormones, which in turn inhibits ovulation and creates a natural period of infertility.
However, it's important to note that this method is part of the behavioral methods of birth control, which are generally the least effective. In typical use, these methods, including natural family planning and withdrawal, have high failure rates, sometimes as high as using no contraception at all. For breastfeeding to be reasonably effective as a contraceptive, it must be exclusive and frequent, and menstrual periods must not have resumed.