The option of contraception is raised by the Talmud (Tractate Yevamot 12b), where the use of a pessary is discussed for women who are too young to get pregnant, presently pregnant, or nursing. In each case, either the woman or her child is at risk for serious complications, and this is the basis for many rabbinic authorities permitting contraception in situations where pregnancy would seriously harm the woman. In those cases, the most "natural" method is preferred; as the use of a condom or pessary creates a physical barrier, "the pill" (or an intrauterine device) is preferred by most authorities. Fertility awareness methods (modern improvements over the rhythm method) are difficult to use, because many infertile days coincide with the ritually impure days of niddah.