We learned in the Mishna (see above daf 7) that there are four women who generally do not menstruate, which leads to Rabbi Eliezer’s teaching that in the event that any of them do bleed, we need not be concerned that she became a nidda prior to the time that she realized that she was menstruating. One of these women was a nursing mother, about whom it is assumed that no menstruation will take place until the baby is weaned.
The Gemara on today’s daf brings a baraita that discusses the case of a nursing mother. According to Rabbi Meir, we assume that a woman who continues nursing even four or five years after her baby is born will not menstruate until the baby is weaned, but if the baby dies, even within the first two years, we treat the mother like any other woman. Rabbi Yehudah, Rabbi Yose and Rabbi Shimon disagree. They rule that we do not anticipate that women who have given birth will menstruate for the first two years, but after that time we expect them to begin menstruating, even if they are still nursing.
In explanation of these opinions the Gemara explains that according to Rabbi Meir “the blood changes and becomes milk.” Therefore if the mother is not nursing, menstruation returns. The Sages who disagree believe that following childbirth, the woman’s “inner organs weaken.” After two years the mother’s body recovers and she will begin menstruating again.
The idea that “blood changes and becomes milk,” is based on the fact that milk – like every glandular secretion in the body – is created by means of the different ingredients supplied by the bloodstream, like calcium, protein, fats, sugar, etc., which are synthesized into milk. A tremendous volume of blood must flow through a woman’s breast in order for a liter of milk to be created. Thus, it is the relationship between the circulatory system and the creation of milk that explains the lack of menstruation in a nursing mother.
The idea that “inner organs weaken” is based on the negative energy balance that exists in a woman who has given birth and is now nursing. In that situation, the woman’s body is using more energy than it consumes which keeps her reproductive system from functioning normally, leading to a cessation of menstruation.