Sotah, Chapter Four, Mishnah Three
This mishnah continues to discuss situations where a man married a woman prohibited to him, and therefore should she become a sotah she doesnt drink the bitter waters or receive her ketubah.
We should note that according to the mishnah it is dangerous for a nursing woman to become pregnant again, for fear that her milk would dry up. This was probably a greater possibility when nutrition was not as good as it is today. Despite this, rabbis did not prohibit pregnant women from having sexual relations with their husbands.
1) [A wife] who was pregnant by a former husband or was nursing a child by a former husband does not drink and does not receive the ketubah, the words of Rabbi Meir.
a) But the sages says: he may separate her [from himself] and take her back after the period [of two years].
2) An aylonit [who is incapable of giving birth], an old woman, and one who is unfit to bear children do not drink and do not receive the ketubah.
a) Rabbi Eliezer says: he may marry another wife and be fruitful and multiply with her.
3) All other women either drink or do not receive the ketubah.
Section one: A man may not marry a widow who is either pregnant or nursing a child from her former husband. The rabbis feared that a new baby with a new husband would cause a problem for the nursing child (and pregnant women will eventually be nursing women), for the mother could not dont nurse two babies at the same time. While a husband may continue to have sexual relations with his wife even when she is nursing, in that case we can assume that if the milk is not sufficient for both children, the husband will supplement it with eggs and goats milk, which were used as supplements before formula existed. However, concering a new husband we cannot be sure that he will also do so (since the first child is not his), therefore the rabbis forbade the marriage. Since the marriage was prohibited, Rabbi Meir says that she doesnt drink the waters, nor does she collect her ketubah. The Sages hold that the marriage is not truly prohibited, because he may remain married to her while at the same time staying separate from her (sexually) for two years. This refers to the standard length of time in which women nursed their children. This way she wont become pregnant while still nursing. Since the marriage can be maintained, she can become a sotah, and drink the water.
Section two: The women listed in this section can all, by definition, not have children. An aylonit is a girl who never develops physical signs of maturity. By definition she cannot conceive. The old woman referred to here is one past child-bearing age. The woman who is unfit to bear children refers to one who drank a sterilizing potion. According to the first opinion in the mishnah, it is forbidden for a man to marry these types of women because they cannot bear children, and he is commanded to have children. Since this is a forbidden marriage, she does not drink the water.
We should note that in Yevamot 6:5 we learned that this prohibition only refers to someone who does not yet have children. A man who has children may marry a woman who cannot have children, since he has already performed his commandment.
Rabbi Eliezer points out that this marriage too, while originally prohibited, may be maintained if he marries another woman with whom he can have children. Therefore, if she becomes a sotah she can drink the water.
Section three: All other women, if they dont want to drink the water, also do not receive their ketubah.