On a Biblical level, a father has the ability to arrange a marriage for his daughter while she is still a minor. Although this is not practiced today, there were periods in Jewish history when for financial or security reasons it was important for a young girl to be married to ensure her future or her safety. Based on this rule being mentioned in our Mishna (2a), the Gemara discusses certain details of such relationships. This is one of the basic sources in the Talmud that deals with issues of birth control.
Rav Beivai taught a baraita before Rav Nahman. Three categories of women may use a mokh (a contraceptive resorbent, or absorbent cloth) while engaged in marital relations – a minor, a pregnant woman and a nursing woman. The minor, because she might become pregnant and as a result might die; a pregnant woman, because she might cause her fetus to become deformed into the shape of a sandal fish; and a nursing woman, because she might have to wean her child prematurely, which may result in its death.
And the baraita continues: Who is considered a minor? It is a girl from the age of eleven years and one day until the age of twelve years and one day. If she was younger than this or older than this, she may go ahead and engage in relations in her usual manner. This is the statement of Rabbi Meir. Since it is assumed that a minor who is less than eleven years old cannot become pregnant, she is considered to be in no danger.
The hakhamim say that all women should carry on marital intercourse in the usual manner, and heaven will have mercy on them (i.e. no harm will come to them), based on the passage that states (Tehillim 116:6) HaShem preserves the simple.
The rishonim differ as to how to understand this baraita, and what its implications are for the halakha. According to Rashi, the discussion is whether a woman can insert a physical barrier into her vaginal canal as a means of birth control. Rabbi Meir’s position is that a woman who has reason to fear that pregnancy will result in a danger to her or to her unborn child is permitted to do so, although it would be forbidden to other women. Tosafot and others reject Rashi’s explanation, arguing that inserting a mokh during relations would be forbidden. They suggest that the mokh is an absorbent cloth that is inserted following sexual relations in an attempt to remove the semen. According to Rabbi Meir, a minor as well as a pregnant or nursing woman would be obligated to use this mokh in an attempt to keep a potentially dangerous pregnancy from developing (a method that is recognized today as being of limited use, if any), while other women would be permitted to do so.