Yevamot

The law is that the rival of a woman incapable of procreation is permitted, even though he  knew her When Rabin came  he stated in the name of R. Johanan: The rival of a mema'eneth,  the rival of a woman incapable of procreation, as well as the rival of a divorced woman who had been remarried to her former husband,  are all permitted. R. Bebai recited before R. Nahman: Three [categories of] women may  use an absorbent  in their marital intercourse:10  A minor, a pregnant woman and a nursing woman. The minor,  because [otherwise] she might  become pregnant, and as a result  might die. A pregnant woman,  because [otherwise]. she might  cause her foetus to degenerate into a sandal.  A nursing woman,  because [otherwise] she might  have to wean her child prematurely  and this would result in his death. And what is the age of such a minor?  From the age of eleven years and one day until the age of twelve years and one day. One who is under,  or over this age  must carry on her marital intercourse in the usual manner. This is the opinion of R. Meir. The Sages, however, say: The one as well as the other carries on her marital intercourse in the usual manner, and mercy will be vouchsafed from heaven,  for it is said in the Scriptures The Lord preserveth the simple. Since it has been stated, 'because she might become pregnant and as a result might die' it may be implied that it is possible for  a minor to be pregnant and not die. But, if so, one could imagine a case where  a mother-in-law should be in a position to make a declaration of refusal,  whereas we learned, ONE CANNOT SAY OF A MAN'S MOTHER-IN-LAW, THE MOTHER OF HIS MOTHER-IN-LAW AND THE MOTHER OF HIS FATHER-IN-LAW THAT THEY WERE FOUND INCAPABLE OF PROCREATION OR THAT THEY MADE A DECLARATION OF REFUSAL! — Read, 'because she might become pregnant and die';  for Rabbah b. Liwai said: She  is subject to an age limitation. Prior to that period  she does not conceive at all; during that period  she dies and her embryo dies; after that period  both she and her embryo survive. But is it really so? Surely, Rabbah b. Samuel recited: One cannot say of a man's mother-in-law, the mother of his mother-in-law and the mother of his father-in-law that they were found incapable of procreation or that they made a declaration of refusal, since they have already given birth to children!  — But [the reading], in fact, is, 'because she might become pregnant and as a result might die'. But, [then, the previously mentioned] difficulty remains!  — R. Safra replied: Children are like marks of puberty.  Others Say: Children are