(18)For thus said GOD,
The Creator of heaven who alone is God,Who formed the earth and made it,Who alone established it—Who did not create it a waste,But formed it for habitation:I am GOD, and there is none else.

As it is stated: “In those days Hezekiah became deathly ill, and Isaiah ben Amoz the prophet came and said to him: Thus says the Lord of Hosts: Set your house in order, for you will die and you will not live” (Isaiah 38:1). This seems redundant; what is the meaning of you will die and you will not live?...Isaiah said to him: Because you did not marry and engage in procreation. Hezekiah apologized and said: I had no children because I envisaged through divine inspiration that the children that emerge from me will not be virtuous. Hezekiah meant that he had seen that his children were destined to be evil. In fact, his son Menashe sinned extensively, and he thought it preferable to have no children at all. Isaiah said to him: Why do you involve yourself with the secrets of the Holy One, Blessed be He? That which you have been commanded, the mitzva of procreation, you are required to perform, and that which is acceptable in the eyes of the Holy One, Blessed be He, let Him perform, as He has so decided.

It is from the roots of this commandment [that it is] because the body is an instrument of the soul — with it, it carries out its activity; without it, it can never complete its work. And hence it is in its shadow truly for its good, and not for its bad; as God does not do bad but does good to all. Thus we find that the body at its command is like a pair of tongs in the hand of a blacksmith: with it he can produce a tool fit for its purpose...

(7) But Er, Judah’s first-born, was displeasing to GOD, and GOD took his life. (8) Then Judah said to Onan, “Join with your brother’s wife and do your duty by her as a brother-in-law, and provide offspring for your brother.” (9) But Onan, knowing that the offspring would not count as his, let [the semen] go to waste whenever he joined with his brother’s wife, so as not to provide offspring for his brother. (10) What he did was displeasing to GOD, who took his life also.

Yehuda and Ḥizkiyya, were twins, but one of them was fully developed after nine months of pregnancy and one was fully developed at the beginning of the seventh month, and they were born two months apart. Yehudit, the wife of Rabbi Ḥiyya, had acute birthing pain from these unusual deliveries. She changed her clothes to prevent Rabbi Ḥiyya from recognizing her and came before Rabbi Ḥiyya to ask him a halakhic question. She said: Is a woman commanded to be fruitful and multiply? He said to her: No. She went and drank an infertility potion.Eventually the matter was revealed, and Rabbi Ḥiyya found out about what Yehudit had done. He said to her: If only you had given birth to one more belly for me, i.e., another set of twins. As the Master said: Yehuda and Ḥizkiyya were twin brothers and became prominent Torah scholars, and Pazi and Tavi, Rabbi Ḥiyya’s daughters, were twin sisters, and became the matriarchs of families of distinguished Torah scholars.

She had pain with childbirth. This means pain without danger, for otherwise, even if she were obligated in pirya ve-rivya, there is nothing that stands before pikuach nefesh. The practical halachic significance regards a man who is prohibited from sterilizing himself since he is obligated in pirya ve-rivya, for on account of pain with intercourse he may not sterilize himself with a potion, but in a case of danger it would be permissible.

...A woman it is permitted to drink this root drink in order that she become impotent so she cannot give birth.

It implies that, even without any particular pain, it is permissible

A woman is permitted to drink it in order that she not bear children. That is to say, it is a given that for the purpose of healing it is permissible, but even if it is not for healing but in order that she not bear children it is also permissible…And it also implies that it is not permissible even from the outset except when she has pain with childbirth like Rabbi Chiyya’s wife in Yevamot, for Rabbi Chiyya permitted it from the outset, but if she doesn’t have pain with childbirth more than other women in general, it is prohibited…For Rambam only said that one who sterilizes a woman is exempt on a Torah level but prohibited rabbinically in a case where he sterilizes her without the need to save her from the danger of difficult childbirth, but for the need that she not bear children with difficulty like Rabbi Chiyya’s wife, it is permissible

Rav Beivai taught a baraita before Rav Naḥman: Three women may engage in relations with a contraceptive resorbent, a soft fabric placed at the entrance to their wombs to prevent conception, despite the fact that this practice is generally prohibited. They are as follows: A minor, a woman who is already pregnant, and a nursing woman. The baraita specifies the reason for each exception: A minor may do so lest she become pregnant and perhaps die; a pregnant woman, lest she be impregnated a second time and her previous fetus becomes deformed into the shape of a sandal fish by being squashed by the pressure of the second fetus. As for a nursing woman, she does so lest she become pregnant and her milk dry up, in which case she will wean her son too early, thereby endangering him, and he will die...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. And the Rabbis say: Both this one and that one, i.e., in all these cases, she may go ahead and engage in relations in her usual manner, and Heaven will have mercy upon her and prevent any mishap, since it is stated: “The Lord preserves the simple” (Psalms 116:2)

Rabbi Yannai said: If an ill person says he needs to eat, and a doctor says he does not need to eat, one listens to the ill person. What is the reason for this halakha? It is because the verse states: “The heart knows the bitterness of its soul” (Proverbs 14:10), meaning an ill person knows the intensity of his pain and weakness, and doctors cannot say otherwise. The Gemara asks: It is obvious that a person knows himself better than anyone else does. Why does this need to be stated explicitly? The Gemara answers: It is lest you say that the doctor is more certain because he has had more experience with this condition. Therefore, the verse teaches us that even so, it is the ill person who knows his own suffering better than anyone else.

A sick person who needs to eat, if there is an expert doctor there, even if he is a non-Jew, who says that if this person is not fed it is possible that the illness will worsen on him, needless to say if he might be in mortal danger, they feed him on the doctor’s orders. If the sick person says that he does not need (the food) we still listen to the doctor, but if the sick person says, “I need (the nourishment),” even if one hundred doctors say that he does not need (the nourishment) we listen to the ill person.

For “the heart knows its own bitterness” applies in all matters, and even if a doctor says [the sick person] doesn’t need any medicine, we don’t listen to him [the doctor]. But if the illness is known, and the sick person says that the medicine works for this illness and the doctor says that it doesn’t, there is no rationale to listen to the sick person and violate Shabbat for no reason [but in any case, it is possible to say that where the sick person says that he knows that the nature of his body is to heal from this illness if he takes this healing treatment, it is possible that we listen to him, for even in this it is slightly applicable to say a person is more expert in his body than a hundred doctors]…

Even if he married a woman and dwelled with her for ten years, we have not been accustomed to force him to divorce her, even though he has not fulfilled pirya ve-rivya...

If children with defects like Down Syndrome will be born…it [Maharshal’s ruling] is not the same matter as what his honor [the questioner] wrote, that he is afraid lest his children be born with defects, because Maharshal deals with a case where a woman already has children who do not go in the proper path and therefore there is a basis to think other children will follow them, which is not the case for mere concern or fear. Only if affected children were already born, or even if they were not yet born but the doctors say that because of the parents’ genetics or age or another reason it is likely that such children will be born, then one should permit contraception even if they have not fulfilled pirya ve-rivya.