Gemara

(Rav Bivi - Beraisa): Three women use a contraceptive wad to (absorb the semen and) prevent pregnancy: a minor, a pregnant woman, and a nursing woman.

A minor does so, lest she get pregnant and die. A pregnant woman does so lest the fetus get mashed. A nursing woman does so lest pregnancy force her to wean her baby, and it will die.

R. Meir says, a girl between 11 and 12 years uses a wad. If she is younger or older, she has relations normally.

Chachamim say, at any age she has relations normally. Shamayim will have mercy on her - "Hash-m guards the simple."

Nidah 13a (Mishnah): The more a woman checks herself (for blood), the better.

This is because she is not Bas Hargashah (this will be explained).

Rishonim

The Rif and Rosh (Yevamos 2b and 1:7) bring Rav Bivi's Beraisa verbatim.

Perush ha'Rosh (Nedarim 35b DH Shalosh): The three women must put a wad there to absorb the semen to prevent pregnancy.

Rashi (Kesuvos 39a DH Meshamshos): The three women may use a wad at the time of Bi'ah. It is not as if they waste seed.

Tosfos (Yevamos 12b DH Shalosh): Rashi says that only these three are permitted. Other women are forbidden, for it wastes the semen, even though women are not commanded about Peru u'Rvu. R. Tam says that a wad before Bi'ah is surely forbidden. Such a Bi'ah is like spilling one's seed onto rocks. A wad after Bi'ah is permitted. The Bi'ah is normal, like Bi'ah with a minor or Ailonis, who cannot become pregnant. She may use the wad afterwards, because she is not commanded about Peru u'Rvu. The three women must use a wad. The Ri supports Rashi from Nidah (13a), which praises one who checks herself often, for she is not 'Bas Hargashah'. I.e. this will not cause her to waste seed. This implies that she may not waste seed. Therefore, other women may not have Bi'ah with a wad. However, we could explain 'she is not Bas Isur Hargashah', i.e. she may waste seed.

Tosfos (Kesuvos 39a DH Shalosh): R. Tam says that the three women must use a wad, i.e. it is good counsel to do so.

Question (Avnei Nezer EH 79:2): Yevamos 42a says that according to R. Meir there is no concern lest the fetus become mashed (due to semen) because she uses a wad. This implies that it is obligatory!

Tosfos ha'Rosh (Yevamos 12b DH Shalosh): Wasting semen is like bringing a flood to the world. It is unreasonable to permit using a wad during Bi'ah for Simchas Onah. It is better than she refrain from Bi'ah! After Bi'ah any woman may use it. Even if she is commanded about wasting seed, this refers to causing seed to leave the body, but not after it left

Hagahos Ashri (Kesuvos 3:4): The Ri permits only a minor or a woman who is pregnant or a nursing to use a wad, but not others. Chachamim do not permit even these, whether the wad is inserted before or after Bi'ah. R. Tam permits.

Tosfos (Megilah 13b DH Toveles): Mordechai had Bi'ah with Esther without waiting three months after she was with Achashverosh for Havchanah (to know from whom is the child) because she used a wad.

Turei Even (DH Toveles): A Medrash says that Daryavesh ha'Acharon was Achashverosh's son from Esther! Perhaps she was afraid to use a wad with Achashverosh, rather she used it to avoid pregnancy from Mordechai.

Chasam Sofer (2:YD:172 DH uvi'Teshuvas): She was afraid to use a wad during Bi'ah with Achashverosh, rather afterwards. Even though we do not rely on this for Havchanah, perhaps she knew that she was very proficient. Shamayim caused that once she 'failed', in order that she would have a son who would authorize building Bayis Sheni. Indeed, she also used a wad to prevent pregnancy from Mordechai, for she knew that Achashverosh would assume that the child is his and would raise him like a Nochri.

Poskim

Pischei Teshuvah (EH 23:2): Doctors said that pregnancy would be a mortal danger for a certain woman. Chemdas Shlomo (46) permitted her to have Bi'ah with a wad. R. Akiva Eiger and the Chasam Sofer permitted a wad only after Bi'ah.

Chasam Sofer (6:Likutim 40 DH Ad): It used to be that a woman could drink a sterility potion or use a wad after Bi'ah. Her husband could marry another woman if he wants children. Nowadays, R. Gershom's Cherem forbids him to divorce her against her will or marry a second wife, so she may not do so without his consent.

Magen Avraham (OC 606): (We immerse on Erev Yom Kipur.) If a woman had Bi'ah within three days before Yom Kipur she should wash the area with hot water to prevent an emission of semen. If the Bi'ah was close to the time of her Tevilah or Veses, the most likely times to become pregnant, she should not wash, lest this prevent pregnancy.

R. Akiva Eiger (Teshuvah 1:71): Seemingly, a wad after Bi'ah should be permitted. Rashi and Tosfos do not argue about the Halachah, only about how to explain the Gemara. The Ri suggested a source for Rashi to say that other women may not waste seed. We do not abandon what was clear to R. Tam due to the Ri's Safek. However, most Poskim forbid. Rashi in Kesuvos connotes that adult women may not use a wad because they waste seed. This applies also after Bi'ah. The Nimukei Yosef asked that wasting seed does not apply when she cannot become pregnant. The Shitah Mekubetzes in Kesuvos brings many Rishonim who hold like the Nimukei Yosef who permit a minor to use a wad for in any case she cannot become pregnant, but forbid an adult for it wastes the seed.

R. Akiva Eiger (ibid., DH v'Hayah): Kesuvos (71b) connotes that a woman is not forbidden to waste seed. Rav Yehudah explains the Mishnah to say that if a husband annulled his wife's vow on condition that she remove the semen after Bi'ah to prevent pregnancy, he must divorce her. If this were forbidden, according to Abaye his stipulation would be Batel, for it is contrary to Torah! However, Abaye could explain the Mishnah differently, like the Beraisa there.

R. Akiva Eiger (ibid., DH v'Nir'eh): The Gemara in Nidah (42a) shows that a woman is forbidden to waste seed. Rava forbids a woman to eat Terumah for three days after relations, for she is prone to become Tamei through emitting semen. He explains that the verse (which implies that she can go the Mikveh and become Tehorah at night) discusses a woman who never sat up, therefore we are not concerned for emissions. If it were permitted for a woman to remove the semen, Rava would not establish the verse in such a bizarre case (she was carried to the Mikveh on the bed)! The Magen Avraham is against the majority of Poskim.

Suggestion: Bedikos show that women may use a wad after Bi'ah! (When there was Efer Parah Adumah, Chachamim enacted that Benos Yisrael check themselves for Dam Nidah every morning and evening for the sake of Taharos (Nidah 4b), even though this removes the semen!)

Rebuttal (R. Akiva Eiger 72): Surely R. Yochanan ben Brokah agrees that Bedikos were encouraged, even though he holds that also women are commanded Peru u'Rvu (Yevamos 65b)! A Bedikah does not necessary prevent pregnancy; perhaps the semen was already absorbed. He must hold like R. Shimon, who permits Davar she'Eino Miskaven. However, even R. Yehudah who forbids Davar she'Eino Miskaven permits Bedikos. This shows that women are not forbidden to waste seed.

R. Akiva Eiger (Drush v'Chidush b'Sof, DH Ma'aseh Hayah): R. Meir and Chachamim argue about whether or not we are concerned for the minority of women in these three categories who can become pregnant. If she were Muchzekes to see blood, even Chachamim would permit. Only a single woman may drink a sterility potion.

Note: This does not explain why R. Meir permits a pregnant woman to use a wad. R. Chiya's wife asked R. Chiya whether or not she is commanded about Peru u'Rvu before drinking a sterility potion (Yevamos 65b). It is a Chidush to say that she did not ask properly to get the correct answer!

Chavos Ya'ir (31 DH v'Al): All agree that after semen has been absorbed she may not ruin it. Yevamos 100b supports R. Tam. It brings the Beraisa to show that a Mema'enes will not give birth (i.e. because a minor is obligated to use a wad). However, one can say that since it is permitted, surely all minors do so to avoid the danger. Rashi forbids other women to use a wad even after Bi'ah, for the man knows that his seed will be wasted. Even though a woman is exempt from Peru u'Rvu and may drink a sterility potion, perhaps she may not do an act to waste seed, due to Lasheves Yetzarah (a Mitzvah mi'Divrei Kabalah to have children). One must be concerned for the stringent opinions.

Chasam Sofer (ibid., DH Emnam): Rashi holds that Chachamim do not permit a wad at the time of Bi'ah even due to the danger. However, perhaps they obligate her to use one afterwards, and not to rely on "Shomer Pesa'im Hash-m".