Yevamot 12B

הלכתא: צרת אילונית מותרת, ואפילו הכיר בה. ואפילו צרת בתו אילונית.

And the halakha is that the rival wife of an aylonit is permitted, and this is the case even if her first husband knew of her status and her marriage was fully valid. The mitzva of levirate marriage does not apply to an aylonit, but her rival wife is not forbidden. And even in the case of the rival wife of his aylonit daughter who was recognized as such, the other wife is not considered the rival wife of a forbidden relative.

ואלא הא דקתני ״שנמצאו״⁠ — תני ״שהיו״. כי אתא רבין, אמר רבי יוחנן: אחת צרת ממאנת, ואחת צרת אילונית, ואחת צרת מחזיר גרושתו ⁠— כולן מותרות.

The Talmud asks: However, with regard to that which the Mishnah teaches: Were found to be, from which it was inferred that there is a difference between an aylonit whose condition was known by the husband beforehand and one who was recognized by him only later, how is this to be explained? The Talmud answers that one should emend this and teach simply: That were, and not: Were found to be. When Ravin came from Eretz Yisrael he said that R' Yoḥanan said: There is one halakha with regard to the rival wife of a girl who refused her husband, and the rival wife of an aylonit, and also the rival wife of one who remarries his divorcée: They are all permitted.

תני רב ביבי קמיה דרב נחמן, שלש נשים משמשות במוך: קטנה, מעוברת, ומניקה. קטנה ⁠— שמא תתעבר ושמא תמות. מעוברת ⁠— שמא תעשה עוברה סנדל. מניקה ⁠— שמא תגמול בנה וימות.

§ Incidental to the case of refusal, the Talmud cites a related halakha. Rav Beivai taught a baraita before Rav Naḥman: 3 women may have sex 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.