Nursing after intercourse:[1]

One who has a nursing child is not to nurse the child after marital relations for approximately two Milin, which is close to an hour [i.e. 36-48 minutes[2]]. [Lack of waiting this amount is dangerous for the child.[3]] At the very least [such as in a time of need[4]] one is to wait the length of one mil [i.e. 18-24 minutes, or according to some Poskim[5] even half a Mil, which is 9 or 12 minutes]. If the child is really crying, then some Poskim[6] rule that one is not to adhere to this matter at all, and may nurse the child right away without fear. Other Poskim,[7] however, are stringent even in such a case that one should always wait one Mil. Practically, one should try to calm the child in other ways, and if one is unable to do so, then one may be lenient to nurse him even prior to the passing of the above amount of time.[8]

[1] M”A 240:29; Bedek Habayis; Zohar Parshas Vayikra p. 119a; Kav Hayashar 17:5; Siddur Yaavetz Mosach Hashabbos Mitos Kesef 7 Chulya Beis 12 [rules that even initially one only needs to wait a Mil]; Kitzur SHU”A 150:16; M”B 240:54; See Yad Ephraim ibid; Kaf Hachaim 240:91; Sheyikadesh Atzmo 25:2

[2] How much is a Mil? Some Poskim rule that a Mil is 18 minutes [1^st opinion in Admur 459:10; Michaber ibid and in Yoreh Deah 69:6; Rama 261:1; Maryu 193; Terumos Hadeshen 123], and hence the total time to wait prior to nursing is 36 minutes [See Misgeres Hashulchan 150:16] Other Poskim rule that a Mil is 24 minutes [2^nd opinion in Admur 459:10; Rambam in Pirush Hamishnayos Pesachim 3:2], and hence the total time to wait prior to nursing is 48 minutes [See Shulchan Hatahor 241:5; Kitzur SHU”A ibid; See Taharas Yisrael 240:20; Misgeres Hashulchan 150:16 concludes to wait 9 and at best 112 minutes] Practically, Admur mainly rules like the opinion of 24 minutes. [Admur ibid; See Ketzos Hashulchan 3 Seder Hachnasas Shabbos 5; Piskeiy Teshuvos 240:29]

[3] Or Yakar Vayikra 16; Toras Nassan Ramak p. 149; Shulchan Hatahor 241:5 that it can lead to the Nichfeh illness.

[4] Taharas Yisrael 240:88

[5] Possible understanding of Yad Efraim on M”A ibid; Taharas Yisrael 240:88; See however Sheyikadesh Atzmo 25:3 footnote 2

[6] M”A ibid according to some prints; Siddur Yaavetz Leil Shabbos 3:12; Divrei Yatziv 1:104-2

Other opinions in M”A ibid: Some have a different Girsa in the M”A ibid which makes it read that if the child is crying one may be lenient like the opinion of one Mil, thus removing the source from the M”A above which claims that one may be completely lenient. [Girsa of Machon Yerushalayim; See Sheyikadesh Atzmo 25:3 footnote 2 that so is implied from wording of the Zohar.]

[7] M”A ibid according to some prints [see previous footnote]; Or Tzadikim 27:18; Elya Raba 240:24; Olas Tamid 240:7; Siddur Yaavetz Mosach Hashabbos Mitos Kesef 7 Chulya Beis 12                                                                                       

[8] Siddur Yaavetz ibid that on this the verse states, “And the child [i.e. Moshe] cried and she had mercy on him”