Nida & Mikveh Purification after birth:[1]

Becoming impure [i.e. Nida]:[2] Every woman becomes impure after a vaginal birth irrelevant of whether she has seen blood. This impurity is called Tumas Leida and is a second form of impurity, asides for her becoming impure due to seeing Nida blood. Practically, since all woman see blood when they give birth, they are hence impure for two reasons, Tumas Leida and Dam Nida.

Becoming pure [Mikveh]-Letter of law: From the letter of the law, a woman is impure for 7 days after the birth of a boy and 14 days after the birth of a girl, even if she did not see blood. If she saw blood, then in addition to needing to wait the above amount of days, she must also perform a Hefsek Taharah and guard Shiva Nekiyim before being able to immerse. From the letter of the law, this Shiva Nekiyim can be performed even within her 7 or 14 days after birth, and at conclusion of the days required to wait, she may immerse. Practically, however, women continue seeing blood after birth for 4-12 weeks and hence cannot perform a Hefsek Tahrah until that time. However, there is no need at all to wait 40 days after the birth of a boy and 80 days after the birth of a girl, so long as she has stopped bleeding and is physically ready.[3] [There are different remedies available to help clear up a woman’s blood. As the weeks go by, the blood colors become lighter and eventually are deemed Halachically pure colors. There is no need to wait until there is no more blood being seen, rather as soon as the color is light enough for a Rav to purify, she can perform the Hefsek Taharah and Shiva Nekiyim.]

When to go to Mikveh-Advice:[4] The general medical advice is for a wife to delay having marital relations from between four to six weeks after birth in order to allow the body to recuperate.[5] [Intimacy prior to this time period, which allows the body to heal, can cause extra and unnecessary tearing and bleeding of her birthing wounds.] Nonetheless, some[6] suggest waiting longer intervals, such as 9 or 10 weeks, or even 12 weeks, from the birth, in accordance with her strength and stamina. All in all, while the wife should discuss the matter with her husband, and hear his perspective before making any decisions, she is not obligated to immerse in the Mikveh and re-begin marital intimacy until she feels physically capable of doing so.[7]

Going to Mikveh even if intercourse cannot take place: Some Poskim[8] rule that a woman may go to Mikveh at night in order so that the couple can engage in hugging and kissing, even if intercourse will not take place that night, such as due to a medical concern, so long as there is no worry of it leading to Zera Levatala. Other Poskim,[9] however, rule that she may not immerse in such a case, due to worry of it leading to Zera Levatala. Practically, one is not to be lenient if the husband is prone to have Zera Levatala due to this.

Dam Tohar:[10] The concept of Dam Tohar, pure blood after birth, is no longer accustomed today and hence a woman becomes impure upon seeing blood, even if she is within 40 or 80 days from her birth. [When going to Mikveh for seeing blood during Dam Tohar, she is to immerse in a Mikveh with a blessing.[11]]

[1] See Nitei Gavriel 107-108

[2] Michaber 194

[3] Michaber Y.D. 194:1; Rama ibid that in a place without a custom, one should not be stringent at all; Teshuvas Rama 94, brought in Taz 194:3, that so is custom today in his provinces; See Taz ibid at length for negation of the stringent opinion and the harsh words of the Bach against one who is lenient; Shach 194:1; Pleisi 194; Noda Beyehuda Kama Y.D. 54; Chacham Tzevi 68; Many Poskim in Nitei Gavriel 107:3 footnote 4

Other opinions: Some are accustomed to delaying intercourse for 40 days after having a boy and for a period of 80 days after having a girl in order to recuperate after birth and not become pregnant right away. [See Rama ibid; Poskim ibid; Shvus Yaakov 3:77]

[4] Shiureiy Shevet Halevi 194; Piskeiy Teshuvos 240:8; Nitei Gavriel 107:2 footnote 2

[5] See Taz Y.D. 197:4 that women after birth would delay going to Mikveh until they felt physically ready

[6] Taharas Yisrael 194 Beir Yitzchak 19

[7] See Taz 197:4; Nitei Gavriel 107:4

[8] Even Shoham 14, brought in Pischeiy Teshuvah 184:22; Sheilas Ya’avetz 2:10; Chut Shani Niddah 197:3; Shevet Halevi 8:271; See Rama 184:10 and Admur 184:33-35; Or Tzadikim 32:4

[9] See Posei’ach Sha’ar 23:27; Beir Moshe 8:91; Piskeiy Teshuvos 240 footnote 44; Regarding prohibition of night of Veses, see: Aruch Hashulchan; Taharas Yisrael; Darkei Teshuvah; Maharam Shick 364; Nitei Gavriel Niddah vol. 2 102:2; So ruled to me Rav Farkash in a phone conversation and so he rules in Taharah Kahalacha 24:109 and footnote 177; See also Piskeiy Teshuvos 240 footnote 44; Sheyikadesh Atzmo 11 footnote 4; Shut 38 in back of Sefer

The reason: As this can lead to them having intercourse. [Poskim ibid; however see Taharah Kahalacha ibid footnote 177 which negates this reason] Alternatively, it is because it is a danger for her to immerse and not have relations, as stated in the below Q&A. [Taharah Kahalacha ibid]

[10] Rama 194:4; Lechem Vesimla 194:13; Pischeiy Teshuvah 196:16; Avnei Tzedek 84; Nitei Gavriel 108:1

[11] Chasam Sofer 191; Sheilas Shmuel 74; Nitei Gavriel 108:3

[12] Admur 326:7; M”A 326:8; Terumos Hadeshen 255; Maharil 139; Rama Yoreh Deah 197:2; Bach 197; Shach 197:3; M”B 326:25

Other opinions: The Beis Yosef [brought in Shach 197:3] rules that there is never a prohibition to immerse on Shabbos in any situation. [Vetzaruch Iyun if this is even if she could have immersed beforehand? Seemingly yes, as so is implied from the fact that the Michaber never rules anywhere regarding any restrictions in immersing on Shabbos, and rather wrote simply that one may immerse on Shabbos.] Others rule that it is forbidden to immerse on Shabbos even if [she had holy reasons to avoid doing so beforehand, such as that] her husband did not arrive until Erev Shabbos. [stringent opinion in Rama ibid; Taz 197:4 based on his understanding of the Terumas Hadeshen] However he too agrees that in a case that she could not immerse due to a physical impracticality [Oness], such as that she is after birth and did not have the strength to immerse beforehand, then it is allowed. Thus, the Taz argues on Rama which holds that it is dependent on the custom and rather rules that it is always forbidden unless it was physically impossible for her to do so. In the Nekudos Hakesef the Shach debates against the ruling of the Taz and supports the ruling of the Rama and Bach.    

Custom of Sephardim: The Sephardim are accustomed to be lenient in this matter and immerse on Friday nights. [Beis Yosef ibid; Rav Poalim O.C. 4:12; Mizbeiach Adam 199; Kneses Hagedola 197:5; Shulchan Gavoa 197:4; Taharas Habayis 2:454; However, see Darkei Taharah p. 170 that writes to be stringent in this matter as rule Admur here and Rama ibid.]

[13] Taz 197:4

[14] See Rama 197:2; Nitei Gavriel 107:8

[15] Daas Kedoshim; Daas Torah; Avnei Nezer 247; Nitei Gavriel 107:7

[16] See Rama 197:2; Nitei Gavriel 107:8

[17] See Getting Closer pp. 69-86; I Am for My Beloved pp. 97-102; The Newlywed Guide to Physical Intimacy pp. 50-53