When a woman conceives and gives birth to a boy, she shall be ritually unclean for seven days… (Leviticus 12:2)
Many things cause ritual impurity; one of these is childbirth. A woman who gave birth to a boy was unclean for a week; if she had a girl, two weeks. The nature of this impurity is similar to that of a niddah (menstruant woman) in that the couple may not engage in marital relations. (We will discuss the laws of a niddah in Mitzvah #181, in parshas Metzora, and Mitzvah #207, in parshas Acharei Mos.)
If a woman who bleeds from her uterus is ritually unclean, it follows logically that one who gave birth is ritually unclean. It’s so obvious that a woman bleeds during childbirth that the Talmud rules, in the unlikely event that blood wasn’t seen during the delivery, the mother is considered unclean anyway. (See Talmud Niddah 664.) The law of the week or two weeks of ritual impurity applies to live births, stillbirths, and miscarriages at least 40 days after conception.
Some people might have the automatic reaction that it’s sexist for the impurity period to be longer following the birth of a girl than for a boy. This is wrong for two reasons:
(1) Don’t look at the period following a girl as longer, look at the period following a boy as shorter. The very next verse (12:3) reminds us that a son is circumcised on the eighth day. It increases the mother’s joy to be purified in time for the bris, something that is not an issue in the case of a daughter. (See Talmud Niddah 31b.)
(2) Look at verses 12:4-5. You’ll see that each period of ritual impurity is followed by a period of ritual purity. Under Torah law, a woman during this period does not become a niddah even if she sees uterine blood. Following the birth of a boy, this clean period is 33 days; for a girl, it’s 66 days. So, doubling the unclean days also doubles the clean days. This actually represents a net gain in the number of clean vs. unclean days following the birth of a daughter!
It may be counterintuitive that a woman might bleed from her uterus and have it not be ritually impure. The Talmud in Niddah (35b) says that the same Torah that tells us certain blood is impure can tell us that other blood is ritually pure. However, our current practice differs based on rabbinic enactments and safeguards adopted by the Jewish people; consult your rabbi for guidance in this area.
This mitzvah applies to men and women in all times and places. (How can it apply to men? A man must act in accordance with the reality of his wife’s status.) The details of this mitzvah are discussed in the Talmud in a number of places in tractate Niddah; see, for example, pages 35b-36a. This mitzvah is codified in the Shulchan Aruch in Yoreh Deah 194; it is #100 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos but it is not in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.