According to the opening of Parashat Tazria (Leviticus 12:1–8), after a woman gives birth to a boy she is impure for seven days. Circumcision takes place on the eighth day, after which she must wait another thirty-three days during which she is pure. At their conclusion she may enter the sanctuary. A simple calculation gives forty days altogether, although the Torah does not explicitly state the total. When a woman gives birth to a girl, however, both her days of impurity and her waiting period are twice as long: two weeks of impurity and sixty-six days of waiting, making eighty days altogether. Only then may the mother enter the sanctuary.

What accounts for the difference between giving birth to a boy and to a girl? Why is the period twice as long after a girl’s birth?

Nahmanides offers two explanations. The first, based on Mishnah Niddah 3:7, attributes the difference to the formation of a male fetus taking forty-one days, and that of a female fetus eighty-one days. This explanation is difficult for two reasons. First, the connection between fetal formation and the waiting period before entering the sanctuary is unclear. Moreover, the cited mishnah says that “a male is completed in forty-one days and a female in eighty-one,” whereas the waiting periods last only forty and eighty days. Another difficulty is that Nahmanides does not explain why the period of impurity is seven days for a boy and fourteen for a girl.

Nahmanides offers another explanation: physiologically, giving birth to a girl affects the mother more than giving birth to a boy, so she needs twice as long to regain her previous physical condition. Yet the basis for this physiological assumption is unclear; even if we accept it, it is not clear why the difference requires twice the waiting time.

An interesting explanation appears in the Book of Jubilees, one of the noncanonical books from the Second Temple period. It addresses both differences: the days of impurity and the waiting period. The explanation seems far from the plain meaning, and I cite it only because of the book’s antiquity: it was written close to the period when the biblical canon took its final form.

In the first week Adam and his wife were created, and in the second week she was brought to him. Therefore [the Torah] commanded that she be impure seven days for a son and two weeks for a daughter. (Jubilees 3:11–12)

According to Jubilees, the difference stems from the interval between the beginning of creation and the creation of each member of the first couple, Adam and Eve. Adam was created after a week, hence a week of impurity. Although the woman was created with him, she was brought to him only another week later, hence two weeks. Jubilees then notes that the total period during which the mother of a boy may not enter the sanctuary is forty days: seven days of impurity followed by thirty-three days of purity. For a girl, the periods are doubled, totaling eighty days. These different totals correspond to the days that elapsed before God brought Adam and Eve into Eden: Adam after forty days and Eve after eighty.

After Adam had lived forty days in the land where he was created, we brought him into the garden of Eden. Therefore it is engraved on the heavenly tablets: this is the law of a woman who conceives and gives birth to a boy. She shall be impure seven days, corresponding to the first week. For thirty-three days she shall remain in the blood of purification; she shall touch nothing holy and shall not enter the sanctuary until these days are completed for the birth of a boy. If she gives birth to a girl, she shall be impure two weeks, corresponding to the first two weeks. For sixty-six days she shall remain in the blood of purification, making eighty days altogether. When the woman had completed eighty days, we brought her into the garden of Eden, for it is holier than all the earth, and every tree planted there is holy. Therefore these days are prescribed for the mother of a son or daughter: she shall touch nothing holy and shall not enter the sanctuary until these days for a son or daughter are completed. This is the law and testimony written for the children of Israel to observe forever. (Jubilees 3:13–21)

The nineteenth-century commentator Rabbi David Zvi Hoffmann suggested that the days of impurity were originally meant to last two weeks after the birth of either a girl or a boy. In his explanation, the Torah reduced the period after a boy’s birth to seven days so that the mother would be pure on the eighth day, the day of her son’s circumcision. He further suggested that circumcision has a purifying effect that halves the pure days before entering the sanctuary as well, reducing them from sixty-six to thirty-three (Hoffmann, Commentary on Leviticus, pp. 251–252). This can draw support from the mention of circumcision immediately after the seven days of impurity, despite its apparent lack of connection to the mother’s laws. The sequence could indicate a connection: impurity ends after seven rather than fourteen days to allow the mother to be pure on her son’s joyful day.

On Hoffmann’s proposal, the two weeks after a girl’s birth are the original period, while the period after a boy’s birth is a shortened version. If so, we would expect the Torah to begin with the “ordinary” laws for a girl’s birth, then describe the boy’s birth, whose periods are halved because of circumcision. Furthermore, contrary to Hoffmann’s explanation, the seven-day period associated with a boy appears standard. Seven, and particularly seven days, recur in laws of impurity and purity: impurity from a corpse, skin disease, and male and female discharges. Seven days are also the duration of menstrual impurity, to which the Torah compares the mother of a boy: “she shall be unclean seven days; as in the days of the impurity of her sickness shall she be unclean” (וְטָמְאָה שִׁבְעַת יָמִים כִּימֵי נִדַּת דְּו‍ֹתָהּ תִּטְמָא; Leviticus 12:2). There is no comparable fourteen-day period elsewhere in the Torah’s laws. As I have shown, the total wait is forty days for a boy and eighty for a girl. Forty recurs frequently in the Torah – in the flood, Moses’ ascents to Sinai, the scouts’ journey, and the years in the wilderness – whereas eighty is uncommon. This suggests that seven days, as for menstruation, are standard; the doubled period after a girl’s birth is the exception.

I would like to propose a different explanation. It rests on the understanding that impurities resulting from bodily discharges are not inherently negative but express life’s normal, cyclical course. Like day and night, summer and winter, waking and sleep, impurity and purity are cyclical states expressing the natural course of life throughout the universe.

During life, the body invests energy and produces substances capable of generating life: a man’s semen, for example, or the uterine lining rebuilt each month, on which life can develop in a woman’s womb. When these vital fluids leave the body, through seminal emission or menstruation, the body has released life energy even if the result is the creation of new life. Separation from that life energy produces impurity. In childbirth, a woman parts with life energy in the form of the child carried in her womb for nine months. Although this is a joyful event for the mother and a new beginning for the infant, her body is emptied of vitality and enters a state of impurity.

We must still explain why a mother is impure twice as long after giving birth to a girl. One possibility, proposed in lectures by Rabbi Professor Jonathan Grossman, is that parting from a daughter involves a greater loss of life than parting from a son, because of the potential life in the newborn daughter’s womb. But this has two main difficulties. First, potential life exists in a newborn son as well as a daughter with a womb, since the son too is needed to create life. One could argue that the daughter’s potential is greater because she is born with all her eggs, whereas the son’s sperm will be produced only at maturity. In my view, however, this is not decisive, since neither reproductive system is yet developed at birth. Second, if parting from a daughter is twice the loss and therefore imposes double impurity and waiting periods, we would expect every female death to cause greater impurity than a male death because of her doubled potential for life. For example, impurity from a female corpse would last fourteen rather than seven days. The Torah contains no such law.

I therefore incline to understand the doubled impurity as arising because two females have undergone childbirth: the mother and her daughter. A daughter, like her mother, has a womb and will eventually form and give birth to a living being within herself. Participating in her own birth affects her too, making her impure like her mother. Since the infant has no independent standing, the Torah transfers the daughter’s days of impurity and purity to the mother. The mother is therefore impure for twice as long and, after those doubled days, remains in the “blood of purification” (עַל דְּמֵי טָהֳרָה) for twice as long before entering the sanctuary. When a boy is born, by contrast, childbirth does not render him impure: the process cannot impart this impurity to someone without a womb, for whom it is foreign to his own body. The mother’s periods therefore retain their original length.

Alternatively, the newborn son may become impure through his encounter with childbirth just as the daughter does. But, similarly to Hoffmann’s suggestion, circumcision on the eighth day could have a purifying effect that completely purifies him. There is then no need to transfer his impurity and waiting periods to his mother, so hers remain unchanged. This alternative has the advantage of explaining why circumcision is mentioned in the childbirth laws despite its apparent irrelevance.

Either way, unlike the mother of a girl, upon whom the Torah places her daughter’s periods as well and thus doubles them, the mother of a boy retains her own original periods: seven days of impurity, like menstruation, followed by thirty-three days completing the forty-day wait before entering the sanctuary.