As we have seen, there is a Torah commandment to have a son and a daughter, and the Sages expanded this and enacted a rabbinic mitzva to have additional children. At first glance, it would seem that there is no limit to the rabbinic mitzva, and that people must do their best to have as many children as possible. However, it seems more reasonable to posit that there are two tiers within the rabbinic mitzva. The first tier is a blanket obligation to have four to five children. The second tier is to have even more children, each couple according to its capability.
Accordingly, ordinary couples of relatively sound mind and body are obligated to fulfill the rabbinic mitzva of having four to five children. Beyond this, they may assess whether they have the energy to continue fulfilling this important mitzva by having more children. For example, if they know that they can raise additional children and educate them in the ways of Torah, mitzvot, and derekh eretz (considerate behavior), they should continue to have as many children as they can. However, if they know that having additional children will be too stressful for them, making anger and irritability their constant companions, there are grounds for them to stop having children; even though each additional child is a mitzva, being in a bad emotional state often leads to transgression, which will likely have a negative impact on the upbringing of their children. Additionally, parents who are interested in channeling their talents in other meaningful ways may choose to do so even if this will leave them without the energy to raise additional children, as we will explain below in section 16. (Concerning permissible contraceptive methods, see sections 17-19; for the need to consult a rabbi, see section 20.)
The basis for dividing the rabbinic mitzva into two tiers is that we find that the Sages often pattern a rabbinic mitzva on a Torah commandment, and since the Torah established an obligation to have two children, it is reasonable to posit that the rabbinic obligation is to have an additional two children. One might even suggest that the requirement is to have a total of two girls and two boys. In most cases, then, in order to fulfill the rabbinic requirement, a couple would need to have five children. Indeed, it is the norm in most religious families today to make efforts to have at least four to five children.
Thus, although a couple fulfills a Torah commandment with each child that they have, there are three tiers of obligation within this mitzva: a) There is a Torah obligation to have a son and a daughter, and even when conditions make it challenging, the couple must make every effort to fulfill this mitzva (see below, sections 13-15, and ch. 6 sections 1-4). b) There is a rabbinic obligation to make efforts to have four to five children. c) There is a mitzva to have even more children, taking into account the parents’ energy (as we will explain below in sections 16 and 20).
In my humble opinion, the reason that this question never arose in the past is because almost half of all children died in childhood and women were infertile for various reasons. For example, miscarriages often caused infertility, in the times before dilation and curettage (D&C), and breastfeeding often had a contraceptive effect for two years. (Nowadays, it seems that it is due to the abundance and varieties of food available that women have the strength to nurse and ovulate simultaneously.) In any case, in the past, the average number of children was at most four or five. It never dawned on those who merited the blessing of many children to try to prevent pregnancy.
Nowadays, with the improvement of medical care and the shortening of time spent breastfeeding, the average woman can give birth to many more children. Thus, the question arises: what are the obligations under this mitzva, and when is contraception permissible? Some are inclined to say that the obligation is to have as many children as possible, giving only minimal consideration to financial issues and physical well-being. They permit birth control only if there is physical or mental illness involved (R. Yosef Messas; Yaskil Avdi; Minḥat Yitzḥak). Others maintain that one may take finances and health into consideration (AHS; Igrot Moshe; Tzitz Eliezer). See n. 15 below. In practice, though, it seems that even those who are stringent will rule leniently if those asking describe their situation as difficult, and even those who are lenient will rule stringently if those asking describe their situation as manageable. Thus, the halakhic ruling will depend a great deal on how the couple presents the question, which in turn depends upon their worldview, their attitude toward having many children, and their perception of how difficult this would be. In such cases, both the questioners and the responders find themselves facing a difficult dilemma.
In order to help these families and the rabbis they consult, it seems to me that it is necessary to divide the rabbinic obligation into two tiers, which will address most questions. It also seems that most poskim rule this way in practice, namely, that before a family has reached the average size in the religious community, rabbis are less willing to be permissive regarding birth control, but from that point on they are more willing. This formulation therefore gives systematic expression to what is already the generally accepted pesak (as explained below in n. 15).
Two rationales can be brought to support this division. First, we know that the Sages often pattern rabbinic laws upon Torah laws, as the Talmud states, “Whatever the rabbis ordained, they ordained on the pattern of the Torah” (Gittin 65a et al.). For example, fasts established by the prophets are patterned after Yom Kippur, which is a biblically mandated fast (Peninei Halakha: Zemanim 7:1). R. Yosef Ottolenghi (Lu’aḥ Dinim Shel Ha-Nimukei Yosef, Yevamot §60) seems to apply this principle to procreation: “On the Torah level, the mitzva of procreation is fulfilled by having a son and a daughter, while fulfilling the rabbinic mitzva requires having two of each.” This requirement of specifically two sons and two daughters can be added to how Ha’mek She’ala understands She’iltot (165:3-4), namely, that in order to ensure the fulfillment of the Torah mitzva, one must have two sons and two daughters (n. 5 above). Statistically, the chance that out of four children two will be boys and two will be girls is 37.5%; out of five children, it is 62.5%; out of six, 78.13%; and out of seven, 87.5%. Thus, the second tier in the fulfillment of the mitzva requires four to five children, meaning, that if one does not have two boys and two girls among the first four children, it is best to have a fifth child, for in most families, the five children will include two boys and two girls. However, we do not assert on the basis of this speculative reasoning that it is obligatory to have more than five children. Even in a family with four children, the second tier obligation is patterned on the Torah with respect to number, and the risk of not fulfilling the mitzva of procreation has been greatly minimized.
The second rationale is not as well defined, but carries great weight. There is a well-known principle that when we are uncertain as to how exactly to perform a specific mitzva, we should look at the prevalent practice among observant Jews. Among families that have no specific physical or mental difficulties (not counting those who enhance the mitzva by having especially large families), the typical number of children is between four and five. Likewise, some Rishonim write that the rabbinic mitzva does not require people to make every effort to have a limitless number of children; rather, it is a mitzva to populate the world in a normal way (Ha-Ma’or; Ramban, Milḥamot Hashem on Yevamot 19b in the Rif pages). Some contemporary rabbis maintain that there is a dispute about whether the rabbinic commandment is a bona fide obligation, like other rabbinic mitzvot, or is a lighter commandment (Bnei Banim 2:38; Ish U-veito, ch. 17, n. 2). Though it seems more likely that this is not a dispute, if we accept their statements, there is room in the division I propose for each view and rationale; having four to five children is obligatory, and having more is an optional mitzva.