And God blessed them, and God said to them, “Be fruitful and multiply and fill the earth and subdue it, and rule over the fish of the sea and over the fowl of the sky and over all the beasts that tread upon the earth.” (Genesis 1:28)
We are used to thinking that the commandment to ‘be fruitful and multiply’ is simply a directive to marry and bear children. It is true that we are obligated to have children, but on closer inspection this mitzvah is much more complex.
The Gemara (Megillah 13a) cites a verse in Chronicles I (4:18) which implies that Basya, the daughter of Pharaoh gave birth to Moses. The Gemara exclaims that this could not be so as Yocheved was the mother of Moses, not Basya. Basya merely rescued Moses from the Nile River and raised him as her son. But from this the Gemara learns that someone who raises a child is like the one who gave birth to the child.
Furthermore, although there are some authorities who claim that the commandment of peru u’revu is nonetheless related only to having children, there is another difficulty to this simplistic explanation which necessitates a different approach. Since there are unfortunately couples who are not able to conceive, it is hard to imagine that God would command them to do something which is not necessarily possible.
Indeed, Rabbi Moshe Feinstein (d. 1986) contends that the mitzvah is in fact also fulfilled through a married couple’s attempt to have children (Iggros Moshe Even HaEzer 2:18). He points to the Mishnah (Yevamos 61b) which states that a man should not refrain from piriya v’riviya unless he has children.
Through closer inspection, there are two essential characteristics of this commandment: firstly for married couples to try for children and secondly to raise children even if they are adopted or fostered. The mitzvah of peru u’revu is therefore less to do with the biological process of having children and more to do with nurturing a healthy family life.