“G-d blessed them and He said to them: Be fruitful and multiply, fill the earth and subdue it; and rule over the fish of the sea, the bird of the heaven, and every creature that moves on the earth” (Bereishit 1:28)
Radak explains that “Be fruitful” refers to giving birth, and “multiply” refers to raising children. Although these words were meant as a blessing, as is understood regarding the fish, our Sages say that regarding people, it is a mitzvah, as well (Yevamot 65).
“G-d blessed Noach and his sons, and He said to them: Be fruitful and multiply and fill the land” (ibid. 9:1)
“You will blessed more than all other nations; there will be no infertile male or infertile female among you or among your animals” (Devarim 7:14)
The Ohr Hachaim states that this blessing will be fulfilled when there will be no infertile person among Bnei Yisrael.
Rashi (ibid. vs. 22) explains that the word פרו (be fruitful) comes from the root פרי, meaning, “Produce fruits!” Regarding the word ורבו (and multiply), Rashi comments, “If He had not said anything but ‘Be fruitful,’ one creature would have begotten one offspring but no more. ‘And multiply’ allowed for the possibility that one can beget many.”
The Sifri states (Berachah 358): None of the tribes was blessed with children as was Asher. From where is this derived? Our Sages explain the words (Devarim 33:24) “Blessed of sons” to mean that Asher will be blessed by sons. He will have numerous children. This is in line with the verse regarding Yosef (ibid. vs. 13), ‘Blessed by Hashem is his land – with the heavenly bounty of dew.’ All the other tribes were blessed either with might or land, whereas Asher alone was blessed with offspring” (Ramban, Devarim 33).
The Shelah (Parashat Vayechi) writes how fortunate is the person who merits not having unsatisfactory offspring. For this, the devout one should constantly pray. One leaves nothing after him in this world besides for his upright children. Concerning Yaakov’s blessing of Yosef’s sons, the pasuk states that “he blessed Yosef.” Their blessings and virtues are attributed to their father. When Yaakov conferred the blessings on Yosef’s sons, he told Yosef, “But progeny born to you after them shall be yours.” This means that if he would beget more children, as fitting as these, who go “after them,” i.e., follow in their footsteps, they “shall be yours,” they would be Yosef’s acquisition forever. How fortunate is the father who leaves behind upstanding, righteous children.
A segulah for a woman who has trouble conceiving is to look at the milah knife while the blood on it is still fresh (Sefer Hamiddot).
