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There is a positive mitzvah to bring children into the world and raise them to be proper Jews. A possible reason behind this mitzvah is that Hashem didn't want the world to be empty. Additionally, it is also the doorway to all mitzvot because only living people can fulfill mitzvot and the Torah was given to man and not the angels.
- The mitzvah is to endeavor to have children according to his ability.
- Technically, only men are obligated in this mitzvah. Nonetheless, women who do have children do fulfill a mitzvah in aiding her husband to have children. Additionally, some say that women are obligated to have children in order to ensure that the world is populated.
- A person fulfills his obligation once he has two children, one son and one daughter.
- Continuing to have more children is also an important mitzvah. Nonetheless, it is important to take into account other values and concerns. 
- Some say that it is a segula for a woman to have an easy labor if her husband opens the aron kodesh during her 9th month. 
- Rambam (Asin), Sefer HaChinuch (no. 1)
- Yishayahu 45:18, Sefer HaChinuch (no. 1)
- Sefer HaChinuch (no. 1)
- The language of the Gemara Shabbat 31a in heaven a person will be asked whether he "involved" himself in Pru Urevu and not whether he actually fulfilled the mitzvah. Rav Moshe Feinstein (Igrot Moshe EH 2:18) writes that the actual mitzvah for men is to have relations with his wife in order to have children. However, when his wife actually gives birth to children, that isn't this mitzvah but an exemption of the mitzvah. The Minchat Chinuch (no. 1, s.v. VeIm Adam), however, assumes that having the children is the actual fulfillment of the mitzvah. Rav Soloveitchik quoted by Rav Schachter in a shiur (Yevamot 47, min 54-9) on yutorah.org had a compromise opinion in which a person fulfills the action of the mitzvah (maaseh ha'mitzvah) with relations and fulfills the completion of the mitzvah (kiyum ha'mitzvah) when his wife actually gives birth to children.
- Yevamot 65b, Sefer HaChinuch (no. 1), Rambam (Ishut 15:2), Shulchan Aruch EH 1:1. The Mishna (Yevamot 65b) states that women are exempt from this mitzvah because the end of the pasuk states that people should conquer the world and that isn't the tendency of women. Meshech Chachma (Beresheet 9:7) suggests that the reason the Torah exempted women is because having children is dangerous and the Torah wouldn't obligate them to put themselves in danger.
- Ran (Kiddushin 16b)
- Tosfot Bava Batra 13a s.v. Sheneemar implies that women are obligated in the mitzvah of "לא תהו בראה לשבת יצרה". Levush YD 249:15 codifies this. See this article by the Schlesinger Institute for more sources.
- Yevamot 61b, Rambam (Ishut 15:4), Shulchan Aruch EH 1:5
- Rabbi Yehoshua in Yevamot 62b states that if one had children when he is younger he should continue to try to have children when he is older because one never knows the outcome of his children. The Rambam (Ishut 15:16) writes that even though already fulfilled his mitzvah of having children, he has a derabbanan mitzvah to have more children and if he does it is as though he built an entire world. Rabbi Melamed adds that there is a mitzvah of VeHalachta Bedrachav to have more children. Tosfot Bava Batra 60b s.v. din writes that if everyone would only have one son and one daughter, the Jewish nation would cease to exist.
- Rabbi Melamed explains that it is important to weigh one's decision carefully. For example, he says, if a person knows that if he has more than eight children they will have a lot of stress, frustration, and difficulty, then it is likely advisable not to continue to fulfill this mitzvah if it will cause one to sin and negatively impact the Chinuch of the children.
- The Chida Avodas HaKodesh; Moreh B'etzba 3:4