Every union creates new souls:
Every marital union between a husband and wife which takes place in a form of holiness creates a Jewish soul. This soul, may enter the body of another child who is born, if that woman does not become pregnant with the soul that they created. It is for this reason that marital unity is permitted between a husband and wife even during times that she cannot conceive, such as during pregnancy, or after menopause. Accordingly, the Zohar explains that although Sarah was physically barren, she merited to have many spiritual children. This is learned from the verse which states “Vethi Sarah Akarah, Ein Lah Velad/And Sarah was barren, she had no children.” This means that although she did not have children, she gave children to others. The explanation is as follows: The souls created by the unity of Avraham and Sarah were conceived within other women’s fetuses and born carrying that soul. The male souls created entered male fetuses, and the female souls entered female fetuses. Accordingly, although Avraham and Sarah had no physical children [until the birth of Yitzchak], they had many spiritual children which were born to babies on earth. These children who were born carrying the holy children souls of Avraham and Sarah eventually converted and became followers of Avraham and Sarah. This is the mystical meaning behind the converts that were made by Avraham and Sarah, as it refers to the souls that they created and were born into the bodies of those who eventually came to convert.
 Shlah Hakadosh Lech Lecha p. 45