The slavers want to limit the amount of children you have, so you cannot rise up against them. That is exactly what Pharaoh did in Egypt. God, on the other hand, has a way of blessing the poor. If the poor are fertile, it doesn't take too many generations before they rise up and overthrow their masters.
"Now there arose a new king over Egypt, who did not know Joseph. And he said to his people, “Behold, the people of Israel are too many and too mighty for us. Come, let us deal shrewdly with them, lest they multiply, and, if war breaks out, they join our enemies and fight against us and escape from the land.” Therefore they set taskmasters over them to afflict them with heavy burdens. They built for Pharaoh store cities, Pithom and Raamses. But the more they were oppressed, the more they multiplied and the more they spread abroad. And the Egyptians were in dread of the people of Israel. So they ruthlessly made the people of Israel work as slaves and made their lives bitter with hard service, in mortar and brick, and in all kinds of work in the field. In all their work they ruthlessly made them work as slaves. Then the king of Egypt said to the Hebrew midwives, one of whom was named Shiphrah and the other Puah, “When you serve as midwife to the Hebrew women and see them on the birthstool, if it is a son, you shall kill him, but if it is a daughter, she shall live.”
Exodus 1:8-16 ESV