It is quite acceptable in many societies to determine the size of one's family based on one's income. I have occasionally heard people express their doubts and worries as to how they will be able to financially support a growing family. Logically, it would make sense to assume that family income should be directly linked to how many children a family shall have.
In the Gemara of Ta'anith (8b) our Rabbis of blessed memory state, that He who gives life, gives sustenance. When G-d blesses a father and a mother with a child, He also provides them with the means necessary to sustain the child. You may have heard the common expression: "Each child brings his own blessings with him".
It is interesting to note that at the time of the Beth HaMiqdash, a woman who gave birth brought two offerings, a sheep and a dove (or a turtle dove). However, if she could not afford the sheep, she brought two doves or turtle doves, (Wayyiqra 12:7).
What is the significance of the two different options?
HaRab Mordechai Eliyahu, 'a"h, explains that though the sheep offering was a more costly offering, G-d who provides the sustenance for each child, would enable the mother to bring a sheep offering. The mother who had faith, would bring the sheep offering. But if the mother did not trust G-d fully and raised questions as to how she would provide for her child, she would bring the less costly offering of two doves.
A profound lesson in Emunah!