[VladimirIvanov]

If you intend to send your child to college then the cost per child is very high. If you don't then the marginal cost per child is very low. Another consideration is whether the mother works and if it's possible for her to keep taking time from work every few years to have additional children. One thing I notice about large families is that they usually do not pay for their children's college education. TLDR - marginal cost per child increases in developed countries.