I have not read through the thread but do have a comment. From what I have read, humans have long had families of 4-5 children. Today in traditional societies in which people have children 3-5 years apart (4-5 years apart from hunter gather times) 5 children is not uncommon. From what I understand, the commercialization of formula as well as birth control, had heavy influence on fertility, and therefore family size, in western culture in the last century. A lot has changed in the past century or so.
I actually view this as a very difficult century in which to have children. We seem to be bridging centuries and so much has changed for the last one hundred years of humanity.
In the 1800's no one would bat an eye at nursing a three year old and now it seems verboten. Before the 20th century, 5-6 children would seem common (actually it went from 7 children to 3.5 between 1800-1900).
It's also important to remember that maybe 2 or 3 of those children would have lived to adulthood. So, there may have been 5-7 births, but some would have died in infancy and others in childhood. Falling birth rates tend to have more to do with infant survival and industrialization (you didn't need so many children to work the farm). How late children were weaned has varied through the centuries.