Do the Math
I just finished a book whose title I will not mention. One of the background details is that the Thinly Disguised Oppressive Xtian Star Empire took the better part of a thousand years to make back the population lost in a war. Another bit of information is that while they suppress some areas of science and technology because they are ungodly, they are in general advanced enough to be competition for Undisguised Islamic Star Empire, the I Thought Pearl S. Buck Was Keeno Chinese Star Empire and the Bland Realm of Reasonable People Star Empire. Yet another bit of information is that women from the TDOX generally have somewhere between five and eight babies.
I would like to say at this point that it takes a lot of courage to develop a future that is so daringly imaginative. Not since Orson Scott Card's ENDER'S RISK GAME have I seen a future so edgy in its speculative nature.
My intepretation of this is that child mortality should be as low as ours (a high rate of child death isn't mentioned, nor is famine or other causes of short lifespans) but we also know the population growth rates we would expect from five to eight children don't seem to be there: that's, what, around 3 to 5 percent growth rate per year, so a doubling time of 23 to 14 years? At those rates, a thousand years is enough for the population to grow by somewhere between 10^13 to 10^22 times the original one and clearly that isn't what happened.
So ignoring the remote possibility that the author just never thought about those particular facts all at the same time, how do we explain the combination of huge numbers of children, low mortality rates and an apparently nearly flat population curve? Defered marriage? Or better, something that keeps most people from marrying at all.