Actually, the biggest problem with your math is that you state that 21,000 children cannot be born in a year, when, in fact, if we assume your 170 million number is correct for 1 A.D. (which is a conventional way of referring to time), and assume that 25% of the population were of child-bearing age (a conservative estimate) and that 50% of that 25% are females, that comes out to 21 million females