Until about a decade ago, the recent trend was for the poor to have more children than the rich, both domestically and internationally. But, this trend has since reversed in the U.S., largely due to the impact of fertility treatments and factors surpressing fertility in the poor like a reduction in teenage pregnancy rates. But, this was a historical anomaly for the century or so that the trend of the poor having more children persisted. Prior to that point, the well to do in places as different as early modern England and China consistently had more surviving children than the poor.
It isn't unreasonable to infer that the rich had more children in part because mothers and children were less likely to die prematurely, for example, due to malnutrition, and that the reversal in the number of surviving children per family was due substantially to the fact that improved medical care caused almost every mother to survive childbirth and almost every child to live to adulthood.