There is a school of thought among some genealogists that one ought not to write about things that would have embarrassed one's ancestors if they were alive to hear or read it. If you are in that school, you should definitely not betake yourself to a good library (or this link) and you should definitely not read historian Gloria L. Main's article "Rocking the Cradle: Downsizing the New England Family," Journal of Interdisciplinary History 37(1):35-58, published in 2006.
OK. Now for the rest of us: like most scholarly articles, this one is part of an ongoing conversation and is very condensed in style (especially as to statistical methods). And so it's always hazardous for a newcomer and non-historian to jump in and quote, because I may not be able to distinguish between what Main says that is new to the conversation and what is old hat, or between the accepted facts and her new and less fully accepted ideas. (Also I don't know if the conversation has advanced in the subsequent four years.) But it is a conversation genealogists might want to have an ear on.
The fact is that New England families average only about half as many children in 1790-1840 as they had before. The mystery is how and why. Main writes,
Ordinary families living in New England's countryside who avoided or terminated pregnancies did so without the aid of any new contraceptive technology or medical knowledge. ... it was far more difficult to prevent babies in the New England of 1800 than it is today in Bangladesh.She draws on the rich well of New England vital records and family histories, noting that "The best of them have been refereed by professionals and published by reputable presses." She also has some interesting things to say about them that affect both their genealogical and historical value: "Since male heads of large households generated longer paper trails, genealogies are inevitable biased toward large families iwth many male descendants." Of course there's also the Revolutionary-era decline in record-keeping and increase in mobility. But "under-recording of births and deaths, especially of females, by town clerks in New England occurred from the outset." She's got the numbers.
Main also has a detailed discussion of the few ways that New Englanders could have limited their family size. For this venue, suffice to say it was not easy or fun. So why did they do it? The statistics she marshals do not suggest that children had become an economic burden, but they may have become a time burden. Fewer babies do correlate with less farming in a county as of 1820, and also with more meetinghouses and libraries than average. Main's suggestion:
Since women outnumbered men in church membership in early New England and were better educated and more politically aware than ever before, rising female status within the family may explain the greater willingness of men to cooperate in helping their wives avoid pregnancy.