Between 1790 and 1840, median completed family size in New England genealogies fell by half. Age at marriage rose only moderately during this period, but the rate of premarital pregnancy plummeted, intervals between births lengthened, and mothers' age at last birth fell to thirty-seven. Analysis of the number of children born to a subset of mothers younger than thirty in various Connecticut and New Hampshire towns c. 1818 suggests that couples' motives for postponing births appear to have been as much cultural as economic.