This paper evaluates age, period, and cohort effects on marital fertility during onset of the Utah fertility transition (1880–1909). Computerized genealogies are used to derive age–period–cohort fertility rates for 49,842 once-married couples. Age, period and cohort effects on marital fertility are then estimated using Johnson's (1985) relational model. Declining marital fertility in Utah is shown to be explained by both lower fertility levels across periods and increasing age-specific limitation across cohorts. Direct cohort effects on fertility are insignificant. These results are consistent with prior research, and the view that fertility levels were adaptive (in part through birth spacing across ages) to immediate contexts of childbearing while age-specific fertility truncation increased across cohorts (in part through the more general diffusion of contraceptive innovations).