In this study, we contrast period and cohort approaches to answering the question: Is US fertility now below replacement? The answer would appear to be an unambiguous “yes” based on period trends in the total fertility rate (TFR). Since 2007, TFR has declined from 2.12, just above the replacement level set by demographic tradition at 2.10 births per woman, to 1.67 in 2022, leading many to speculate that the United States has now entered a sustained period of below-replacement fertility. A quite different picture emerges from cohort trends in the cumulative fertility rate (CFR), a cohort measure that is not subject to biases that can distort period TFRs. For older birth cohorts of US women—those born between 1959 and 1987 and who were thus age 33 or older in 2020—observed or projected CFRs at age 45 vary between 2.00 and 2.24 births per woman. For younger cohorts—those born between 1988 and 2010 and who were 10 to 32 as of 2020—we project CFRs at age 45 that are below 2.00, with these declines attributable to falling fertility at younger ages. We thus conclude that from a cohort perspective, the question “Is US fertility now below replacement” should be replaced by the question “Will lifetime fertility fall below replacement for the youngest cohorts of US women?”, with the answer to this latter question depending on the extent to which decreases observed at early ages in these cohorts will or will not be offset by future increases at later ages.