A decline in the availability of oxidized nicotinamide adenine dinucleotide (NAD+) is an important trigger of aging across diverse species, including yeast, nematodes, and mammals. Decreased concentrations of this metabolic molecule drive progressive losses of physiological function across various tissues (1). On page 1194 of this issue, Ciampa et al . (2) report that NAD+ depletion in the placenta is not merely a marker of tissue fatigue. Rather, NAD+ decline is a regulatory component of a precisely orchestrated metabolic countdown that determines when a new life is brought into existence. More generally, this discovery reframes the role of NAD+ in metabolism, development, and aging.