After fertilization, fathead minnow eggs are approximately 1.4 mm to 1.6 mm in diameter. Following several well-defined embryonic stages, eggs hatch within 4 to 5 days at 25°C. Upon hatching, fathead minnow larvae absorb the yolk sac within 1 to 2 days, afterwhich larvae become active feeders preying upon live food. These protolarvae range in length from 4.0 mm to 5.2 mm and can be characterized by an incomplete mouth, dark eyes, rudimentary pectoral fins, melanophores, which are widely distributed on the yolk sac and concentrated in regions posterior to the vent. Contrastingly, mesolarvae and metalarvae possess high concentrations of melanophores on the ventral surface of gill covers. The number of myomeres between protolarvae and mesolarvae, and metalarvae differ only slightly, with the metalarvae possessing a slightly more in the predorsal region and a few less in the postanal region. All larval stages have rounded rather than flattened eyes, similar to 'bluntnose minnows Pimephales notatus'.
Reproductive maturity in fathead minnows is identified by a number of morphological changes in both males in females. For example, males develop a dorsal pad, tubercles on their lower jaw, and banding changes. Just prior to maturation, females develop urogenital papillae. Fathead minnows reach sexual maturity within 4 to 5 months after hatching in optimal habitat conditions (e.g., water temperature of 25°C and photoperiod of 16 hours of light). Once mature and under appropriate seasonal conditions, minnows can spawn continually for a period of several months.