Maternal and child exposomics can characterize complex exposure profiles, but measurement remains uneven across the developmental continuum. Although prospective preconception cohorts and studies of minipuberty demonstrate the feasibility of early enrollment and infant hormone phenotyping, these windows are not routinely harmonized across cohort designs. We propose developmental-event-guided exposomics, combining core pregnancy and birth assessments with targeted preconception recruitment and infant sampling during biologically defined windows. Sampling frequency and specimen type should reflect chemical toxicokinetics, with paired external and internal measurements used to calibrate exposure estimates. Computational models should quantify uncertainty rather than obscure unobserved developmental states. Standardizing a limited set of sentinel windows could reveal modifiable sources while biological responses remain measurable, providing a stronger basis for timely exposure reduction before critical opportunities for prevention are lost.