Circadian rhythms in gene expression and hormones are ubiquitous across species and differentiated cell types, yet their developmental origins remain poorly understood. This study aimed to determine if daily rhythms can be detected in utero and if they synchronize to the mother. We developed methods to longitudinally monitor PERIOD2 (PER2), a core circadian clock protein, from embryonic day (E)8.5 to E17.5 by restricting PER2::LUCIFERASE expression to the mouse fetoplacental unit (fetus and feta...