We propose a simple method of measuring polymerization-shrinkage evolution during curing in photopolymer. The real-time spectral fringe analysis of a broadband beam transmitted through a Fabry-Pérot etalon supported by a photopolymer film provides the shrinkage evolution during curing. For the proof-of-principle demonstration a blue-sensitized nanoparticle-polymer composite material is used. It is shown that the measured shrinkage dynamics are well correlated with the photo-calorimetric conversion dynamics of monomer to polymer. We also discuss a discrepancy in steady-state shrinkage between our proposed and holographic Bragg-angle detuning measurements.