The photophysical properties of lead halide perovskite nanocrystals (NCs) are critical to their potential application in light emitting devices and other optoelectronics, and are typically characterized using optical spectroscopies. Measurements of nuclei and nascent NC photophysics during synthesis provide insight into how the reaction can be changed to control the properties of the resulting NCs. However, these measurements are typically only performed ex situ after growth is halted by centrifuging the reaction mixture for several minutes. Here, a method is reported to rapidly sample the reaction mixture during a solvation-limited synthesis to enable multiple spectroscopic measurements during nucleation and NC growth. Absorbance and fluorescence measurements of a reaction mixture during the formation of methylammonium lead triiodide perovskite NCs are reported. The changing positions of spectral features as a function of reaction time show the expected weakening of exciton confinement during NC growth. The evolving fluorescence spectra demonstrate that the capping and surface passivation of nascent NCs changes during the reaction. The species in the reaction mixture, particularly during the early stages of the synthesis, are shown to be unstable. This indicates that, even for a relatively slow solvation-limited reaction, the photophysics of the reaction mixture can only be accurately captured if spectroscopic measurements are completed within seconds of sampling. The common use of centrifugation to quench NC syntheses prior to spectroscopic measurement biases the NC population towards more stable, well-capped NCs and does not accurately report on the full NC population in a reaction mixture.