When immiscible fluids flow in microchannels, monodisperse droplets or jet are obtained depending upon the flow rate of the aqueous phase and the oil phase. In this work, we present experimental data showing that the confinement and the shape of the geometry play a fundamental role. We analyze the stability of the jet in the framework of the lubrication at low Reynolds number. In cylindrical geometry, we relate the transition between the droplets regime and the jet regime to the absolute/convective transition of the Rayleigh Plateau instability and reach a remarkable agreement with the data. In rectangular geometry, we follow the same analysis and point out the existence of an absolutely stable bidimensional jet.