We propose a 2D simulation of the formation of twins, in correlation with their microstructure observed in a Fe-22 wt.% Mn-0.6 wt.% C austenitic steel deformed at room temperature. TEM observations show that microtwins of a few tens of nanometer thick develop in between grain and twin boundaries, and are stored into stacks of a few tenth of micron wide. We first use a model of the emission of an isolated microtwin, based on the critical stress required to develop successive Shockley dislocation loops along parallel slip planes. As the first loop drags a stacking fault, while the following ones only thicken it, when the critical stress is reached, several dislocations are emitted until the backstress shuts down the source at the equilibrium state. After, the twin thickens stably with the increase of the applied stress. The same model is reproduced to simulate numerically the simultaneous formation of interacting microtwins in a stack. We give a general law correlating the average thickness of the twins with the stacking fault energy, their length, their number and their distance. The thickness is a key parameter in our physically based model presented during this congress.