Material properties of the placenta under dynamic loading conditions.

Trauma during pregnancy especially occurring during car crashes leads to many foetal losses. Numerical modelling is widely used in car occupant safety issue and injury mechanisms analysis and is particularly adapted to the pregnant woman. Material modelling of the gravid uterus tissues is crucial for injury risk evaluation especially for the abruption placentae which is widely assumed as the leading cause of foetal loss. Experimental studies on placenta behaviour in tension are reported in the literature, but none in compression to the authors' knowledge. This lack of data is addressed in this study. To complement the already available experimental literature data on the placenta mechanical behaviour and characterise it in a compression loading condition, 80 indentation tests on fresh placentae are presented. Hyperelastic like mean experimental stress versus strain and corridors are exposed. The results of the experimental placenta indentations compared with the tensile literature results tend to show a quasi-symmetrical behaviour of the tissue. An inverse analysis using simple finite element models has permitted to propose parameters for an Ogden material model for the placenta which exhibits a realistic behaviour in both tension and compression.