A simple two‐dimensional analytic model is evaluated for transmitted cardiac motion in fetal lung. The model treats the latter as being an incompressible viscoelastic medium. The mean radial deformation in an elastic medium is demonstrated to depend on a length parameter , where μ, ρ, and ω corespond to elastic shear modulus, mass density, and frequency of cardiac motion, respectively. Digitized M‐mode images are demonstrated as a feasible method to measure such deformations invivo. Data for two patients are presented to illustrate the technique.