This work presents the dynamic study of the plantar pressure in a group of 15 pregnant women. An instrumented insoles (ECnsole) developed by the research group were used in order to measure the plantar pressure distribution. A questionnaire that described their perception of the back pain were filled by the participants. This test were repeated in weeks 12, 20 and 32 of pregnancy. From our preliminary results, the centre pressure in the Y-axis in both foot were slightly displaced towards the heel in week test 20 and 32, and it agrees with the onset of back pain in pregnant women.