Weight gain and stretched and weakens abdominal muscles by an enlarging gravid uterus are remarkable features during pregnancy. These changes elicit postural and movement instability and place strain on various body segments. In general, agonist and antagonist muscles of body segments act simultaneously to increase joint stabilization during human movements. The co-contraction might be well observed in pregnant women because of their unstable body joints. Musculoskeletal models are well used to investigate muscle load. However, very few studies have been conducted on the model for pregnant women. Additionally, it is difficult to estimate the co-contraction of muscles by using musculoskeletal models. Therefore, the purpose of this study is to construct musculoskeletal model for pregnant women and estimate co-contraction of trunk muscles. At first, motion analysis of sit-to-stand for a pregnant woman was conducted to obtain the motion and force data for inputting to musculoskeletal model. Simultaneously, muscle activation of rectus abdominis and longissimus were measured by using surface electromyography. On the other hand, the size and mass of body segment of a musculoskeletal model were changed to meet pregnant women. Then, stretched abdominal muscle was modeled. At last, the co-contraction of rectus abdominis and longissimus was estimated from EMG data and joint torque that was calculated from the musculoskeletal model using genetic algorithm.