BACKGROUND CONTEXT: The kinematics of the lumbar region and the activation patterns of the erector spinae muscle have been associated with the genesis of low back pain, which is one of the most common complications associated with pregnancy. Despite the high prevalence of pregnancy-related low back pain, the biomechanical adaptations of the lumbar region during pregnancy remain unknown. PURPOSE: This study analyzes lumbar spine motion and the activation pattern of the lumbar erector spinae muscle in healthy pregnant women. STUDY DESIGN: A case-control study. PATIENT SAMPLE: The study involved 34 nulliparous women (control group) and 34 pregnant women in the third trimester (week 36 § 1). OUTCOME MEASURES: We recorded the parameters of angular displacement of the lumbar spine in the sagittal plane during trunk flexion-extension, and the EMG activity of the erector spinae muscles during flexion, extension, eccentric and concentric contractions, and the myolectrical silence. METHODS: The participants performed several series of trunk flexion-extension movements, which were repeated 2 months postpartum. The position of the lumbar spine was recorded using an electromagnetic motion capture system. EMG activity was recorded by a surface EMG system and expressed as a percentage of a submaximal reference contraction. RESULTS: Antepartum measurements showed a decrease (relative to control and postpartum measurements) in lumbar maximum flexion (52.5 § 10.5° vs 57.3 § 7.7° and 58.7 § 8.6°; p < .01), the percentage of lumbar flexion during forward bending (56.4 § 5.6% vs 59.4 § 6.8% and 59.7 § 5.6%; p < .01), and the time keeping maximum levels of lumbar flexion (35.7 § 6.7% vs 43.8 § 5.3% and 50.1 § 3.7%; p < .01). Higher levels of erector spinae activation were observed in pregnant women during forward bending (10.1 § 4.8% vs 6.3 § 2.4% and 6.6 § 2.7%; p < .01) and eccentric contraction (12.1 § 5.2% vs 9.4 § 3.1% and 9.1 § 2.9%; p < .01), as well as a shortened erector spinae myoelectric silence during flexion. CONCLUSIONS: Pregnant women show adaptations in their patterns of lumbar motion and erector spinae activity during trunk flexion-extension. These changes could be associated with the genesis of pregnancy-related low back pain, by means of biomechanical protection mechanisms against the increase on abdominal mass and ligamentous laxity.