A case is reported of a lumbosacral plexus compression by the fetus in a young 34-weeks pregnant woman, who had low-back pain and progressive muscular weakness of the leg. Neurological examination showed a grade IV motor weakness of the iliopsoas, quadriceps femoris and biceps femoris muscles. Mechanical stretch manoeuvers were negative. Electromyography revealed denervation activity in L4 and L5 muscles. Lumbosacral plexus radiculopathy was diagnosed. Although fetal compression appears to be an uncommon cause of lumbosacral radiculopathy during teenage pregnancy, both neurosurgeons and obstetricians should be aware of the possibility.