Sciatica can be caused by a number of underlying conditions, including piriformis syndrome (the tightening of the piriformis muscle, located in the buttock), a herniated disc in the back, sacroiliac joint dysfunction (a weight-bearing joint in the lower back), and others. Sciatica is especially common in pregnant women since the expanding uterus places extra pressure on the sciatic nerve, and in people whose jobs require them to drive long distances or sit for long periods of time.