138 Women with previous lumbar problems or chronic back pain conditions are more likely to develop back pain during pregnancy, with pain occurring twice as often as in those with no prior complaints. They are also more likely to have severe and long lasting pain [6]. Along those same lines, women who experience back pain during one pregnancy have an 85% chance of experiencing back pain during a subsequent pregnancy [7]. Activity is also related to the onset of back pain during pregnancy [3, 8]. A sedentary lifestyle increases risk of back pain compared to patients who engage in a more active lifestyle. Nonetheless, patients who have occupa- tions described as ‘mostly active’ and ‘physically demanding’ also have a higher risk of developing pain during pregnancy suggesting that extremes of activity are probably not ideal. Increased body mass index (BMI) may be a risk factor but study results are conﬂicting. Orvieto et al. found that BMI was signiﬁcantly higher in those who experience pain compared to those who did not [9]. These results agree with those found by Mogren et al. who found a mean pre and end BMI of 24.57 and 30.10, respectively, in those who had low back pain compared to BMIs of 23.30 and 28.56 in those who did not [3]. However, Mens et al. found no signiﬁcant difference between BMI before pregnancy in those with back pain and the general population [7]. Younger age and multiple parity also increase the inci- dence of back pain [3, 10]. Associated symptoms include stiffness and limited motion in the back or legs. Pain and associated symptoms may be constant or may only occur in certain positions or after extended activity. Approximately one-third of patients report that pain increases as the day goes on while another one-third report that the pain worsens during the night and often disturbed sleep [2]. Approximately 10% of women claim that it prevented them from working [11] and more than 80% report that it affects their daily routine including housework, childrearing, and job performance [7]. Nore´n et al. report back pain as the leading cause of sick leave during pregnancy in Scandinavian coun- tries, with an estimated cost of $2.5 billion in 1990 [12]. Etiology Low back pain in pregnancy is generally ascribed to the many changes in load and body mechanics that occur during the carrying of a child. It is normal to gain between 20 and 40 pounds during pregnancy. This clearly shifts the body’s center of gravity anteriorly and increase the moment arm of forces applied to the lumbar spine. Studies suggest that an anterior shift is associated with pubic symphysis problems. Furthermore, postural changes may Curr Rev Musculoskelet Med (2008) 1:137–141 be implemented to balance the anterior shift, leading to lordosis, and increase in the natural inward curvature of the spine, further increasing stress on the lower back [13]. The intervertebral discs respond to axial loading by expelling ﬂuid, resulting in decreased height and an overall compression of the spine [14]. Rodacki et al. showed that the spines of pregnant women with low back pain compress more after activity than pregnant women without back pain and those who are not pregnant, 4.57, 4.23, and 3.99 mm, respectively. Pregnant women with low back pain also take longer to recover from activity related compression [15]. The abdominal muscles also stretch to accommodate the expanding uterus. As they stretch, they lose their ability to perform the function of maintaining body posture, causing the lower back to support the majority of the increased weight of the torso. Studies comparing pregnant women enrolled in an exercise program designed to address core strength, ﬂexibility, and muscular endurance, particularly abdominal strength, with those involved in no exercise program, demonstrated a decrease in postural changes and severity of pain in the exercise group [16]. A signiﬁcant portion of women ﬁrst experience pain during the ﬁrst trimester, when mechanical changes do not yet play a signiﬁcant role in the etiology of pain [3]. This suggests that some pain may be secondary to hormonal changes rather than physical stresses. Hormonal changes during pregnancy also cause inﬂammation and pain in the back. For example, some studies have found back pain to correlate with increased levels of relaxin which are pro- duced during pregnancy [17] even though others have not [18, 19]. It has been suggested that the hormone relaxin increases 10-fold in concentration during pregnancy. As the structures of the pelvis and lower spine soften and become more pliant, discomfort may result particularly not only in the sacroiliac joint but also generalized over the entire lower back. Another theory has been put forward that low back pain during pregnancy, especially pain that worsens at night and is severe enough to wake the patient up, is the result of venous engorgement in the pelvis. The expanding uterus presses on the vena cava, particularly at night when the patient is lying down. This combined with the increased ﬂuid volume from ﬂuid retention during pregnancy leads to venous congestion and hypoxia in the pelvic and lumbar spine. Fast et al. cor- related this condition to the low back pain experienced at night in patients with congestive heart failure [20]. Sciatica, caused by herniation or bulging of an inter- vertebral disc resulting in nerve compression is often thought to be the cause of low back pain. Associated symptoms of pregnancy, like leg pain and faulty bladder control, can point to this diagnosis. However, during pregnancy this is very unlikely and only presents in about 1% of pregnant women.