The pubic symphysis, altered by the physiologic and hormonal influences of pregnancy, exhibits changes which are only now beginning to be understood. Among the changes incidental to relaxation and widening is the appearance within this articulation of a roentgen shadow of gas or vacuum density. This phenomenon, commonly overlooked or misinterpreted, has only recently become the subject of discussion. Each opposing surface of the pubic bone is covered by a thin layer of hyaline cartilage joined to the bone by a series of nipple-like processes (7, 8). The facing surfaces are connected with each other by fibrocartilage of varying thickness, often containing a longitudinal cavity or cleft, unlined by synovial membrane. It is said to be this potential space, ordinarily invisible on a roentgenogram, which under the influence of pregnancy becomes visible as the shadow described above. History Utilizing an intravaginal film applicator, Dale (3), in 1930, studied the pubic symphysis of pregnant women and noted varying degrees of widening. In non-pregnant women and those who had been pregnant some time previously, the average joint width was 4 mm. In 35 women, pregnant but not complaining of pubic pain, the average width was 6 mm. In 19 patients complaining of discomfort, the average width was 7 to 8 mm. Nearly all of this last group showed a fissure centrally located in the pubic articulation. Magnusson (12), in 1937, observed the gas or vacuum phenomenon in the pubic articulation and other joints but did not believe that any practical importance could be attached to it. Williams (17), in an excellent and definitive investigation, studied 3,000 roentgen pelvimetry films and found the phenomenon to be common. In a specific analysis of 232 roentgen pelvimetric studies done near term, the incidence of “gas” streaks in the symphysis was 41.5 per cent. No correlation was observed between pain in the symphysis and the presence or absence of “gas.” In most cases the gas disappeared within one week following delivery, although in 2 cases it could still be seen twenty days later. Material and Findings We have observed this gas shadow for some time and it was therefore decided to check our findings on a sample of 100 cases. The films were taken consecutively, and completely at random, from a drawer which contained pelvimetric studies, unfiled according to any pattern. They were unselected except that they represented examinations of women near term who had been studied for the usual obstetrical indications. A clear and unmistakable dark streak running vertically near the center of the pubic articulation was interpreted as positive (Fig. 1). All doubtful shadows, including probable ones, were called negative. Even with these rigid criteria, 30 of the 100 cases showed this phenomenon. There was no notable difference between primiparae and multiparae. The size or shape of the pelvis did not playa significant role.