Changes in symphysis pubis width during labor.

We studied changes in the width of the symphysis pubis in 32 women examined serially by ultrasound during labor. Measurements were made at the superior border of the symphysis and at its narrowest breadth in the latent phase, the active phase, and the second stage of labor. There was a significant increase in the width of the symphysis between the first and second stages of labor at both measured levels. Widening was observed in 94% at the superior symphyseal breadth and in 59% at the narrowest. Of those cases in which the width of the symphysis increased, there was a large spectrum of change, ranging from 9 to 98% of the original width at the narrowest measurement site and from 2 to 139% at the superior breadth. There was a strong inverse correlation between maternal age and the degree of symphyseal widening in nulliparas, but not in multiparas. We conclude that labor is associated with a substantial widening of the symphysis pubis in most, but not all women.