The effect of the urinary bladder in obstetrical sonography

Observations of random obstetrical cases out of a large-volume practice are presented to demonstrate the often overlooked effects of filling the urinary bladder for obstetrical sonography. The effects, predominantly evident in the second trimester, include: perssure effects on the lower uterine segment, displacement of the cervix and/or corpus of the uterus, rotation, flexion, version, and tilting of the uterus. These events may affect the perception of placenta location and fetal position. This is especially important in the diagnosis of placenta previa in the second trimester. Many instances of placental "migration" may be due to this phenomenon. Implications for amniocentesis are briefly discussed.