A number of problems beset the indirect diagnosis of posterior placenta previa using transabdominal ultrasound. We add a new potential complicating factor. In 128 pregnancies at or beyond 30 weeks, gestation, measurements were taken from the fetal skull to the maternal sacrum before and after compression. Up to 69% compressibility of the placenta was found in vivo and in vitro. Modified fetal skull to maternal sacrum measurement criteria were devised from the results. Placenta previa is highly unlikely if the measurement from the fetal skull to maternal sacrum is less than 10 mm before compression or less than 7 mm after compression. Placenta previa is probable if the measurement is greater than 20 mm before compression or greater than 15 mm after compression. In 40% of the cases, posterior placenta previa could not be excluded. We conclude that placental compressibility is an additional confounding problem for indirect ultrasound assessment of posterior placenta previa and that indirect assessment should be attempted only if maneuvers to image the lower uterine segment directly are unsuccessful.