The fallacy of placental migration: effect of sonographic techniques

A study of 233 consecutive second-trimester sonographic examinations performed between 15 and 20 weeks showed no cases of apparent placenta previa and only one case (0.4%) that appeared to be a marginal placenta previa. This differs from previously reported incidences of false-positive placenta previa of 5.6%-45%. The difference is thought to result from the technique of studying patients with completely empty bladders, taking special views of the internal os, and recognizing the temporary changes produced by myometrial contractions, which can give the false-positive diagnosis of placenta previa.