Prenatal diagnosis of hydrometrocolpos in a Down syndrome fetus.

We report the prenatal sonographic diagnosis of hydrometrocolpos caused by an imperforate hymen in a Down syndrome fetus, with spontaneous evacuation on the third day of life. In this case, sonographic evaluation in the 37th week revealed a large retrovesical, sharply marginated, heterogeneous, unilocular cystic structure within the fetal abdomen extending to the left side of the umbilicus.