X-linked ichthyosis (XLI) is a relatively common genetic disorder that occurs in about one in every 2000-6000 male births. Clinically, XLI is characterized by a generalized scaling of the skin, with large, polygonal, dark brown scale, and more prominent on the extensor aspects of the limbs. It is known that an undetectable maternal serum, unconjugated estriol, associated with placental steroid sulfatase (STS) deficiency, may be the cause of cause of XLI. In most case, STS deficiency is caused by a complete or partial deletion of the STS gene mapped on chromosome Xp22.3. We describe here the prenatal detection of a male fetus affected with STS deficiency as a result of an undetectable unconjugated estriol in the second-trimester maternal serum screening. Microdeletion of the STS gene was confirmed by fluorescence in situ hybridization analysis of cultured amniotic fluid.