Diagnosis of fetal infection has depended on identification of pathogens by means of microbiological cultures, immunologic techniques, and special molecular biology techniques that can identify organisms known or suspected of being associated with adverse outcomes of pregnancy. Rubella, cytomegalovirus (CMV), herpes simplex virus (HSV), and human immunodeficiency virus (HIV), for example, are capable of gaining access to the amniotic cavity and producing fetal infection, even when amniotic membranes are intact. Intrauterine invasion by viruses can be associated with maternal symptoms of infection or can be completely silent. In many instances extensive fetal compromise with irreversible structural damage or fetal death will have occurred by the time infection is confirmed by culture or other histopathological methods. The evidence of fetal infection may be as subtle as nascent intrauterine growth restriction (IUGR), mildly inappropriate calcification of fetal organs, placenta, cord, and membranes, and failure to adequately develop fetal fat reserves. The evidence of infection may be as dramatic as obvious fetal malformation, severe central nervous system structural damage, or fetal death. Sonography is capable of detecting most of the grave alterations and some of the subtle effects that are typical of fetal infection.