In utero infections of the fetus can lead to significant morbidity and mortality in the newborn child. The signs and symptoms of clinical disease, however, do not always suggest a given pathogen. The laboratory must be able to provide an early and accurate diagnosis of the causative agent so that prompt and appropriate antimicrobial therapy and medical care can be initiated. The scope of this article includes the methods employed by the laboratory to assist in the diagnosis of bacterial, fungal, parasitic, and viral infections of the fetus. Where appropriate, detection methods were addressed for the diagnosis of the major pathogens responsible for infection during the birth process.