PROBLEM: Microbial infections of the chorioamnion and amniotic fluid have devastating effects on pregnancy outcome and neonatal morbidity and mortality. The mechanisms by which bacterial pathogens cause adverse effects are best addressed by an animal model of the disease with a naturally-occurring pathogen. METHOD OF STUDY: Intrauterine infection in humans as well as genital mycoplasmosis in humans and rodents is reviewed. We describe a genital infection in rats, which provides a model for the role of infection in pregnancy, pregnancy wastage, low birth weight, and fetal infection. RESULTS: Infection of Sprague-Dawley rats with Mycoplasma pulmonis either vaginally or intravenously resulted in decreased litter size, increased adverse pregnancy outcome, and in utero transmission of the microorganism to the fetus. CONCLUSION: Mycoplasma pulmonis is an ideal model to study maternal genital infection during pregnancy, the impact of infections on pregnancy outcome, fetal infection, and maternal-fetal immune interactions.