Sexual intercourse is a risk factor for both pregnancy and the acquisition of a sexually transmitted infection (STI). Therefore, STIs represent a fairly common diagnosis in pregnant women. The Centers for Disease Control and Prevention (CDC) estimate that approximately 20 million cases of STIs are diagnosed annually in the United States. Pregnancy complications such as preterm labor are also associated with some STIs. Effective prenatal screening, diagnosis, and treatment of these infections will reduce most of the pregnancy complications that are attributable to STIs. This chapter presents the general clinical features of STIs in pregnancy, followed by more specific, detailed information regarding the most prevalent and clinically significant STIs. It discusses presenting symptomatology, history and data collection, physical examination, laboratory and imaging studies, differential diagnosis, clinical management and follow-up of STIs such as chlamydia, gonorrhea, bacterial vaginosis, genital herpes simplex virus (HSV), HIV, syphilis, Zika virus and pelvic inflammatory disease.