Vaginal bleeding is the most common cause of emergency care in the first trimester of pregnancy and accounts for the majority of premenopausal bleeding cases. Ultrasound evaluation combined with a quantitative beta human chorionic gonadotropin test is an established diagnostic tool to assess these patients. Spontaneous abortion because of genetic abnormalities is the most common cause of vaginal bleeding; ectopic pregnancy and gestational trophoblastic disease are other important causes and in all patients presenting with first trimester bleeding, ectopic pregnancy should be suspected and excluded, as it is associated with significant maternal morbidity and mortality. A thorough knowledge of the normal sonographic appearance of intrauterine gestation is essential to understand the manifestations of an abnormal gestation. Arteriovenous malformation of the uterus is a rare but important cause of vaginal bleeding in the first trimester, as it has to be differentiated from the more common retained products of conception, with which it is often mistaken.