In the past, many patients have been referred to a FHS participant site for evaluation and treatment of maternal or fetal abnormalities. Until recently, most studies regarding maternal or fetal disease have been single-center studies. Many maternal and fetal abnormalities are exceedingly rare. Due to the relative rarity of many forms of fetal congenital malformations, little collective short- and long-term data on outcomes exist. The Fetal Heart Society aims to collect data on a large cohort of patients and improve best clinical practice and improve outcomes for rare diseases.