Abstract: Objective: Determine the related causes of stillbirth and reduce the unexplained cases Design: Observational cohort study Setting: 14 selected hospital from 12 provinces (14 cities) of Iran Population: A total of 105,562 births and 762 registered stillbirths at 22 completed weeks of gestation or more Methods: Iranian Maternal and Neonatal Network (IMaN) registers information about almost all births (live & dead) around the country, but this network does not collect data about stillbirth causes. In this study, we developed the stillbirth evaluation protocol with experts’ cooperation, and we designed forms for the stillbirth registration system electronically. Then we trained related individuals in 14 selected hospitals around the country. After a year, we extracted, analyzed, and, based on the Relevant Condition of Death Classification (ReCoDe), interpreted the collected data. Main outcome measures: Related causes of stillbirth Results: In 742 registered stillbirth cases in 14 selected hospitals, the relevant causes were identified in 65.4% of cases, while 34.6% of cases remained unclassified. The most frequent relevant conditions were fetal (33.2%), maternal (9.1%), amniotic fluid (8.8%), placenta (7.7%), and umbilical cord (6.2%). Conclusions: Our registration decreased the percentage of stillbirth with an unexplained cause from about 70% to 34.6%. Funding: Iran University of Medical Sciences Key words: stillbirth, cause of stillbirth, Iran Tweetable abstract: The most frequent relevant causes of stillbirth in Iran were fetal, maternal, and amniotic fluid conditions.