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Background: We aimed to establish the prevalence of interatrial septal openings (IASOs) in newborns, to define their natural course during the first year of life, and to find the predictor factors for their spontaneous closure., Methods: One thousand and seventy two newborns were examined with echocardiography in early postnatal period. The length of interatrial septum (IASL) and IASO diameter were measured, and the "IASO/IASL" ratio was calculated for each case. According to the IASO diameter, study population was divided into 4 groups as Group 1 (< 3 mm), Group 2 (> or = 3-5 mm), Group 3 (> or = 5-7 mm) and Group 4 (> or = 7 mm)., Results: The overall prevalence of IASOs was 78.6% (83.3% in preterms (105 of 126) and 78.1% in full-terms (738 of 946)). The mean diameter of IASOs was 3.1+/-1.1 mm. The overall spontaneous closure rate of IASOs at the end of the first year of life was 96.1% (99.3% in Group 1, 94.8% in Group 2, 64.8% in Group 3 and 50.0% in Group 4). The spontaneous closure was earlier in Group 1 compared to other groups (p=0.011). There were positive correlations between the closure time and the diameter of IASOs, and closure time and "IASO/IASL" (p<0.001 and p<0.001 respectively). The most important factors affecting the spontaneous closure time were the initial diameter of IASO and the "IASO/IASL" ratio (p<0.001 and p=0.029 respectively). When the "IASO/IASL" ratio is < or = 33.3%, its sensitivity and specificity in predicting the spontaneous closure at the end of the first year were 99.1% and 88.2% respectively., Conclusions: The prevalence of IASOs in newborns and spontaneous closure rate of the IASOs with a diameter of less than 5 mm are very high. In addition to the initial diameter of IASO, the "IASO/IASL" ratio is also an important predictor for spontaneous closure.