Introduction Patency of the oval foramen (PFO) and atrial septal defects (ASDs), i.e. interatrial communications, have been reported to be present in 24–92% of newborns. For isolated ASDs, the reported prevalence varies from 0.3–2.4 per 1000 livebirths. This broad range partly reflects the absence of a clear and universal clinical classification of interatrial communications. On this basis we have recently proposed a clinically applicable echocardiographic algorithm for classification of interatrial communications (figure 1), which proved superior to standard assessment by experts in terms of inter- and intraobserver agreement. Purpose To determine the prevalence of interatrial communications in newborns. Methods Echocardiograms (TTEs) from newborns (0–30 days) included in a population study (N>25,000) were assessed/analyzed for interatrial communications according to the new algorithm and categorized into three PFO subtypes and three ASD subtypes. Results TTEs from 16,420 newborns were analyzed; 3,694 (22.5%) were excluded due to suboptimal image quality and 9 (<0.1%) were excluded due to concurrent severe congenital heart disease. Of the remaining 12,718 included newborns (median age 12 days [8; 15], 48.1% female), an interatrial communication was present in 10,033 (78.9%) cases; 9,274 (72.9%) cases were classified as PFO while 759 (6.0%) were classified as ASD. In the ASD group, 368 (48.5%) had a defect size ≥4 mm, 364 (48.0%) had multiple interatrial communications, and 27 (3.6%) had the defect located in the lower 1/3 part of the septum (figure 2). Conclusion An interatrial communication was present in almost 4 out of 5 newborns. ASD was diagnosed in 6% of the newborns and the prevalence of PFO was 12 times higher than the prevalence of ASD. Follow-up studies of these children are expected to provide clinically useful information on the long term structural and hemodynamic impact of these well categorized ASD and PFO subtypes. Funding Acknowledgement Type of funding sources: Foundation. Main funding source(s): Candy's Foundation, The Danish Children's Heart Foundation