Introduction Complete atrioventricular septal defect, is a complex heart malformation, characterized by a large, unrestrictive ventricular septal defect associated with an atrial septal defect ostium primum like. Together these defects can give rise to a large communicating area in the inferior region of the interatrial septum and a single atrio-ventricular valve, with different degrees of regurgitation at the right or left valvular part. This type of defect is frequently associated with Trisomy 21 (Down Syndrome). This type of malformation represents 2-5% of all congenital heart defects. Objective Establishing the importance of early diagnosis of Down Syndrome and the possible complications that can occur. Results Boy, postterm newborn, GA=43 weeks, BW=4300g, 4th pregnancy, 4th birth, born to a mother without medical supervision during her pregnancy, presents at birth generalized cyanosis, trisomy 21 phenotype, postductal and preductal blood oxygen saturations 80-85% while receiving oxygen under tent. Laboratory testing reveal severe trombocytopenia which was maintained over the course of hospitalization, despite platelets micro-transfusions. On the 11th day of life Prednison is introduced. On the 15th day of life he was transferred in the Pediatric Cardiology Ward of Emergency Hospital "Sf.Maria" Iasi, where he received diuretics (Spironolactona,Furosemid), conversion enzyme inhibitors (Ednyt). His evolution, from a cardiac stand point, was slow but good. He was released after 15 days in good health, stable hemodynamically, pulmonary, digestive, and satisfactory weight gain. Conclusion Fetal ultrasound is an important tool in detecting congenital defects. In regard to congenital heart defects, they can appear in pregnancies without risk factors, which entails the necessity of prenatal diagnosis and a multidisciplinary approach which should include pediatric cardiologists and neonatologists for the manangement of the newborn with congenital heart defects. Screening tests should be done before conception to identify if risk factors exist for certain diseases or if there is a risk of passing on to the child the disease. These screenings combined with genetic counseling could help in reducing the morbidity and mortality of genetically transmitted defects.