Weakness of the heartbeat in newborns is often caused by diseases, and common causes include congenital heart disease and hypothyroidism. 1. Congenital heart disease: Congenital heart disease is a cardiovascular disease in which the heart and blood vessels develop abnormally during fetal life and are present at birth. Different shunt direction of congenital heart disease will lead to different symptoms, some common symptoms include generalized cyanosis, dyspnea, heartbeat weakness, etc. 2. Hypothyroidism: Congenital thyroid defects in newborns or lack of iodine in the mother’s diet during pregnancy can lead to congenital hypothyroidism, which is often characterized by a weak heartbeat, hypothermia, cold extremities, and slow weight gain. When a child develops a weak heartbeat, he or she should go to the hospital immediately for treatment.