Blood Circulation in the Fetus and Newborn During pregnancy, the fetal lungs are not used for breathing—the placenta does the work of exchanging oxygen and carbon dioxide through the mother's circulation. With the first breaths of air the baby takes at birth, the fetal circulation changes. Patent Ductus Arteriosus (PDA) PDA is a heart problem in which the connection between the aorta and pulmonary artery remains open after birth, instead of closing as it should. Congenital Heart Disease Heart problems are the most common kind of birth defects. While children with some heart defects can be monitored by a doctor and treated with medicine, others will need to have surgery.