At the end of pregnancy, the fetus must take the journey of childbirth to leave the reproductive mother. Upon its entry to the air-breathing world, the newborn must begin to adjust to life outside the uterus. This is true for all viviparous animals; this article discusses humans as the most-researched example. The outside environment is a drastic change for the neonate, therefore the neonate must be assessed frequently and thoroughly. The apgar scale is an assessment performed immediately following birth. It consists of the assessment of heart rate, respiratory effort, muscle tone, reflex irritability, and generalized skin color. Apgar scoring is performed one minute and five minutes after birth. Scoring ranges from 0 to 10, with 0 indicating severe neonatal distress and 10 indicating a smooth transition to extrauterine life. Newborns transitioning into extrauterine life will undergo periods of reactivity. These periods are divided into three stages. The first stage occurs in the first 30 minutes of life; during this stage the infant is alert and responsive with heart rate peaking at 160-180 beats per minute and then stabilizes to a baseline rate of 100-120 beats per minute. Crackles upon auscultation and irregular respirations are a normal finding. In the second stage, there is a decrease in responsiveness and motor activity which is often manifested as sleep. This period can last from 1–2 hours. The third stage marks the second period of reactivity. This period can occur anywhere in the first 2 to 8 hours after birth and lasts anywhere from 10 minutes to several hours. Tachycardia and tachypnea may be present during brief periods. Passing of meconium also occurs. Cardiac Physiology: In utero, the placenta delivers oxygenated blood to the fetus through the umbilical vein. Upon delivery, the umbilical cord is cut. The cardiovascular system must now adapt. Blood CO2 rises because it is now not removed by the placenta. This is a powerful stimulus for the infant to start breathing. Breathing sharply increases O2 in the lungs, thus quickly reverting hypoxic pulmonary vasoconstriction that had held the pulmonary vascular resistance high during the uterine life. Lung ventilation also extends the so far convoluted, shrunk pulmonary vessels, also contributing to the quick and marked drop in the pulmonary vascular resistance. As a result, much higher proportion of the right ventricle output flows into the pulmonary vessels than into the systemic circulation through the ductus arteriosus. The detachment of the