In obstetrics, an initial episode of activity, alertness, and responsiveness to interaction, characteristic of the physiological and social responses of newborns to stimuli. The first period of reactivity begins with birth, lasts approx. 30 min, and ends when the infant falls into a deep sleep. Common assessment findings include transient tachypnea, nasal flaring, sternal retraction, crackles, tachycardia, and irregular heart rhythms. The second period of reactivity begins when the infant awakens and usually lasts 4 to 6 hr. Common assessment findings include signs of excessive respiratory and gastric mucus, hunger, apneic episodes, and the passing of a meconium stool.