Although aerial respiration only begins at birth, it is currently widely recognized that breathing has a long preparatory history during the 9 months preceding parturition. Several authors purposely omit the idiomatic expression "first breath" and propose talking about a shift from periodic respiration to the establishment of continuous respiration when describing the changes that occur at birth. In addition to having preparatory functions essential for the shift to continuous aerial respiration at birth, animal studies have shown that fetal breathing movements also play a fundamental role in the anatomical and functional development of the lungs [26,27].
Up to mid-pregnancy, breathing can be considered an "either/or" event. Fetuses either "breathe" or move, swallow and hiccup. During the first half of pregnancy breathing and swallowing are not conjugated. This physiological phenomenon acquires pathological significance in the premature infant that is having to cope with the different requirements of extrauterine life. The fine coupling of these activities, necessary for feeding after birth, is not yet established by mid-gestation.
Growth is associated with an increasing capacity of the fetus to sustain prolonged episodes of breathing. Moreover, bouts of breathing become less fragmented. Parallel to this, the intervals between bouts also increase in duration: long apneic intervals become a consistent feature of fetal breathing around mid-gestation. It is established that breathing movements are the first movements to disappear in compromised and growth-restricted fetuses, where energy and oxygen consumption have to be spared for more delicate areas . A concomitant increase in blood flow to the brain, heart and adrenals is usually observed in such cases and is taken as a clinical indicator of fetal compromise . The increased spacing between episodes of breathing could also acquire pathological significance in the premature infant and occasionally also in the neonate. In only too many ways a premature infant is a fetus born at an untimely age. Under particular circumstances the premature infant, but also the neonate, could revert to a fetal form of "breathing" with long apneic phases separating breaths. Apneic phases and the suppression of breathing during hypoxia are functional during the fetal stage and represent an effi cient sparing method. This same sparing could have dire consequences if applied at a later, inappropriate stage. Hypoxia of whatever origin could expose the premature infant and some vulnerable neonates to the danger of reverting to a fetal form of breathing with long apneic spells which in turn would increase hypoxia and cause the cessation of breathing. Hypoxia and obstruction of the upper airways (with consequent hypoxia) have been postulated to be at the basis of some cases of sudden infant death syndrome.
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