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Physiologic transition from intrauterine to extrauterine life

Caraciolo J Fernandes, MD
Section Editor
Leonard E Weisman, MD
Deputy Editor
Melanie S Kim, MD


The successful transition from intrauterine to extrauterine life is dependent upon significant physiologic changes that occur at birth. In almost all infants, these changes are successfully completed at delivery without requiring any special assistance. However, about 10 percent of infants will need some intervention, and less than 1 percent will require extensive resuscitative measures at birth [1].

The physiologic changes that occur in the transition from intrauterine to extrauterine life are reviewed here. The indications and principles of neonatal resuscitation are discussed separately. (See "Neonatal resuscitation in the delivery room".)


Prior to delivery, the human fetus depends upon the placenta for gas and nutrient exchange with the maternal circulation. A discussion on the development of the placenta, which is essential for normal fetal growth and development, is found separately. (See "Placental development and physiology".)

The low vascular resistance of the placenta and the high vascular resistance of the fluid-filled fetal lungs result in right-to-left shunts characteristic of the fetal circulation (figure 1).

Fetal circulation — In the fetus, the placenta has the lowest vascular resistance and receives 40 percent of the fetal cardiac output, which results in a low systemic pressure (figure 1). In contrast, the fetal lungs are filled with fluid, resulting in a high vascular resistance and less than 10 percent of the cardiac output going to the lungs (algorithm 1). In contrast to earlier studies in lambs, subsequent studies using magnetic resonance imaging (MRI) and Doppler ultrasound to measure fetal blood flow suggest pulmonary blood flow may be higher (11 to 15 percent of combined ventricular output) [2,3].

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Literature review current through: Nov 2017. | This topic last updated: May 12, 2017.
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