Newborn Development: Post-Parturition Changes, Thermoregulation & Standards

Instructor: Nika Nakia

Nika Nakia is a family medicine doctor who loves to study and teach human anatomy & physiology.

In this lesson, you'll learn how a newborn's organ systems adapt and how body temperature is regulated. We'll also explore clinical tools used to assess newborn development and detect potential diseases. Updated: 04/13/2021

Fetal and Newborn Physiology

When examining a newborn baby, medical professionals perform many maneuvers, including touching the soft fontanelles of the scalp, listening to the heartbeat to check for murmurs, and examining skin to look for any abnormal markings or signs of dehydration. This sort of examination is an important aspect of gathering clinical information to determine whether the newborn is making appropriate developmental progress following delivery. One reason why it's important for healthcare providers to understand newborn physiology is so they can differentiate between physiologic norms and abnormalities and make clinical decisions based on this knowledge.

In this lesson, you'll learn some important physiologic changes that occur in the newborn body after delivery, but first let's explore a little about the fetus' physiology before it officially becomes a newborn. Here's a chart to summarize some key comparisons:

Organ System Fetal Physiology Newborn Physiology
Cardiovascular Maternal placenta transports blood and thus oxygen to the fetus

PVR high, SVR low

R-sided heart pressures high, L-sided heart pressures low

Three 'detours' or shunts that bypass the lungs and liver since they are not fully functional in the womb: foramen ovale, ductus arteriosus, ductus venosus
Umbilical cord clamping disrupts placental blood flow; newborn must develop independently functioning CV system

PVR low, SVR high

R-sided heart pressures low, L-sided heart pressures high

Three shunts eventually close so that adequate blood flow reaches the lungs and liver
Respiratory Lungs are collapsed and essentially non-functioning

Gas exchange (O2, CO2) is placenta-based
Lungs are expanded and thus able to participate in respiration

Gas exchange is lung-based
Gastrointestinal Nutrients delivered to the fetus by the placenta

Ingestion of amniotic fluid aids in growth & development of GI tract
GI tract is functional but limited in its ability to digest and provide immunity; its microbiome is still developing as well
Renal Wastes eliminated by way of the placenta Kidneys are still maturing during neonatal period but are able to perform some excretory function

PVR = pulmonary vasculature resistance; SVR = systemic vasculature resistance

Please refer to this chart as needed as we discuss the physiologic adaptations that a newborn fetus is challenged to make in order to thrive.

Cardiovascular and Respiratory System Changes

Having transitioned from the milieu of a warm, fluid-filled womb to a colder, air-filled environment, the newborn body undergoes unique transformations following expulsion from its mother's body. So, how does a fetus make the physiologic transition from 'inhaling' amniotic fluid in the womb to successfully inhaling the air of the delivery room? Well, one aspect of the transition involves a very well-engineered interplay between the heart and lungs. These two organs have a 'hand-in-glove'-like relationship; what affects one typically affects the other. Following the delivery of the newborn, two major events occur that affect the function of both the cardiovascular and the respiratory systems:

  • The baby takes its first breath
  • The umbilical cord is cut

The First Breath

Imagine a newborn baby (let's call her Whitney) takes her first breath. The lungs, which were essentially collapsed and non-functional in the womb, are now expanded and being depended upon to deliver oxygen to the newborn.

Imagine baby Whitney's lungs as two small sponges inside her chest, perfectly 'sandwiching' and cradled around her heart. The holes in the sponges are like the blood vessels that course through the lungs. When the spongy lungs expand after she takes her first breath, the holes open up, which is analogous to the blood vessels expanding in diameter because they are no longer being compressed by surrounding lung tissue. The expansion of the lungs allows blood to flow more freely through the pulmonary vessels within them, and the pulmonary vessels also vasodilate so that the blood flow (perfusion) will match the increased oxygen that is being inhaled (ventilation) by the lungs. Combined, these actions lead to a decrease in resistance of the pulmonary vasculature. Lower resistance and thus pressure within the pulmonary vessels means that the right side of Whitney's heart does not have to generate as much force to push blood through the pulmonary vessels, so the pressure in the right side of the heart decreases.

When Whitney was a fetus, she had two shunts in her heart that helped to divert blood away from her nonfunctional lungs: the foramen ovale and the ductus arteriosus. She also possessed a functional ductus venosus, a shunt that allows highly-oxygenated blood coming from the placenta to bypass the minimally functional liver. The foramen ovale is a valve-like conduit that allows blood to flow from the right atrium to the left atrium, instead of the traditional route from the right atrium to the left ventricle and then to the pulmonary artery. The ductus arteriosus allows blood to flow from the pulmonary artery to the aorta, instead of moving into the lungs from the pulmonary artery.

As a fetus, the pressures on the right side of the heart were higher than those on the left side because the right side had to work harder to push blood through the highly resistant pulmonary vessels being compressed by collapsed lungs. However, now that Whitney has exited the womb and the right-sided heart pressures have become lesser than the left-sided pressures, the higher pressures within the left atrium and the aorta apply a pressure that pushes back on these openings, causing them to eventually fibrose and close shut. Furthermore, the ductus venosus typically closes within days after birth, in response to changes in systemic blood pressure that occur after the clamping of the umbilical cord.

The Umbilical Cord

Another important event that affects newborn physiology is the clamping of the umbilical cord that occurs shortly after delivery. In addition to a decrease in pulmonary vasculature resistance (PVR), the systemic vasculature resistance (SVR) increases in response to the cessation of blood flow from the placenta to the fetus, which has been interrupted by umbilical cord clamping. Since vasoconstriction is a typical physiologic response to decreases in blood flow and thus blood pressure, it makes sense that SVR would increase in an effort to maintain adequate perfusion of the fetal organs. In summary, PVR causes lower pressures on the right side of the heart, while an increase in SVR causes higher pressures to be generated on the left side of the heart. This pattern of a right-sided heart with lower pressures relative to the left side is what is seen in the adult heart, while the opposite scenario occurs in the fetal heart.

Other factors that influence Whitney's ability to adapt to a new environment include whether her lungs are producing enough surfactant, which is a lipid-rich substance that aids in decreasing surface tension of water that lines the alveoli; thus the surfactant helps keep the alveoli inflated.

And what becomes of Whitney's umbilical cord, which connected the fetus to the mother's placenta and consisted of the umbilical vein and artery that carried oxygenated blood to the fetus and carried deoxygenated blood away from the fetus, respectively? Well, because newborn Whitney now has her own independently functional circulatory system and is able to acquire oxygen with the use of her own lungs, the blood vessels within the umbilical cord are no longer necessary and eventually they fibrose.

CC by OpenStax
Neonatal Circulatory System

Renal System Changes

The adjustments of the cardiovascular and respiratory systems within the newborn are rapid; however, other aspects of the newborn's physiology adjust more gradually to being out of the womb. For example, the urinary tract, which begins forming urine at ~9 weeks and has completed the nephrogenesis of ~1 million nephrons/kidney by 35 weeks, is still maturing post-delivery.

At birth, all of Whitney's nephrons have been formed, but neither the glomerulus or the tubules that comprise them are fully functional after birth. They still need to mature and grow in size. Due to the immaturity of nephron, Whitney is not able to either concentrate or dilute the urine with the precision of an adult kidney. As such, she is at an elevated risk for becoming dehydrated or hypervolemic, respectively.

Along with the challenge of maintaining fluid (water) balance, her neonatal kidneys must maintain the homeostasis of electrolytes like sodium. Immediately after the birth, her kidneys can reabsorb 97% of the sodium that is filtered, leaving 3% to be excreted in the urine. This relatively high amount of sodium in the urine has a diuretic effect, contributing to the typical pattern of infant weight loss following birth. However, within days, a term infant is better able to concentrate its urine and can increase its reabsorption of sodium to 99%, which is more similar to the kidney function of adults.

Gastrointestinal System Changes

Now, you might be wondering, 'how is Whitney going to be able perform gastrointestinal (GI) functions (e.g., digestion, absorption and elimination) on her own, now that she can't rely on her mom and the placenta to do these jobs?'

Prior to receiving her first meal as a newborn, amniotic fluid was the main substance to which her GI tract was exposed. However, if she's going to be breastfed, for example, Whitney must now rely on her mother's milk for nutrition. Though fully formed by 25 weeks' gestation, the GI tract of the newborn is limited in various functions, including its ability to digest macronutrients, white blood cell activity, and antibody production.

Mature breastmilk, consisting of carbohydrates protein, fat and various vitamins, minerals and enzymes, is considered the best nutritional option for newborns because it is designed to facilitate infant digestion. It also contains antibodies and leukocytes which enable the immune component of the GI system to defend against ingested antigens that have the potential to cause disease. Through her milk, Whitney's mother helps to confer a healthy microbiome to her infant daughter's GI tract. The GI microbiome consists of various colonies of bacteria, viruses, archaea and fungi, all of which collaborate in the functions of infant immunity and digestion.

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