Nika Nakia is a family medicine doctor who loves to study and teach human anatomy & physiology.
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.
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.
Lastly, manufacturers of infant formula attempt to create it such that it mimics breastmilk as much as possible; however, it typically contains high amounts of a protein called casein, which can make it more difficult for the infant GI tract to digest than breastmilk.
So, since we've either watched enough movies depicting the event or watched a baby being delivered in real life, we know that the newborn is typically swaddled and capped with one of those knitted beanies very shortly after being delivered in an effort to keep it warm. However, since steady body temperature is a very important homeostatic parameter for the body to maintain, you might wonder if the baby has other built-in means of staying warm, and if you have guessed that it does, you are correct!
In addition to limb movement, 'brown fat is where it's at' regarding how Whitney generates heat for herself. Newborns are at risk of hypothermia because the surface area of their skin, the main organ by which heat loss occurs, is relatively high in comparison to their weight. Furthermore, their subcutaneous fat is provides suboptimal insulation and they are not able to shiver for heat production until about three months of age. Brown fat, a source of what's called 'non-shivering thermogenesis,' is found in the neck, mediastinum, axillae and around the kidneys, to name a few locations. Thermal receptors within the body can relay low body temperature to the central nervous system, which then stimulates lipolysis (breakdown of fat) by way of neurotransmitters. Brown fat is high in mitochondria, which are then able to use the metabolites of brown fat lipolysis for heat and energy production.
After a newborn is delivered, it is typically shuttled into the hands of a nurse, who performs a focused health evaluation. One important aspect of assessing a newborn is the calculation of the Apgar score. The Apgar score was created in 1953 by Dr. Virginia Apgar, an anesthesiologist at Columbia Presbyterian Hospital in New York. Although it was developed almost 70 years ago, it continues to be used as standard of care for newborns to assess how well they are adapting to extrauterine life. A helpful mnemonic, listed below, has been used to memorize the criteria of the score. Each criteria is given a score of 0-2 points. The maximum total score is 10.
- A = Appearance (skin color relates to cardiorespiratory health)
- P = Pulse (heart rate relates to cardiorespiratory health)
- G = Grimace (response to physical stimulation relates to neurologic health)
- A = Activity (muscle tone and movement relates to neuromuscular health)
- R = Respiration (breathing rate relates to respiratory health)
Typically, an Apgar score is calculated at 1 minute and 5 minutes post-delivery. It can help determine whether the newborn needs assistance with breathing, i.e., resuscitative support. An Apgar score above 7 is considered normal, while scores below 7 can be indicative of neonatal complications, such as infection, respiratory distress or congenital heart disease. For example, if Whitney contracted HIV from her mother and developed congenital HIV pneumonia, she might present with a low Apgar score as a clinical manifestation of the infection. Or if she is born prematurely (i.e., before 37 weeks' gestation), she might develop respiratory distress due to inadequate development of her lungs, which could present as a low Apgar score.
The journey of newborns from intra- to extra-uterine life involves major physiologic adaptations that their bodies are equipped to accommodate under normal circumstances. For professionals involved in delivering healthcare to newborns, it's important to understand both fetal and neonatal physiology so that the appropriate clinical interventions can be implemented if the transition fails to progress smoothly.
Some of the changes that occur post-parturition in the newborn include changes to breathing, circulation, digestion, and urinary function. Newborns have a built-in means of staying warm. They generate heat with limb movement and brown fat, which is found in the neck, mediastinum, axillae and around the kidneys. Brown fat can be broken down and used by mitochondria to generate heat.
An Apgar score is calculated 1-5 minutes post-delivery. It can help determine whether the newborn needs assistance with breathing. An Apgar score above 7 is considered normal, while scores below 7 can be indicative of neonatal complications. Some of the conditions that negatively impact newborn development include teratogens, premature birth, heart defects, and acquired illnesses such as HIV.
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