
Viability and Advances in Neonatal Medicine
Defining Viability
The concept of viability, referring to the stage at which a fetus can survive outside the womb, has undergone profound shifts over the past few decades due to remarkable advancements in neonatal medicine. These changes not only redefine the boundaries of life’s earliest survivability but also highlight the inherent sanctity and value of every human life.
The following video, though labeled as showing “viability,” depicts an ultrasound at 11 weeks gestation revealing a clearly visible heartbeat. In this context, viability is used differently than its medical definition of surviving outside the womb, here, it reflects the undeniable evidence of a living, developing human being long before the traditional threshold of external survival.
Defining Viability
Traditionally, viability was defined at around 28 weeks of gestation, when a baby's organs were thought to be developed enough to sustain life outside the womb. This definition was largely shaped by the medical limitations of the time, as neonatal technology could not adequately support infants born earlier. However, even at this stage, survival was not guaranteed, and long-term health outcomes were often uncertain. Despite these challenges, the ability to survive outside the womb signaled the baby’s potential for independent life, a milestone that reinforced its intrinsic value.
The Changing Threshold of Viability

Advances in neonatal care have dramatically shifted the threshold of viability earlier, with cases of survival as early as 22 weeks now becoming increasingly common. Innovations such as advanced respiratory support, improved incubator technology, and meticulous neonatal intensive care have made this possible. Studies show that while survival rates for infants born at 22 weeks are still modest, they improve significantly with each additional week of gestation, underlining the importance of life-sustaining efforts even at the earliest stages.
This changing threshold challenges society to reconsider how we define and value life. When infants born at 22 or 23 weeks can not only survive but thrive with appropriate care, it raises profound ethical and moral questions about the protection of unborn life at every stage of development. If medicine can save the lives of these fragile infants, how can we fail to recognize their humanity and inherent worth?
The Pro-Life Implications
From a pro-life perspective, these advancements affirm what has always been true: human life is valuable from its very beginning. The shifting viability threshold underscores the idea that our worth is not dependent on gestational age, size, or degree of independence. Every life, no matter how small or vulnerable, has dignity and purpose.
Moreover, the expanding capacity to save preterm infants highlights the contradiction of a society that invests immense resources into neonatal care while simultaneously permitting the destruction of unborn life through abortion, often past the point of viability. It begs the question: How can we work so hard to save some lives while allowing others to be ended?
The evolving science of viability calls us to advocate for life with renewed urgency. As thresholds continue to move earlier, it becomes increasingly clear that life begins well before viability can be scientifically or medically defined. This reality challenges laws, policies, and cultural attitudes that fail to protect the most vulnerable among us.
Advances in neonatal medicine are a testament to human ingenuity and compassion, offering hope to countless families. They also serve as a powerful reminder of the value of every life and the responsibility we have to nurture and protect it, from conception to natural death. Let us continue to embrace a culture of life, where every child is seen as a miracle, and no life is deemed too small to matter.
Medical Breakthroughs in Neonatal Care
The concept of viability, the point at which a baby can survive outside the womb, has long been a cornerstone of discussions surrounding the sanctity of life. Over the past decades, advances in neonatal medicine have dramatically improved the survival rates of preterm infants, challenging outdated notions of viability and underscoring the inherent value of every human life, no matter how small or fragile.
Surfactant Therapy
Surfactant Therapy has been a groundbreaking development in the care of preterm infants. Before its discovery, respiratory distress syndrome (RDS) was a leading cause of neonatal mortality, as premature lungs lacked the surfactant necessary to keep the airways open. Surfactant therapy revolutionized neonatal care by significantly reducing mortality rates and complications. This medical innovation serves as a powerful reminder that every life deserves a fighting chance, even when born under the most challenging circumstances.
Extracorporeal Membrane Oxygenation (ECMO)
Extracorporeal Membrane Oxygenation (ECMO) is one of the most advanced life-support technologies available for critically ill newborns, providing temporary respiratory and circulatory support when the heart or lungs cannot function adequately on their own. By oxygenating and removing carbon dioxide from the blood outside the body, ECMO gives fragile infants time to recover from severe respiratory failure, persistent pulmonary hypertension, congenital heart defects, or complications from prematurity.
While ECMO does not determine fetal viability on its own, its success demonstrates how medical innovation continues to push the limits of survival for vulnerable infants. Each life saved through ECMO underscores the profound value of investing in technologies designed to protect and preserve life, even in its most delicate and precarious moments.

High-Frequency Oscillatory Ventilation (HFOV)
High-Frequency Oscillatory Ventilation (HFOV) offers another life-saving option for infants with severe respiratory failure. Unlike traditional ventilators, HFOV minimizes lung injury by delivering rapid, small breaths while maintaining constant lung inflation. This technology embodies the pro-life ethos, demonstrating how human ingenuity and compassion can protect and sustain the most vulnerable among us.
Therapeutic Hypothermia (Cooling Therapy)
Therapeutic Hypothermia (Cooling Therapy) has proven to be a life-changing treatment for newborns experiencing hypoxic-ischemic encephalopathy (HIE), a condition caused by oxygen deprivation during birth. By lowering the body temperature, cooling therapy reduces brain injury and improves long-term developmental outcomes. This treatment underscores the pro-life principle that every effort should be made to preserve and nurture life, particularly in moments of crisis.
The rapid advancements in neonatal medicine offer a profound affirmation of the sanctity and worth of every human life. These breakthroughs challenge arbitrary definitions of viability by proving that with the right care, even the smallest and most vulnerable lives can thrive. As we continue to push the boundaries of science and medicine, we are called to protect and cherish every life as a gift from God, demonstrating that no life is too small or too fragile to be valued and fought for.
From surfactant therapy to ECMO, these innovations remind us of the incredible potential within the fields of science and medicine to uphold the pro-life message. They inspire us to advocate for policies and practices that celebrate and protect life at all stages, affirming the truth that every human being is fearfully and wonderfully made.
Real-World Survival Data

The concept of viability, the point at which a preterm infant can survive outside the womb, has profoundly changed over the past few decades. Advances in neonatal medicine have pushed the boundaries of survival, affirming the inherent value of even the tiniest and most fragile lives. Today, real-world survival data and geographic variability in survival rates at 22 and 23 weeks' gestation highlight both the progress made and the challenges that remain in safeguarding these vulnerable lives.
Survival Rates at 22-23 Weeks
In the past, infants born before around 24 weeks’ gestation were often considered beyond the reach of survival. However, recent advances in neonatal care have pushed those boundaries. For babies born at 22 weeks, some centers now report survival‐to‐discharge rates in the ballpark of ~25–30% under active treatment. For those born at 23 weeks, survival rates under care can reach ~50–60%, depending on the hospital and treatment protocols.
These improvements reflect innovations such as antenatal steroids to support lung development, surfactant therapy and improved respiratory care, and advances in neonatal intensive care practices. Each infant who survives, despite the odds, serves as a testimony to how medical innovation can protect life in some of its most vulnerable beginnings.
That said, it remains true that survival at such early gestational ages is very uncertain, and long-term risks remain substantial. These realities underscore both the fragility of early life and the profound moral weight of offering care whenever possible.
Geographic Variability
Despite these successes, survival rates vary significantly based on geography. Differences in hospital policies, access to NICUs, and medical staff training contribute to disparities in outcomes. In the United States, some hospitals provide active treatment to infants born at 22 weeks, leading to higher survival rates, while others do not, leaving these infants without the chance for life-saving interventions. Globally, survival rates differ even more dramatically, influenced by healthcare infrastructure and cultural attitudes toward extremely preterm infants.
These disparities highlight the moral and ethical imperative to ensure equitable access to life-saving care. Every infant, regardless of location, deserves a chance at life. Standardized protocols, greater investment in neonatal care, and a commitment to valuing life can help bridge this gap.
Viability and the Pro-Life Argument
Earlier Viability Strengthens the Pro-Life Argument
The ability of infants born at 22 weeks to survive challenges the notion that life begins only when a child can sustain itself independently. Studies show that up to 30% of these preterm babies, with adequate medical care, can survive and thrive. This earlier viability redefines societal and legal perspectives on the unborn, emphasizing their potential for life and personhood. For pro-life advocates, this serves as a powerful argument for extending legal protections to the unborn earlier in pregnancy. If medical science recognizes the capability of survival at such a young gestational age, it follows that society should afford these lives the same dignity and rights as those born later.
Personhood and Viability
Personhood lies at the heart of the abortion debate. Advances in neonatal medicine reveal the intricate development of a baby long before birth, including the capacity for pain, responsiveness to stimuli, and even rudimentary cognitive processes. For pro-life advocates, these characteristics affirm the inherent value and dignity of the unborn, regardless of gestational age. Earlier viability demonstrates that the line between dependency and independence is fluid and influenced by technology, not intrinsic humanity. Therefore, tying personhood solely to viability is problematic, as it shifts with medical progress rather than reflecting the consistent value of human life from conception.
The advances in neonatal medicine that have lowered the threshold of viability reinforce the pro-life argument by demonstrating the humanity, potential, and survivability of the unborn. These medical breakthroughs challenge outdated legal and ethical frameworks, urging society to reconsider its treatment of the unborn. Personhood and dignity are not contingent on technology but are inherent from conception, and earlier viability highlights this truth with unmistakable clarity. Through these advancements, the pro-life movement finds renewed strength and urgency in its call to protect life at all stages.
This reality is echoed in the powerful testimony of Dr. Beverly McMillan, whose interview with Lila Rose captures her transformation from abortion provider to passionate defender of life.

The Abortion Process
Medical Abortion (Abortion Pill)
Medical abortion refers to the termination of an early pregnancy through a combination of pharmaceutical drugs rather than a surgical procedure. Most commonly used up to the 10th week of pregnancy, it involves two medications, mifepristone and misoprostol, that work together to first block the hormone needed to sustain the growing child and then induce uterine contractions to expel the developing baby. Although it is increasingly promoted and widely administered, the process carries documented physical, emotional, and ethical concerns. Understanding how these drugs function, as well as the possible complications and broader implications, is important for accurately addressing the realities of medical abortion.

How Medical Abortion (Abortion Pill) Works
The medical abortion process begins with the administration of mifepristone, a drug that blocks progesterone, the hormone responsible for maintaining the uterine lining and supporting the pregnancy. When progesterone is inhibited, the uterine lining breaks down, causing the embryo to detach from the nourishment it needs to survive.
After mifepristone, a second drug, misoprostol, is typically taken within 24 to 48 hours. Misoprostol induces strong uterine contractions that force the body to expel the developing baby and pregnancy tissue. This regimen is designed to complete the abortion outside a clinical setting, often in the woman’s home.
During this process, women commonly experience heavy bleeding and severe cramping as the uterus contracts. Additional symptoms may include nausea, diarrhea, fever, and other side effects that may last for several days. The physical effects, combined with the emotional and psychological impact, highlight the seriousness of the procedure. If you have taken the first abortion pill and are reconsidering your decision, it may not be too late, help may still be available. Click the button below to find out more.
Surgical Abortion
The following content is extremely disturbing to most viewers.
Surgical abortion refers to procedures that intentionally end the life of an unborn child and are typically carried out in clinical settings by medical personnel. The most common methods include suction aspiration (vacuum aspiration) and dilation and evacuation (D&E), each used at different stages of pregnancy. These procedures involve significant physical and emotional consequences, and understanding what they entail, as well as the risks and long-term impacts, is crucial for anyone seeking to grasp the seriousness of these interventions.
Suction Aspiration (Vacuum Aspiration)
Suction aspiration is primarily performed during the first trimester of pregnancy, typically up to 13 weeks gestation. The procedure involves dilating the cervix to allow the insertion of a thin tube connected to a suction device. The vacuum suction gently removes the uterine contents, including the developing baby. The procedure is generally quick, lasting about 10 to 15 minutes, and is often done on an outpatient basis.
Potential risks include uterine infection, heavy bleeding, and cervical injury. In rare cases, incomplete removal of pregnancy tissue may occur, requiring further medical intervention.
The video that follows (warning extremely graphic) is an example of a Manual Vacuum Aspiration (MVA) abortion, a variation of suction aspiration that uses a hand-held syringe instead of an electric machine. While both methods rely on suction to remove the developing baby from the uterus, MVA produces suction manually, whereas standard suction aspiration uses a powered device, allowing for stronger and continuous vacuum pressure.
Dilation and Evacuation (D&E)
Dilation and Evacuation (D&E) is a surgical abortion procedure used in the second trimester, generally after 13 weeks of pregnancy. The process begins with forced dilation of the cervix, which is gradually opened over one or two days using medications or mechanical dilators. Once dilation is achieved, the practitioner uses surgical tools, including forceps and suction, to disassemble and remove the developing baby and associated tissue from the uterus.
Because this procedure takes place at a more advanced stage of pregnancy, it is more invasive and carries significant risks. These can include uterine perforation, heavy bleeding, infection, and other complications, which become more likely as the pregnancy progresses. The seriousness and complexity of D&E highlight the gravity of second-trimester abortions and the physical toll they can take on the woman.
Warning the following video is extremely graphic. This promotional video by AbortionNo.org offers a stark and unfiltered look at what a D&E abortion entails, revealing the tragic reality of ending an unborn life.
Induction Abortion
Induction abortion is a late-term procedure, usually performed in the second or third trimester, in which labor is deliberately induced to deliver a deceased baby. Because these abortions occur at stages when the unborn child is significantly developed, and often capable of surviving outside the womb with medical support, the procedure carries profound medical, ethical, and emotional implications.
The process typically begins with a fetal injection of digoxin or potassium chloride, administered through the woman’s abdomen or into the amniotic sac to stop the baby’s heart prior to delivery. Once fetal death is confirmed, the cervix is softened and dilated using laminaria or medications, a process that may take many hours. Labor is then induced using additional medications, and the mother delivers the baby in a process similar to childbirth, though the outcome is tragically different. After delivery, the placenta is removed, and medical staff monitor for complications such as hemorrhage, infection, or retained tissue.
Because induction abortion resembles the physical process of giving birth, it carries risks such as heavy bleeding, uterine rupture, and significant psychological trauma. There is also the possibility of a live birth if the fetal injection fails, an event that raises substantial legal and ethical concerns, as infants may show signs of life for a brief period after delivery.
Abortion Victim Photography
The following content is extremely disturbing to most viewers.
The following section presents a series of photographs documenting the lives lost to abortion, from 7 weeks to 26 weeks gestation. These images, though difficult to witness, reveal the undeniable humanity of the unborn in every stage of development. Each tiny face, hand, and form testifies to a life that had begun, a life with purpose, and a life that was never given the chance to flourish.
Our goal in sharing these photographs is not to shock, but to bring truth into the light. Abortion is often spoken about in abstract terms, procedures, choices, rights, yet behind every statistic is a child with a beating heart, a developing body, and God-given worth. By seeing what abortion ends, we are confronted with a reality that words alone cannot convey.
Please prepare your heart as you enter this section. These images are graphic and heartbreaking, but they are also important. They honor the lives that were lost and remind us why the fight to protect the most vulnerable is urgent, compassionate, and necessary.
Warning: the following images are extremely graphic.
7 to 9 Weeks Gestation
Between 7 and 9 weeks gestation, the unborn child undergoes rapid and astonishing development. The heart beats steadily, brain waves are measurable, and tiny arms, legs, fingers, and toes begin to take shape. Facial features form, major organs develop, and the child even begins small, early movements. Though still very small, these weeks reveal unmistakable human life with growing complexity and individuality.
10 to 11 Weeks Gestation
By 10–11 weeks gestation, the unborn child shows remarkable refinement. Fingers and toes are fully formed, nails begin to appear, and the baby can bend, stretch, and move freely in the womb. The heart is beating strong, vital organs are functioning, and distinct facial features, eyes, nose, and lips, are clearly visible. Though still small, this stage reveals a child who is undeniably human in form and activity.
22 to 26 Weeks Gestation
By 22–26 weeks gestation, the unborn child has reached a stage where medical science now recognizes a real chance of survival outside the womb with proper care. The baby responds to sounds, practices breathing movements, opens and closes the eyes, and develops distinct facial expressions. Fine hair covers the skin, fingerprints are set, and the heartbeat and movements are strong and noticeable. These weeks reveal a child who is active, aware, and growing rapidly in both body and brain. The images in this section bear witness to the heartbreaking truth that even at this stage, when the child’s humanity is undeniable, many lives are still lost to abortion.

Ethical Challenges in Modern Science
Ethical Challenges in Science
Modern scientific advancements have opened new frontiers in medicine and biology, offering unprecedented opportunities to improve human health and understanding. However, these developments also present significant ethical challenges, particularly concerning abortion, stem cell research, and genetic manipulation. From a pro-life perspective, these issues necessitate careful consideration to uphold the sanctity of human life.
Abortion and the Value of Life
The pro-life stance asserts that human life begins at conception, rendering abortion morally unacceptable. Technological advancements have led to prenatal screenings capable of detecting genetic anomalies, sometimes resulting in decisions to terminate pregnancies. This raises ethical concerns about the potential devaluation of lives deemed "imperfect" and underscores the need to protect all human beings, regardless of their developmental stage or health status.
Stem Cell Research and Embryo Ethics
Embryonic stem cell research raises serious ethical issues because it requires the destruction of human embryos, organisms already undergoing organized, directed development as distinct human beings. These embryos are frequently obtained from abortions or from IVF procedures where “excess” embryos are created and later discarded. In both cases, human life at its earliest stage is treated as experimental material rather than as a developing person with inherent value.
Scientifically, embryonic stem cells are prized for their pluripotency, the ability to become any cell type, but acquiring them necessarily ends the embryo’s life. This creates a direct ethical conflict: the pursuit of medical progress depends on intentionally destroying human organisms at a stage where their biological identity is unmistakably human.
Pro-life bioethicists argue that no potential medical benefit can justify the deliberate termination of developing human life, especially when viable and ethically sound alternatives exist. Adult stem cells, cord blood stem cells, and induced pluripotent stem cells (iPSCs) already provide therapeutic benefits without harming embryos, demonstrating that scientific advancement does not require the destruction of nascent human life.

Chinese Scientist, He Jiankui
Genetic Manipulation and Human Dignity
CRISPR-Cas9 enables highly accurate edits to DNA, opening possibilities for correcting genetic diseases. However, germline editing—altering embryos, eggs, or sperm—poses serious ethical and scientific concerns because these changes are permanent and passed to future generations. Current research shows risks such as off-target mutations, mosaicism, unpredictable developmental effects, and long-term health consequences that cannot be fully assessed before birth.
The 2018 case involving Chinese scientist He Jiankui, who created genetically edited embryos that resulted in live births, highlighted how quickly this technology can be misused. His experiment violated international research standards, lacked proper oversight, and exposed children to unknown lifelong risks. The scientific community widely condemned the work as premature and unsafe.
Pro-life scientists caution that germline editing also opens the door to trait selection and modern eugenics, shifting genetics from healing to designing “preferred” humans. This would fundamentally alter how society views human value, reducing children to products of engineering rather than individuals with inherent worth.
In Vitro Fertilization (IVF) and the Ethics of Human Embryos
In vitro fertilization has become a common solution for couples facing infertility, offering hope for those longing to become parents. Yet from a pro-life perspective, IVF raises profound ethical concerns rooted in the dignity and value of every human life created in the process. IVF typically results in the creation of multiple embryos, many of which are never implanted. These “excess” embryos are often frozen indefinitely, discarded, or used for research, each outcome representing the loss or instrumentalization of a human life at its earliest stage.
Pro-life ethicists also raise concerns about embryo selection, where embryos are graded or screened for perceived health, genetic “fitness,” or preferred traits. This practice can reinforce a utilitarian mindset that values certain lives over others, echoing the eugenic tendencies already seen in selective abortion. Additionally, the process of IVF can blur the moral boundaries surrounding conception by separating procreation from the marital act, transforming the creation of human life into a laboratory procedure rather than a natural expression of love between husband and wife.
The widespread acceptance of IVF has also fostered a commercial industry around human reproduction. Frozen embryos are treated as property, bought, stored, and discarded through contractual agreements rather than recognized as human beings with inherent dignity. This commodification of nascent human life underscores the need for ethical reflection, especially as technologies such as embryo genetic testing and artificial wombs continue to advance.
Women’s Rights vs. Fetal Rights

The ethical debate surrounding abortion is deeply rooted in the complex interplay between a woman's right to bodily autonomy and the baby's right to life. From a pro-life perspective, this discourse emphasizes the inherent value of human life from conception and the moral obligations that arise from it.
Violinist Thought Experiment
Pro-choice advocates often invoke a woman's right to control her own body as a justification for abortion. However, pro-life arguments challenge this by highlighting the unique relationship between a mother and her unborn child. Unlike the analogy of a stranger requiring bodily support, as presented by Judith Jarvis Thomson in her famous violinist thought experiment, pregnancy involves a natural biological connection where the mother has a special responsibility toward her offspring. Critics argue that this parental obligation supersedes the general right to bodily autonomy, making abortion ethically impermissible.
The Ethical Imperative to Protect Unborn Life
From the pro-life viewpoint, society has a moral duty to protect the most vulnerable, including unborn children. This perspective contends that legal frameworks should reflect the intrinsic value of all human life, ensuring that unborn-babies are afforded the same rights and protections as individuals who have been born. This approach underscores the belief that the right to life is fundamental and should not be contingent upon developmental stages or external circumstances.
In the ethical discourse on abortion, the pro-life position maintains that the right to life of the unborn child is paramount. While acknowledging the significance of a woman's autonomy, this perspective emphasizes the moral responsibility to protect and preserve innocent human life from conception onward. Balancing these rights necessitates a societal commitment to uphold the sanctity of life, even in the face of complex personal and ethical dilemmas.




Page References
Viability and Advances in Neonatal Medicine
https://www.mdpi.com/2227-9067/10/5/784 https://cadenaser.com/murcia/2024/08/25/el-hospital-de-la-arrixaca-incorpora-tecnologia-para-la-atencion-de-los-recien-nacidos-mas-fragiles-radio-murcia/ https://www.acog.org/clinical/clinical-guidance/obstetric-care-consensus/articles/2017/10/periviable-birth https://www.nichd.nih.gov/research/supported/EPBO https://www.recent-advances.com/ https://publications.aap.org/pediatrics/article-abstract/121/1/e193/71032/Perinatal-Care-at-the-Threshold-of-Viability-An https://www.hse.ie/eng/about/who/acute-hospitals-division/woman-infants/neonatology/perinatal-management-of-extreme-preterm-birth-at-the-threshold-of-viability.pdf https://www.degruyter.com/document/doi/10.1515/jpm-2024-0432/html https://bcmj.org/articles/management-newborn-delivered-threshold-viability https://www.jpeds.com/article/S0022-3476(24)00246-4/abstract https://academic.oup.com/pch/article/26/1/35/6125698 https://link.springer.com/chapter/10.1007/978-3-319-39839-6_43 https://cardiothoracicsurgery.biomedcentral.com/articles/10.1186/s13019-024-03011-3 https://bmcpediatr.biomedcentral.com/articles/10.1186/s12887-018-1040-z https://fn.bmj.com/content/109/1/10 https://publications.aap.org/pediatrics/article-abstract/154/4/e2024065963/199459/Survival-of-Infants-Born-at-22-to-25-Weeks https://www.wsj.com/health/healthcare/premature-babies-hospitals-pregnancy-fa1f091f
Ethical Challenges in Modern Science
https://journalofethics.ama-assn.org/article/genome-editing-ethics-and-politics/2019-12 https://stemcellres.biomedcentral.com/articles/10.1186/scrt474 https://pmc.ncbi.nlm.nih.gov/articles/PMC2726839/ https://www.asrm.org/practice-guidance/ethics-opinions/ethics-in-embryo-research-a-position-statement-by-the-asrm-ethics-in-embryo-research-task-force-and-the-asrm-ethics-committee-2020/ https://www.eurostemcell.org/embryonic-stem-cell-research-ethical-dilemma https://scholarworks.iu.edu/journals/index.php/jsriue/article/download/31847/36487/82117 https://home.csulb.edu/~cwallis/382/readings/160/marquis.html https://www.thepublicdiscourse.com/2022/06/82963/ https://pmc.ncbi.nlm.nih.gov/articles/PMC9793437/ https://religion.llu.edu/bioethics/adventist-guidelines-genetic-engineering https://www.churchofscotland.org.uk/about-us/our-views/science-and-technology/reports-and-leaflets/moral_and_ethical_issues_in_gene_therapy
The Abortion Process
https://academic.oup.com/edited-volume/28138/chapter-abstract/212904815 https://my.clevelandclinic.org/health/treatments/21899-medical-abortion https://www.bpas.org/abortion-care/abortion-treatments/surgical-abortion/dilatation-and-evacuation/ https://www.ucsfhealth.org/treatments/surgical-abortion-second-trimester https://www.healthline.com/health/surgical-abortion https://abortionno.org https://www.apa.org/pubs/journals/features/amp-64-9-863.pdf https://nap.nationalacademies.org/read/24950/chapter/7

































