Are Humans Born Premature Compared To Other Mammals? This is a complex question COMPARE.EDU.VN will help you answer, exploring gestation length and its implications. We delve into the science behind human birth compared to other mammals, discussing the evolutionary pressures and developmental factors that shape gestation. Discover how human prematurity stacks up against other species and gain a deeper understanding of this fascinating aspect of mammalian biology. Let’s explore fetal development, cognitive development, and comparative biology.
1. What Is the Definition of Premature Birth, and How Does It Vary Across Species?
Premature birth, generally defined as birth occurring before the expected completion of gestation, manifests differently across species. In humans, it’s typically defined as birth before 37 weeks of gestation. However, generalizing this to other mammals requires considering relative developmental stages and gestation periods. This variation is crucial when comparing human prematurity to that of other animals.
The World Health Organization (WHO) defines human preterm birth as “babies born alive before 37 weeks of pregnancy are completed” [26]. This definition, supported by the International Federation of Gynecology and Obstetrics, is rooted in a statistical analysis of gestational age at birth, calculated from the first day of the last menstrual period [27].
1.1. Why is Standardization Important?
The standardization aims to provide a common language for discussing and studying preterm birth. However, it’s important to note that “preterm” and “premature” have distinct meanings. “Preterm” refers specifically to the timing of birth, while “premature” describes a lack of completed fetal development [28].
Premature birth in the hospital
1.2. How is Prematurity Defined in Other Species?
Given the lack of a universally accepted definition of preterm birth for non-human species, it’s interesting to consider how the gestational length-based definition used in humans might apply to other animals. One approach is to generalize the human definition of preterm birth as “parturition prior to 92.5% (259 days or 37 weeks/280 days or 40 weeks) completed gestation.” Assuming that gestational length follows a normal distribution [25], this allows us to examine the occurrence of preterm birth in any placental mammal species for which population gestational length data are available.
Applying this percentage-based cutoff to data from diverse species reveals that parturition before 92.5% completed gestation occurs in many organisms, including primates like chimpanzees and gorillas (Fig. 1). One study defined preterm birth in chimpanzees as birth occurring 2 standard deviations below the mean [29], resulting in an estimated 16% of chimpanzees being born preterm. This suggests that the prevalence of preterm birth in chimpanzees, our closest relatives, may be similar to that in humans [2]. While it’s often unclear whether preterm birth in these animals is spontaneous or induced, evidence from horses indicates that placental infections can lead to preterm birth in non-human mammals [30], a known cause of preterm birth in humans [31].
2. How Does Human Gestation Length Compare to Other Mammals in Terms of Body Size?
Human gestation length, at approximately 280 days, aligns with the allometric scaling observed in other mammals. This suggests that human gestation is proportionally related to body mass, fitting within expected biological constraints. Factors such as brain size and metabolic rate further influence these allometric relationships.
Allometry, the study of how traits scale with one another [32], provides a framework for understanding how traits function and vary across ecological and evolutionary time. It also helps identify outliers and the ecological or evolutionary reasons behind them [33]. Allometric studies have been widely used to predict morphological, ecological, and physiological traits based on an organism’s body size, measured by body mass [34–41].
2.1. What Are Some Examples of Allometric Traits?
Examples of allometric traits that scale with body mass include vertebrate brain size [35, 39], longevity [38, 40], and basal metabolic rate [34, 36, 37, 41]. For instance, both basal metabolic rate and brain size scale with mammalian body mass to the three-fourths power [37, 42].
2.2. What About Mammalian Reproduction?
Allometric relationships have also been described for various mammalian reproductive traits, such as litter weight [38, 43, 44], neonate weight [38, 43–45], neonate brain weight [42, 43, 46], and per capita growth rate (Malthusian parameter) [38, 45]. These relationships provide insights into the evolution of pregnancy-associated traits in mammalian species and help identify trends and constraints. For example, studying the relationship between neonatal brain mass and body size has revealed an evolutionary trend toward larger brain size relative to fetal body mass in primates compared to non-primates [42].
2.3. How Does Gestation Length Scale with Body Mass?
Gestation length has been found to scale to maternal body mass by 1/4 [38, 43, 47, 48], although subsequent studies using phylogeny-informed statistics support a scaling exponent closer to 0.10 [49, 50] (Fig. 2). This relationship suggests that the timing of gestation in mammals is either constrained by maternal body mass or that the two traits are under a shared constraint. Recent work suggests that human gestation length may be primarily constrained by metabolism [51], raising the hypothesis that gestation length and maternal body mass, which also scales allometrically with metabolism, may be under a shared metabolic constraint.
3. What Role Does Brain Development Play in Determining Gestation Length?
Brain development significantly influences gestation length, particularly in humans. The “brain growth spurt,” which defines the period of most rapid brain growth, occurs perinatally in humans. This timing is unique compared to other primates and may contribute to the vulnerability of premature human infants.
One notable difference between humans and our close primate relatives is the increased immaturity at term birth, largely due to substantial postnatal brain growth necessary for normal human development [56, 107]. The “brain growth spurt” defines the developmental window when the brain undergoes its most rapid growth period [108, 109], visualized as a sigmoidal curve when brain growth is plotted against age [110].
3.1. What Are The Categories of Growth Spurts?
Growth spurts can be roughly categorized into three types:
- Prenatal
- Perinatal
- Postnatal
Altricial young undergo prenatal growth spurts, precocial young experience postnatal growth spurts, and organisms with intermediate development between altriciality and precociality undergo perinatal growth spurts.
3.2. How Does the Human Brain Grow Compared to Other Primates?
The perinatal growth spurt in humans places the species in an intermediate state of development, previously described as “secondarily altricial” [111]. This is evidenced by the typically singleton births of neonates with open eyes and ears at birth (precociality) combined with the relative helplessness of human babies compared to other primates (altriciality) [56, 107], and by the reduced neonate brain size relative to adult size in humans compared to chimps [112].
3.3. Why is the Brain Growth Spurt Important?
The brain growth spurt is not only a period of dramatic brain size increase but also a period of heightened vulnerability of the growing brain and organism to endogenous and exogenous insults [105, 108, 109, 113]. Humans experience the most rapid brain growth during the perinatal and into the postnatal phase of development [113, 114], a pattern not shared by our closest extant relative, chimpanzees [114]. Rapid brain growth in chimpanzees continues to approximately 22 weeks of the 34–35 week gestation period, whereas rapid human brain growth continues to at least 32 weeks [114]. Therefore, unlike our close primate relatives, the period of enhanced vulnerability resulting from the brain growth spurt overlaps with parturition in humans.
4. What Are the Fitness Consequences of Premature Birth in Humans and Other Mammals?
Premature birth carries significant fitness consequences, particularly in humans. Increased mortality rates and higher incidences of chronic health conditions reflect the challenges faced by premature infants. While data on fitness outcomes in other mammals are scarce, the available evidence suggests similar trends.
Fitness, a complex measure that accounts for numerous life-history traits in age-structured populations, has been equated with reproductive success [65, 66]. In evolutionary genetics, fitness measures the rate of increase in individuals possessing specific genotypes or phenotypes [67]. Individuals with increased rates of survival and reproductive success are expected to have increased fitness [66, 68].
4.1. What Is The Difference Between “Preterm” and “Premature”?
Discussing the fitness consequences of preterm birth requires distinguishing between “preterm,” which denotes an earlier than expected timing of parturition, and “premature,” which denotes a lack of completed fetal development and the source of any fitness consequences associated with preterm birth [28].
4.2. How is PTB Divided?
In humans, preterm birth is defined by gestational age and is often divided into three subcategories:
- Extremely preterm (birth before 28 weeks completed gestation)
- Very preterm (birth before 32 weeks completed gestation, but after 28)
- Moderate/late preterm (birth before 37 weeks completed gestation, but after 32)
Each sub-category has associated complications and levels of prematurity [69], allowing fitness differentials between preterm and full-term offspring to be interpolated by the differential survival rates of each group.
4.3. What Does Full Term Birth Look Like?
Neonate mortality is lowest in infants born at full-term, between 38 and 41 weeks of gestation, with mortality rates rising inversely to gestational age in preterm infants [70, 71]. Preterm birth is the leading direct cause of neonate mortality (deaths within the first 4 weeks of life) worldwide, with approximately 27% of the 4 million neonatal deaths in 2000 attributed to complications from preterm birth [72].
4.4. What Are Some Complications With Premature Births?
Preterm infants have higher rates of:
- Cerebral palsy [73–75]
- Chronic lung disease [76–78]
- Necrotizing enterocolitis [79–81]
- Retinopathy [82–84]
- Hearing impairments [75]
- Hospital readmissions [85, 86]
compared to full-term infants. These complications arise from immature organ systems that are not yet developed to support life outside the intrauterine environment [15].
4.5. Are There Fitness Records of PTB in Primates?
In the few records of fitness outcomes for preterm birth in primates, both chimpanzees and pigtail macaques experience decreased survival rates resulting from preterm delivery [29, 87]. In chimpanzees, all but one chimpanzee in 17 recorded preterm deliveries (≤208 days as defined by Wildman et al.) were aborted, stillborn, or died during the neonatal period [29]. In pigtail macaques, greater than 95% of “high-risk newborns” (including premature, low birth weight, and maternally rejected offspring) die if left in maternal care. However, if provided care in a nursery environment, the mortality rate of high-risk newborn pigtail macaques is reduced to only 20%. Premature pigtail macaques not only have decreased survival but also exhibit complex patterns of behavioral traits that differ from full-term offspring [87].
4.6. What Are The Long-Term Effects of PTB?
The fitness consequences of preterm birth continue to be noticeable in early childhood [73, 88, 89], with complications from preterm birth being the largest cause of mortality in children under 5 [2]. For example, mortality rates in early childhood (ages 1–5) in a large Swedish cohort showed a strong, significant inverse relationship with gestational age [90]. This suggests reduced fitness compared to full-term children, as well as a significant association between decreasing gestational age and the severity of the fitness cost.
Fewer studies have examined the long-term effects of prematurity in young adults [90–96]. Mortality rates in young adulthood (ages 18–36) also show an inverse relationship with gestational age [90, 96]. The outcomes of extremely low birth weight (ELBW) infants, whose average gestational age was 27.1 weeks, include a substantially larger number of incidents of neurosensory impairments (NSI), including cerebral palsy, mental retardation, blindness, and deafness, and were more likely to include multiple impairments [96]. Additionally, male ELBW infants have an increased prevalence of physical conditions including seizures, asthma, and recurrent bronchitis [92, 95, 96].
4.7. What About The Quality of Life?
Even though the prevalence of NSI was higher in young adults with low birth weights, studies support that young adults born with low birth weights are only slightly disadvantaged regarding participation in sports and other social activities, as well as romantic and sexual relationships [92, 95, 97, 98]. However, the argument that preterm-born adults have a similar quality of life is surrounded by dissension in both the medical community [97, 98] and among parents [99, 100]. The precise degree to which fitness is affected in preterm infants that survive to adulthood, especially those impacted by NSI, remains unclear.
5. Are There Specific Cognitive Impairments Related to Premature Birth in Humans?
Cognitive impairments are a significant concern in human premature births. Studies consistently report neurodevelopmental sequelae, including cognitive deficits, in preterm infants. The human-specific timing of the “brain growth spurt” during the perinatal phase may contribute to these impairments.
Defining preterm birth by a percentage-based cutoff leads to the inference that many placental mammals experience preterm delivery. However, understanding the impact, if any, of variation in gestation length on offspring fitness in non-human mammals is challenging due to the lack of studies [12, 29]. One area where the relationship between fitness and parameters associated with reproduction has been well-studied in both humans and other mammals [101–104] is in the context of changes in the environment. In humans, offspring born during the Finnish famine experienced decreased survival rates, but the increased mortality rates did not persist in later life [104]. This suggests that the fitness differential due to maternal exposure to environmental hardships lessens once the offspring reaches reproductive age. In wild mammal populations, differences in birth weights of red deer are associated with seasonal temperature fluctuations during the final months of gestation. Lower birth weights in cooler temperatures are linked to decreased neonatal survival and increased age at first reproduction [101]. In Soay sheep, increased population density, which probably leads to competition for limited food resources and increased competition for mates, is associated with reduced birth weights and neonatal survival [102].
5.1. Cognitive Deficits in Humans
Synthesis of the currently available data suggests that although the fitness consequences of preterm birth in the early years of human life are very large in infancy and early childhood, the consequences may be smaller in adulthood. Furthermore, neurosensory impairments (NSI) are the most consistently reported difference between preterm and full-term neonates. Cognitive deficits, in the absence of major motor defects, are the dominant neurodevelopmental sequelae in preterm birth infants [105]. The degree to which human-specific or human-elaborated adaptations contribute to these cognitive deficits is unknown. Although little data exist on either the rate of incidence of prematurity in other mammals or the resulting cognitive impairments of preterm birth in other mammals, the results to date indicate that neonate fitness is linked to gestational age, environmental fluctuations, and reduced birth weights [29, 87, 101, 102].
6. What Research Is Being Done to Help Us Understand Premature Birth and Develop Potential Treatments?
Current research focuses on identifying the genetic and molecular mechanisms that control gestation length and birth timing. Comparative studies across mammals, particularly those with unique allometric relationships or brain development patterns, offer promising avenues for understanding the biological basis of preterm birth.
Gestation length is normally distributed and scales proportionally to body mass across a wide diversity of placental mammals, suggesting that not only is this trait correlated with body size, but also that many mammals give birth before the ‘optimal’ time. Thus, humans are not unique in the variation or in the length of gestation relative to other mammals. Prematurely born humans suffer numerous neurosensory impairments (NSI), but knowledge of cognitive impairments in other placental mammals is lacking because studies are rare and difficult, given the drastic mortality rate of preterm birth in mammals outside of humans. The timing of the human ‘brain growth spurt’ has the potential to explain increased cognitive impairments in premature humans as the trajectory of growth is unlike closely related primates and directly overlaps with the parturition time window.
6.1. What Three Ways Are There To Advance Our Understanding of This Syndrome?
This evolutionary perspective provides guidance for furthering our understanding of the molecular basis of preterm birth. There are three ways to advance our understanding of this serious syndrome.
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First, the widespread occurrence of preterm birth in wild mammal populations argues for decoupling preterm parturition from premature parturition. All mammals could be useful, at least through comparative and functional genomics experiments, for understanding gestation length and birth timing, even if they are poor models for understanding the pathogenesis of the syndrome. A mechanistic understanding of the regulation of mammalian gestation length would contribute to understanding preterm birth pathogenesis, albeit not by direct inference.
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Second, the significant deviations in the allometric relationships between gestation length and body mass of organisms in the orders Chiroptera and Cetacea (relative to the relationships observed in most other mammals) raises the hypothesis that the evolution of these two traits (gestation length and body mass) might be less correlated or decoupled in these two orders. Species in these two orders can be viewed as outliers that harbor promise for elucidating the molecular mechanisms that control mammalian gestation length and timing.
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Third, a comparative perspective across development at the tissue level provides a way to identify organisms that better model disease aspects of the preterm birth syndrome. For example, the resemblance between the lung histopathology of premature lambs to that of chronic lung diseases in preterm infants has led to the development of lambs as a model for bronchopulmonary dysplasia [119]. Similarly, the brain growth spurt in pigs (like humans) spans the prenatal, perinatal, and postnatal development, leading to suggestions that it has potential as an appropriate model for human infant brain development [120, 121]. Finally, the presence of similar patterns of cerebral injury in premature baboons and humans [116] suggests that non-human primates may be useful models of NSI that result from human preterm birth.
7. Why Is It Important to Study Premature Birth?
Studying premature birth is critical due to its significant impact on infant mortality and long-term health outcomes. Understanding the underlying mechanisms of gestation length and the consequences of prematurity is essential for developing effective prevention and treatment strategies. By exploring these issues across species, we can gain valuable insights into the complex biology of mammalian reproduction and development.
Placental mammals experience “non-optimal” birth timing and that early parturition results in fitness costs through increased mortality in both human and non-human primates. The fitness cost of prematurity in survivors remains elusive. The combination of brain growth timing as well as the secondarily altricial nature of human offspring may be features that make human parturition unique to experience preterm birth as a syndrome of complications, but continued comparative studies in gestation length, birth timing, and brain development may reveal additional similarities between humans and other placental mammals.
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8. What Are Some Common Misconceptions About Premature Birth?
One common misconception is that premature birth is solely a human issue. While humans have a high rate of certain complications associated with premature birth, many mammals also experience preterm births. Another misconception is that all premature babies will have significant health problems; while risks are higher, many premature infants thrive with appropriate care.
9. How Does the Environment Affect Gestation Length?
Environmental factors, such as nutrition and stress levels, can influence gestation length. Studies have shown that maternal malnutrition or exposure to toxins can lead to shorter gestation periods and increased risks of premature birth. Understanding these environmental influences is crucial for developing preventive strategies.
10. What Are the Ethical Considerations in Premature Birth Research?
Ethical considerations are paramount in premature birth research. Ensuring the well-being of both the mother and the infant, obtaining informed consent, and protecting the privacy of participants are essential. Additionally, research involving animal models must adhere to strict ethical guidelines to minimize harm and ensure humane treatment.
FAQ
1. Is premature birth unique to humans?
No, while humans have specific complications associated with premature birth, many mammals also experience preterm births.
2. How is premature birth defined in humans?
It is defined as birth occurring before 37 weeks of gestation.
3. What factors influence gestation length?
Factors include body size, brain development, and environmental conditions.
4. What are the main risks associated with premature birth in humans?
Risks include increased mortality rates, cerebral palsy, chronic lung disease, and cognitive impairments.
5. How does the human brain growth spurt compare to other primates?
The human brain growth spurt occurs later in gestation compared to other primates, potentially increasing vulnerability.
6. Can environmental factors affect gestation length?
Yes, factors such as nutrition and stress can influence gestation length.
7. What research is being done to understand premature birth?
Research focuses on genetic and molecular mechanisms controlling gestation length, comparative studies, and tissue-level analysis.
8. Are there long-term effects of premature birth?
Yes, long-term effects can include neurosensory impairments and physical conditions.
9. How can I support premature infants and their families?
Support can include providing financial assistance, emotional support, and advocating for better healthcare.
10. Where can I find more information about premature birth?
You can find more information at compare.edu.vn and reputable medical websites.
