Currently, a serious healthcare problem is the premature birth of children, which contributes significantly to global morbidity and mortality. Preterm birth is defined as birth occurring between 22 and 36 weeks and 6 days of gestation. When diagnosing a "preterm newborn," the gestational age in weeks at which the birth occurred is specified, i.e., the newborn's gestational age. This clarifies the baseline situation, allows for a correct assessment of the preterm infant, and helps determine management tactics.

According to statistics, approximately 10% of all newborns worldwide are born preterm, with the highest prevalence in low- and middle-income countries. In Russia, the preterm birth rate is about 6%, although it is higher in some regions due to socioeconomic factors. The highest percentage of preterm births is observed in African and Asian countries, where it reaches 18-20% [1, 2].

A child's health is determined by their level of physical, mental, and functional development. Most children born prematurely have lower physical development indicators compared to full-term infants throughout the first year of life. This problem has become particularly relevant in recent decades due to the introduction of new care methods and the transition to new criteria for live birth, leading to an increase in the number of infants with extremely low birth weight. Many factors influence the low rates of physical development gain: the severity of the child's condition in the perinatal period, due to pronounced morphological, metabolic, and functional immaturity of all organs and systems under stress. This significantly complicates the adaptation process in the neonatal period and requires substantial energy expenditure [3].

Currently, the problem of creating a monitoring system for the physical development of preterm infants, especially those with low and/or extremely low birth weight, is becoming increasingly relevant [4]. Anthropometric indicators at birth reflect the characteristics and often the pathology of intrauterine development and, together with other factors, determine the harmony of the child's development in subsequent years. Assessing the physical development of a preterm infant is one step in evaluating the child's overall condition. Physical development is subject to fluctuations depending on geographical, ethnic, climatic, social, and environmental factors; therefore, standards for physical development, including for those born prematurely, require regular updates for each region (at least once every 5-10 years).

Preterm infants are characterized by features such as delayed growth and weight gain compared to their peers, as well as low muscle mass and muscle hypotonia. It is worth noting that the skeletal system in preterm infants develops more slowly, which can lead to reduced bone density and increased fragility. Furthermore, they often exhibit impairments in the development of the respiratory, cardiovascular, and nervous systems, affecting their overall physical adaptation [5].

The anatomical and physiological characteristics of preterm infants inevitably influence their physical and morphofunctional development. Extremely preterm infants constitute the most vulnerable group in terms of impaired physical development. The degree of impairment directly correlates with birth weight and gestational age. Physical development indicators and their dynamics include body length, weight, and head and chest circumferences.

Moreover, preterm infants have specific gastrointestinal tract features that affect the processes of nutrient absorption and assimilation from the first days of life, also leading to an energy deficit. One manifestation of this can be low gains in physical development parameters during the adaptation period and later stages [6]. There is an interconnection between physical and psychomotor development. For instance, 67% of children with significant delays in physical development during the first year of life (low height, weight, head circumference) have a more than two-fold increased risk of neurological impairments [7, 8]. Data suggest that the number of adverse psychomotor development outcomes in preterm children, specifically cerebral palsy, intellectual disability, and mental disorders, is inversely correlated with weight gain during the first year of life.

A prognostic indicator for preterm infants is postnatal growth in early infancy, which is associated with metabolic and chronic diseases later in life [9]. Postnatal growth failure is diagnosed when a child's weight is below the 10th percentile or below -1 z-score for post-conceptional age. It has been established that more aggressive nutritional approaches in the ICU reduce the incidence of postnatal growth failure in preterm infants [10]. Currently, it is believed that intrauterine growth restriction primarily reflects genetic components and placental function, whereas postnatal growth failure is mainly associated with nutritional deficits, conditions, and diseases experienced by the child in the first months of life. Compared to previous decades, postnatal growth failure has decreased worldwide, although it remains a concern.

The physical development of preterm infants is characterized by higher rates of weight and length gain during the first year of life (except for the first month). These rates can be considered relatively intensive until the child reaches the length and weight values typical for full-term, mature newborns. Subsequently, the absolute weight gains become greater than in full-term infants. The age at which indicators of a normal full-term infant are reached (i.e., weight 3200-3500 g and length 50-51 cm) is approximately 2-2.5 months for children with a birth weight of 1500-2000 g, and 3-3.5 months for children with a birth weight of 1000-1500 g. The smallest newborns (under 1000 g) may reach these indicators within 4-7 months. In the catch-up development of preterm infants, the highest rate is characteristic of head circumference growth, followed by chest circumference, then body length, and finally, weight normalizes last. By 2-3 months, preterm infants double their initial birth weight; by 3-5 months, they triple it; and by one year, they increase it 4-7 times. However, extremely immature infants lag significantly in absolute growth and weight indicators ("miniature" children) – corridors 1-3 on centile charts. In subsequent years, extremely preterm children may maintain a specific harmonious delay in physical development. The rates of increase in height, head circumference, and chest circumference in preterm infants during the first year of life also differ from those of full-term newborns [11].

By the end of the first year of life, head circumference increases by 15-19 cm and reaches 44.5-46.5 cm. The "crossover" of head circumference percentiles in healthy preterm infants occurs between the 3rd and 5th months. Slow head growth can be an early sign of deviations in neuropsychic development. Generally, preterm children catch up with their full-term peers in weight and height indicators by 2-3 years of age, while children with a birth weight of less than 1000 g only do so by 6-7 years. The timing of catch-up depends on the degree of low birth weight and can vary from 9 months to 3-4 years [12].

Typically, children with extremely low birth weight grow poorly in early childhood, and this problem often persists. By 5 years of age, 30% may have weight deficit, and 50% height deficit among children born before 30 weeks of gestation. By 8-9 years, about 20% still lag in height. Growth spurts in this group of children begin 1-2 years later.

Given a favorable medical and social environment, the physical development indicators of preterm children almost always reach the norm by 17 years of age. With age, a decrease in the dependence of physical indicators on the influence of biological factors is observed in children born prematurely [13].

Extremely preterm infants with a gestational age of less than 32 weeks are characterized by impairments in the motor function of the central nervous system of varying severity. For example, several studies have established that preterm newborns achieve motor skills such as sitting, crawling, standing, and walking more slowly, most likely due to an imbalance between trunk flexor and extensor muscles [14].

Results from many magnetic resonance imaging studies of preterm infants with perinatal CNS lesions have revealed an increase in structural changes in the brain several months after birth. Preterm infants maintain high risks of developing CNS pathology, including the formation of motor disorders such as cerebral palsy [15].

Among pathological conditions in preterm infants, respiratory disorders hold a leading and particularly important position.

Another factor significantly contributing to reduced rates of physical development is bronchopulmonary dysplasia (BPD). BPD is a polyetiological chronic disease of morphologically immature lungs, developing in newborns, mainly extremely preterm infants, as a result of intensive therapy for respiratory distress syndrome (RDS) and/or pneumonia.

Bronchopulmonary dysplasia has a multifaceted impact on the child's somatic development, affecting many functional systems:

The prognosis and severity of impaired physical development in BPD depend on the disease's severity, the presence of comorbid pathology, and the adequacy of therapy. Early diagnosis, timely treatment (including respiratory support, medication therapy, and nutritional support), and comprehensive rehabilitation play a key role in minimizing the negative consequences of BPD and optimizing the child's physical development [16-18].

The main prerequisites for the formation of postnatal deficits in weight and height indicators are the lack of reserves of macro- and micronutrients. The main problem in feeding preterm infants is that the capabilities of their immature digestive, endocrine, and immune systems do not meet their high nutrient requirements. Gastrointestinal immaturity manifests as reduced and uncoordinated intestinal motor function, low motilin activity, reduced activity of digestive enzymes (lactase, lipase, pepsin, etc.), increased permeability of the intestinal mucosa, and imperfect immune defense. Currently, most specialists support the notion of the need to provide a preterm infant after birth with a growth rate close to the intrauterine rate [19].

Prematurity is a problem rooted in the intrauterine period, as one of the causes is an incorrect approach by the woman to carrying the baby. Consequently, the prevention of prematurity begins with parents planning the pregnancy, which includes consulting a doctor before conception regarding the correction of chronic diseases, infections, and deficient states that may increase the risk of preterm birth. Preterm infants born with low and extremely low birth weight require individualized care and periodic developmental monitoring to prevent various pathologies.