In recent years, the life quality of preterm infants has been remarkably improved with continuous development of the healthy condition and close cooperation between obstetrics and pediatrics [5]. Relevant researches manifest that there is a vital "opportunity window" for the growth rates of preterm infants within 2 to 3 months of corrected age, which will play an extremely important role in future growth and development. According to the recommendation of the American Academy of Pediatrics (AAP), the physiological functions of various system organs of preterm infants are not mature enough after birth. The goal of the growth rates of preterm infants is to approach the growth rates of full-term infants at the first year of age. However, the growth rates of preterm infants are significantly different from those of full-term infants since preterm infants are prone to cumulative malnutrition and growth restriction [6]. Therefore, continuously monitoring of the growth rates of preterm infants is a critical field in pediatric clinic and child health by utilizing the appropriate evaluation criteria [7, 8].
Most scholars convinced that birth length and birth weight were positively correlated with the growth rates of length and weight of preterm and full-term infants [9]. Birth length was the main indicator to monitor fetal linear growth, and it was closely related to infant growth retardation and even adult short stature. Meanwhile, birth weight was the common index to monitor fetal growth rate and recent nutritional status, and it was also correlated to infant growth rate and future health status [10]. The research results of Vieira et al. showed that the correlation between birth length and growth retardation or adult short stature of preterm infants was significantly higher than that of full-term infants. For example, the risk of adult short stature of preterm infants with birth length less than -2SD was 7 times higher than that of full-term infants. Simultaneously, the risk of adult short stature of preterm infants with birth weight less than -2SD was 4 times higher than that of full-term infants. In other words, it was suggested that birth length and birth weight could effectively identify the high-risk infants with growth retardation [11].
The growth trajectories of length and weight of preterm infants were obviously different from those of full-term infants from birth to 3 months [12]. This study showed that the growth rates of length and weight of preterm infants were significantly lower than those of full-term infants from birth to 3 months. Toftlund et al. also reported that the catch-up growth of preterm infants mainly occurred from birth to 3 months [13, 14]. As for the catch-up growth, Miller et al. manifested that the advent of preterm formula or breast milk fortifier might be helpful in a more customized approach to the growth rates of preterm infants. Consequently, preterm infants could supplement additional calories in time, and present an obvious trend of catch-up growth [15]. Relevant studies further showed that some minerals such as zinc, copper, iron, and magnesium regulated heat metabolism through oxidative phosphorylation pathway, and additional heat could significantly promote the growth rate of preterm infants [16].
However, the growth trajectories of length and weight of preterm infants were rather close to those of full-term infants after 3 months [17]. This study showed that the growth rates of length and weight of preterm infants gradually reached the level of full-term infants after 3 months. Arora1 et al. reported that there was no significant difference in the growth rates between preterm and full-term infants after 3 months. It depended on continuous breastfeeding and timely addition of weaning food after 6 months. Therefore, Continuous breastfeeding and timely weaning food could effectively accelerate the growth rates, and control the potential risk of adult diseases in preterm infants [18, 19].
During the first year of age, there were significant differences in the growth rates of length and weight of preterm and full-term infants at different stages. The growth rates of preterm infants were significantly lower than those of full-term infants from birth to 3 months, and gradually caught up to the level of full-term infants after 3 months [20]. Specifically, the growth rates of preterm and full-term infants were attributed to the regulatory mechanism of growth hormone (GH), insulin-like growth factor-1 (IGF-1), and insulin-like growth factor binding protein-3 (IGFBP-3). As for the regulatory mechanism, GH was an important regulator of the growth rates of preterm infants in a pulsed pattern, and IGF-1 formed a complex with IGFBP-3 which was closely related to the growth rates of preterm infants. Furthermore, Amina et al. reported that the serum levels of IGF-I and IGFBP-3 in preterm infants were lower than those in full-term infants (P < 0.001), and showed a dose-response relationship. Meanwhile, the serum levels of IGF-I and IGFBP-3 expressed the first secretion peak at 2 to 3 months, and then a slow downward trend in preterm infants [21].
Based on this study, the perinatal factors were attached more importance to the growth rates of preterm infants from birth to 3 months [22]. This study also manifested that there were some variables of birth length, birth weight, gestational weeks, delivery times, and pregnancy-induced hypertension syndrome which were the influence factors by the contribution to the growth rate of preterm infants. Zhang et al. revealed that the perinatal factors were related to the growth rates of preterm infants [23]. Besides gene polymorphisms, various perinatal factors jointly participated in the regulation mechanism of IGF-1 and IGFBP-3, and finally affected the growth rate of preterm infants [24]. Therefore, multidisciplinary interventions should be developed to promote the growth rates of preterm infants within the first 3 months.
There are some limitations to this study. First, we haven’t recruited enough samples to unveil more risk factors of the growth rate of preterm infants due to the lost health visits at the corresponding points of physical measurement. Second, we haven’t compared the growth rate of preterm and full-term infants in urban and suburb areas of Hubei province.
In a future study, we should establish a birth cohort study of the growth rates of preterm and full-term infants with increased duration of the physical follow-up, and detailed documentation of the potential factors. Furthermore, we should construct a multicenter study of the growth rates of preterm and full-term infants in a larger sample size stratified by gender and region.