The present study demonstrated that full-term SGA infants grow rapidly upon birth, especially in the first three months of life. In addition, catch-up was better for weight compared with length in the first year of life in SGA infants.
The definition of small for gestational age has been controversial so far. As a standard, this study used the INTERGROWTH-21st project, which assessed data in 8 middle and high-income countries[12]. Although no statistically significant differences were found among the 8 nations, using this standard in other countries or regions may reveal differences. A study in Hong Kong, China[13] showed that when the INTERGROWTH-21st standard is employed to determine birth weight in infants, the proportion of SGA infants is greatly increased compared with local standards. Therefore, a different diagnosis can be obtained for the same baby just because of differences in diagnostic criteria. Although China established a birth weight curve for newborns with different gestational ages in 2015[14], it is not convenient to determine full-term small-fetal weights at different gestational ages since these data do not provide a standard for length in newborns based on gestational age. We assessed infant lengths within one year after birth, and the above standard was not appropriate for this study. In order to define SGA more reasonably in China, the birth weight and length curves of newborns of different gestational ages should be generated as soon as possible, so that clinicians can make a more accurate assessment of birth conditions in order to facilitate SGA diagnosis. This would improve future follow-up and interventions.
Considering that the birth weights of twins are generally different from those of singletons, only singletons full-term SGA babies were examined according to the INTERGROWTH-21st standard. It is challenging to assess birth head circumference. Therefore, we compared the birth lengths of 210 full-term infants with matched sex and age, and found different Z scores distributions. As shown above, the percentages of boys and girls whose Z scores were less than − 2 were higher than those of counterparts with Z scores below − 2. This indicates that defining SGA based on birth weight alone does not fully reflect the intrauterine development level. Therefore, the 2007 International Pediatric Endocrinology Society and the Growth Hormone Research Society[15], as well as the 2011 Latin American Consensus[16] both adopted the following definition for SGA infants: newborns whose crown and heel lengths are 2SD lower than the average values for newborns of the same gestational age. However, due to the low operability of body length measurement at birth, its accuracy is relatively lower compared with weight assessment, making birth weight more clinically relevant. However, attention should be paid to the assessment of birth length in the clinical evaluation of children under gestational age. We should also pay enough attention to those without low birth weight but showing birth length 2SD lower than average, strengthening long-term follow-up.
Bocca-Tjeertes et al.[17]followed up full-term SGA infants, and found that catch-up growth is mainly completed during infancy. A prospective study with birth weight ≤ 10th percentile of the same gestational age defined as SGA[18] found that 60% of full-term infants younger than gestational age complete catch-up at 1 year of age. A study showed that the best growth trajectory for full-term SGA infants is to catch up to the 30th percentile in the first few months, and then to a milder catch up speed, reaching the 50th percentile at the age of 7 years and maintaining that level[19]. This study performed a one-year follow-up monitoring of the growth and development of 210 singleton full-term infants smaller than matched gestational age babies in the first year after birth, and measured their 1, 3, 6, and 12-month-old body lengths, weights and head circumferences. Average Z scores for various indicators in full-term infants of both genders after birth continuously increased at various months of age. Most SGA infants showed rapid growth. Although growth rates were different in both genders, growth in the first three months was the most significant in all infants. In addition, boys and girls both showed a faster increase in weight compared with length and head circumference. The rates of body weights, lengths and head circumferences 2SDs below averages at 12 months of age were 1.90%, 10.48%, and 2.86%, respectively. This corroborated findings by Karlberg and Albertsson-Wikland[20]. Studies used weight, body length and head circumference for evaluation, with catch-up growth defined as the corresponding physical index Z scores increasing by > 0.67[21]. Full-term SGA babies tend to have rapid catch-up growth after birth. Although the definition of full-term SGA babies uses birth weight, the core of their catch-up growth lies in height. The international consensus[22] defines catch-up growth as a growth rate (cm/year) greater than the median of children of the same age and sex; alternatively, the child’s length (height) exceeding those of matched age and sex counterparts can be considered to reach catch-up growth at the 3rd percentile or -2 standard deviation. A recent study by Shi et al.[23] demonstrated that in full-term SGA infants, catch-up growth is maintained at more than 2 cm in the first few months, from < P10 to P3 ~ P25-p50, with a median catch-up maintained until age 2. It may be optimal to catch up with such growth trajectory and minimize the risk of adverse health consequences for children. Therefore, more attention should be paid to the length of full term SGA infants. Full term SGA infants have specific growth and development patterns, and catch up growth within 6 months is independent of feeding patterns and average daily energy intake[24]. A 2006 WHO study in developing and developed countries confirmed that the nutritional needs of SGA infants are similar to those of counterparts with appropriate for gestational age (AGA). Therefore, feeding strategies for children younger than gestational age should mainly be based on gestational age rather than birth weight; this should not only promote moderate growth, especially linear growth, to ensure a good nervous system outcome, but also avoid overfeeding to reduce the risk of long-term metabolic syndrome[25].
When assessing the growth level of children, the potential effects of genetic factors should be considered. In this study, body lengths at birth and 12 months of age in full-term SGA infants of both genders were analyzed in relation to father’s, mother’s, and combined parents’ heights. As shown above, there was no associations of birth length in boys and girls with father's height, mother's height, and combined parents’ heights. However, at the age of 12 months, Pearson correlation coefficients between height and father's, mother's and parents’ heights were 0.397, 0.484, and 0.565 in boys, respectively, with statistical significance, indicating that the parents’ heights affected child height. The greater the sum of parents’ heights, the higher the boy's height at 12 months of age. Among girls, Pearson correlation coefficients between child height and mother's and combined parents’ heights were 0.188 and 0.199, respectively, indicating that the greater the mother's and combined parents’ heights, the greater the girl’s height at 12 months of age. This is consistent with the positive correlation between the height of the offspring and those of parents in the normal population.
This study had limitations. First, its retrospective design carries inherent shortcomings. In addition, the sample size was relatively small, which may explain why correlations between child and parents’ heights in full-term infants of both genders had different coefficients. Finally, the study was performed in a single institution, and the findings have limited generalizability.