The term “small for gestational age” (SGA) refers to neonate who is smaller in size than normal neonates with the same gestational age. Impaired fetal growth has many causes, including fetal, placental, maternal, and environmental factors.^1,2) Infants born SGA have a wide spectrum of clinical characteristics. SGA infants are at higher risk of perinatal morbidity, coronary heart disease, type 2 diabetes, and being short later in life.^3,4)

The definition of SGA is confusedly used by neonatologist and endocrinologist. In some publications, SGA is defined as birth weight (BW) or length below 3rd percentile.^5,6) In others, it is defined as below 10th percentile for gestational age.^7,8) In practice, SGA is usually defined as BW below 10th percentile by neonatologists according to neonatal care guideline of Korea. However, endocrinologist defines SGA as BW or length below 3rd percentile or two standard deviations for a reference population due to health insurance standards for growth hormone therapy for those with short stature.^9,10)

Catch-up growth of SGA infant is an early postnatal process. A common definition of catch-up growth is a growth that is over 10th percentile of normal Korean infant's growth curve.¹¹⁾ The majority of children who are born SGA experience catch-up growth by the age of 2.^12,13) However, approximately 10% of children born SGA fail to show catch-up growth. They may remain short during childhood and after the age of 2.^14,15) Thus, monitoring growth profile provides valuable information for children born SGA. Prompted identification of failed catch-up growth is important for management of these short children.

The aim of the study was to evaluate the difference in postnatal catch-up growth rate until the age of 2 between SGA groups with different definitions used by neonatologist and endocrinologist. Because of the variety of definitions of SGA, this study aimed to evaluate the catch-up growth rate of infant with BW between 3rd and 10th percentile for their gestational age. Perinatal factors related to catch-up growth in SGA infants were also evaluated in this study.

A total of 212 SGA infants were admitted at Korea University Anam Hospital and Ansan Hospital during the study period. Based on the exclusion criteria, a total of 83 patients were excluded. The remaining 129 SGA infants who met the inclusion criteria were enrolled for this study (Fig. 1). Of these infants, 60 were preterm infants while 69 were term infants. A total of 122 (94.5%), 106 (82.1 %), and 43 SGA infants (33.3%) were followed up at 6, 12, and 24 months, respectively, for weight.

Our study compared catch-up growth rate longitudinally for two years between the 2 SGA groups (group A, BW below 3rd percentile; group B, BW 3rd–10th percentile). At 24-month-old OPD follow-up visit, 66.7% and 80.0% in group A and 63.6% and 80.0% in group B showed catch-up growth for weight and height, respectively. A total of 20% of SGA infants in both groups did not show catch-up for height up to 2 years old.

The definition of catch-up growth was used variously in previous studies. Lim et al.¹⁶⁾ have defined catch-up growth as those whose growth parameter is over 3rd percentile of normal Korean full term infant's growth curve. Ma et al.¹¹⁾ have defined catch-up growth as weight over the 10th percentile at 2 years of corrected age based on the Standard Korean Infant's Growth Curve. Likewise, growth failure has been defined as weight and height below the 10th percentile at 24 months using fetal growth charts in another study.¹⁷⁾ We defined catch-up growth as growth parameter over 10th percentile of normal Korean infant's growth curve. Approximately 10%–25% of children born SGA fail to show catch-up growth and remain short as adults.^18,19) This wide variation may be due to various definitions of SGA in different studies.

For both group A and B, catch-up growth was defined as growth parameter over 10th percentile of normal Korean infant's growth curve. It was expected that group A (BW below 3rd percentile) would have greater difficulty to achieve catch-up growth than group B (BW 3rd–10th percentile). However, significant difference in the rate of catch-up growth for height was only observed at 6-month-old OPD follow up visit in the present study. At 6-month-old follow-up and thereafter, there was no statistically significant difference in the rate of catch-up growth for height between the 2 groups.

Catch-up growth can be affected by environmental factors such as mode of postnatal diet. Lucas et al.²⁰⁾ have suggested that breastfeeding may promote faster growth in SGA infants due to specific nutrients in breastmilk, better nutrient absorption, and nonnutrient factors in milk. According to the study of Davies,²¹⁾ there is no significant growth differences between term SGA infants fed either banked breast milk or formula with different protein contents. In the aspect of nutrition, we analyzed the mode of diet until the age of 6 months when babies are usually on liquid diet. With regard to feeding habit (breast milk, formula milk, or mixed) in our study, most infants were mixed-fed. The percentage of mixed-fed infants was 85.4% in the group that showed catch-up for weight at 6 months old and 85.0% in the group that failed to show catch-up. Such difference was not statistically significant between the 2 groups. The percentage of formula-fed infants was 13.4% in the catch-up group and 15% in the failed to catch-up group. The percentage of breastfed only infants was 1.2% in the successful group and 0% in the failed group.

In our study, difference in postnatal catch-up growth rate between the 2 groups showed mostly in the early period (≤6 months) of OPD follow-up visits. This result is in concordance with findings of other studies ^21,22,23) which showed that catch-up occurs during the first 6 postnatal months. Postnatal catch-up growth at 24 months old OPD follow-up visit showed no statistically significant difference between the 2 SGA groups in our study (groups A and B: 80%). We reviewed the perinatal characteristics of the catch-up group and the group failed to show catch-up at 6 and 24 months for possible differences (Table 2). In regard of weight, at 6 months old, gestational age was 37.0±2.5 weeks for the catch-up group and 35.9±2.7 weeks for the failed to catch-up group (P<0.05). Preterm infants had more difficulty in gaining early postnatal catch-up growth of weight. At 6 months old, mean BW was 1,999±460 g for the catch-up group and 1,719±451 g for the failed to catch-up group (P<0.05). Infant born with lower BW had more difficulty in achieving early postnatal catch-up growth. At 24 months old, there was no significant difference in perinatal characteristics between the 2 groups.

Regular OPD follow-up visit is important for SGA infants to prevent consequences such as short stature, metabolic syndrome, and altered puberty. Catch-up is typically an early postnatal process that is completed by the age of 2 years in most SGA infants. If a short child who was born SGA shows no evidence of catch-up growth over 3 years of age, the child is less likely to catch up later on.¹⁾ These children should be referred to pediatric endocrinologist for interventions such as growth hormone therapy to achieve optimal growth acceleration. A child with metabolic disease or altered puberty should be referred to endocrinologist at an appropriate time through regular follow-up visits. Monthly follow-up until 3 months of age, quarterly follow-up until one year of age, and then biannually until age of 8 years are recommended as a follow-up plan for SGA infants.²⁴⁾

The major strength of our study was its longitudinal approach for SGA children from birth. Moreover, there was no study of SGA comparing catch-up growth by different definitions of SGA in Korea. However, this study has some limitations. First, it was a retrospective study by collecting medical data of children based on hospital records. There was a decrease in the number of enrollment over time due to different OPD follow-up times. In addition, not all infants were included at each follow-up time. Secondly, we experienced OPD visit follow-up losses of children. It was another cause of small sample size as time passed by. Among infants who did not came to OPD follow-up visits until 2 years of age, 23 infants showed catch-up growth for 2 consecutive times until 12 months old. Furthermore, this study was based on data from only 2 institutions. No data were available after 2 years of age. Therefore, further prospective longitudinal study with information on catch-up growth of SGA children is needed to validate our findings.

In conclusion, there were no significant differences in postnatal catch-up growth rate until the age of 2 between the 2 SGA groups with different definitions except height at 6 months old. Low gestational age and BW was the risk factor for failed catch-up growth for weight at 6 months old compared to infants with catch-up growth. Regardless of SGA definition, approximately 20% of SGA children did not show catch-up in height until 2 years old. Our results suggest that a broader definition for SGA, such as infant with BW below 10th percentile for gestational age, would help us identify SGA infants with growth impairment earlier.