Early diagnosis of abnormal penile size is important both medically and psychologically (123), and the exact measurement of penile size and current references of penile size are most important in diagnosing penile problems such as micropenis. Studies on the penile length of children have been very rarely conducted, especially in Korea (45). In 1987, a study on the penile and testicular size was conducted with 1,071 Korean children (6). Since then, only a few studies have been conducted with small numbers of children.

The authors previously evaluated the mean penile length of Korean boys and neonates in order to establish the reference of the current Korean children’ penile length (78). However, the preliminary study included a relatively small number of boys (n = 233) and neonates (n = 33) and limited to only Ulsan area in Korea. Therefore, the current study was conducted with more boys (n = 909) in Busan, Changwon, and Ulsan area. This study also compared the penile length and anthropometric measures such as height, body weight, and the testicular size of the current study and the previous Korean study conducted in 1987 to evaluate the association between them.

This cross-sectional study was conducted in the outpatient clinic of 5 hospitals between September 2013 and May 2015. The subjects of this study were 909 Korean boys aged 0–14 years. Exclusion criteria were penile diseases including hypospadias and concealed penis, cryptorchidism, varicocele, and other growth problems such as chronic renal failure and endocrinologic disorders. Therefore, most children who were brought for general periodic examination, urinary tract infection, enuresis and lower urinary tract symptoms were included in the current study (7).

For all boys, penile length and the testicular size were measured by a pediatric urologist of each hospital. The stretched penile length (SPL) was measured and the testicular size was evaluated using orchidometry (mL) with the presence of their parents in a warm room. SPL was measured with a stiff ruler by compressing the fat tissue with one end of the ruler through the pubic ramus; then, the penis was fully stretched and the distance to the glans of the stretched penis was plotted (78). Foreskins of the uncircumcised children were not involved in the measurement.

SPL gradually increased until the age of 13 and the most rapidly increased during 13 years old (Table 1). As compared with those reported in 1987, there was no significant change in SPL in most age groups although there was in some age groups (Fig. 1). The result showed that the age of rapid growth of SPL has been 1–2 years younger.

Fig. 2A showed that the height had a continuously increased pattern and as compared with those as of 1987, the increased change was more significant in children aged over 6 years old although the height of infant aged below 1 year old decreased.

Body weight and testicular size (Table 2) significantly increased in most age groups as compared with those reported in 1987. Although there were no significant differences in body weight of infants aged below 2 years old, it significantly increased afterwards, compared with data in 1987 (Fig. 2B). There were no significant changes in testicular size of children aged 0–9 years but it increased afterward and more rapidly during age of 12–14 years (Fig. 2C).

The changes of the testicular and penile sizes have a similar pattern. There was a rapid growth at 12 or 13 years old and the significant change of the age has become 1–2 years younger, compared with data reported in 1987. Nevertheless, in the current study, there were no changes in the testicular size of children aged 0–9 years while the penile size continuously increased.

The accurate measurement of the penile length is very important in children with abnormal genital development such as micropenis, buried penis and hypospadias and is essential to find out the sign of underlying severe endocrine and chromosome disorders (9). Another important thing is age specific standard data to compare the accurate measurement of SPL. However, studies on SPL in boys have been very rarely conducted, especially in a wide range of age groups. Therefore, the authors previously evaluated the mean SPL of Korean boys in order to establish the reference of the current Korean children’ penile length and compare with that of reported in 1987 (7). The study showed increases of SPL, height, body weight and testis size comparing with those reported in 1987, however, the preliminary study included a relatively small number of 233 boys. Therefore, current study was conducted with a larger sample including 909 boys and it revealed that there was no significant change in SPL except in some age group as compared with those reported in 1987 (Fig. 1) in the contrary to the previous preliminary study results. Unfortunately, the current study do not provide the data regarding the penile length of neonates so, it cannot be compared with other studies.

The reasons for the inconsistent result are as follows. First, the sample sizes were different. The preliminary study included 30 children in each age group while the current study included more subjects in order to bring more creditability to the statistics. Second, there is a regional difference. The current study was conducted in Busan, Ulsan and Changwon, Korea while the preliminary study was conducted in only Ulsan, Korea. However, the cohorts included in the preliminary and current study were all Korean people and it is thought to be a minor factor. Third, penile measurement was checked by different physicians in each hospital and in each study. Although SPL was measured in the same way in both studies, the results may differ depending on the examiners because the measurement process of SPL can be affected by the strength of examiners (10). Although the differences may significant, it is accounted to be only a few millimeters. A previous Korean study reported that there was no difference in SPL between two observers (3.3 ± 0.2 cm and 3.2 ± 0.3 cm, P = 0.165) in newborn infants with normal weight while there was a significant difference between two observers in newborn infants with low weight (2.9 ± 0.4 cm and 2.7 ± 0.4 cm, P = 0.001) (8). In a recent study, no significant fixed or proportional bias could be found for inter-observer variation (11). The reliability of the measurements of SPL remains a subject of debate (1213). Nevertheless, it is suggested that SPL is one of the most reliable methods in children to compare the penile size (1415).

In the current study, SPL gradually increased until 13 years old and the most rapidly increased during the age of 13 years (Fig. 1). It is thought that penile size had little change in boys aged 4–14 years since 1987 (6). The previous study reported that penile size increased slowly till 4 years old followed by a steady phase. It then increased rapidly during the puberty. Those patterns of penile growth are similar to the penile growth reported in previous Turkey or Brazil studies (1617). However, in current study, penile size increased continuously till 13 years old. Gradual penile growth curve before the puberty was also revealed in previous studies conducted in other countries such as Turkey, India and Japan (11181920).

In a recent Japanese study, SPL increases continuously during the prepubertal period, and the most rapidly increases during the first 4 months of life (11). This was explained by Boas et al. (21). They showed that penile growth rate during the first 3 months of life was positively correlated with serum testosterone levels. In the current study, the testicular size also increased after 9 years old and more rapidly in boys aged 12–14 years. This means that the age of rapid SPL growth has become 1–2 years younger as the age of puberty has become younger over the last quarter of century.

Fig. 2A showed that the height had a continuously increased pattern and as compared with those as of 1987, the increased change was more significant in children aged over 6 years old although the height of infant aged below 1 year old decreased. It may be suggested that Korean mothers today intend to keep fetal weight under control for safe and problem-free delivery. The exact causes of decreased height of infants are unknown and have to be evaluated later.

With the great economic development over the last quarter century in Korea, height, body weight and testicular size of children significantly increased but there was no significant change in SPL except penile growth pattern. The results of the current study may be used as the new reference values for boys aged 0 to 14 years. However, these results should be readjusted by the further epidemiologic studies with more children in a community-based population.