Introduction

The first year of a child's life, known as infancy, marks a crucial phase of growth and development. Examining growth indicators during this period not only gauges infant nutrition but also reflects a country's economic status. These growth indicators primarily encompass anthropometric measurements, such as length, weight, and head circumference1. Research suggests that the primary risk factors influencing infant length and head circumference mirror those affecting other growth indicators, indicating the growth of skeletal bones and skull development2. The growth curve plays a vital role in evaluating child health and nutrition. Presently, the global standard for assessing infant growth is the World Health Organization (WHO) standards, which established reference values for length in 20063 and head circumference in 20074. Our country formerly relied on reference values established in 2009, but we now employ the most recent "Growth Standards for Children Under 7 Years of Age (WS/T423-2022)"5 released by the National Health Commission in 2022 to assess infant growth.

Length denotes the vertical measurement from a child's head's top to their heel, encompassing the combined lengths of the head, trunk, and lower limbs. A child's body length is closely linked to nutrition, endocrinology, and other factors and is commonly assessed in children under 3 years old. A Chinese study has revealed that the first year after birth is when children experience the most rapid height increase6. Therefore, monitoring length during infancy is critical. Head circumference refers to the measurement taken around the upper edge of the eyebrow arch, encircling the head and extending to the occipital tubercle. This measurement is associated with brain volume, cognitive function, and cranial growth7. It serves as a significant indicator of brain capacity, cognitive function, and cranial development2,8. A child's head circumference at age 3 is approximately 90% of that of an adult; hence, head circumference is typically assessed in children under 39. The American Academy of Pediatrics (AAP) also recommends measuring head circumference eight times before age 210. Gale's research has demonstrated that a child's brain volume at age 1 can be a predictor of their intelligence in later life8. Therefore, it is especially important to measure infants' head circumference. Monitoring the length and head circumference of infants from birth to 1 year old is of great significance in assessing their individual nutritional and neurological development8.

Hainan, the southernmost province and China's second-largest island, boasts distinct geographical, climatic, and lifestyle conditions compared to other regions in China. A study by Dou and her colleagues11 has underscored that children in Hainan exhibit lower growth rates in terms of length and head circumference compared to the national average. However, specific percentile reference values for infants aged 0–1 year have not been explicitly provided, and the underlying causes for these conditions in Hainan infants' length and head circumference remain unexplored. Child growth is influenced by a combination of genetic and environmental factors. Given Hainan's unique geographic environment, it becomes crucial to assess the disparities in infants' length and head circumference between Hainan, the national standard, and WHO guidelines. This study introduces comprehensive percentile reference values and charts for the first time, laying the foundation for the evaluation of infant length and head circumference in this region.

Methods

Study sample

The data utilized in this study were gathered from the Growth and Development Monitoring Survey of Children aged 0–18 years in Hainan Province, conducted between December 2020 and April 2022. For this specific investigation, data regarding infants up to one year old were acquired from 18 cities in Hainan. According to the sample size formula, n = Zα/22 × p (1 − p)/δ, with α = 0.05, Z0.05/2 = 1.96 ≈ 2, and δ = 0.1 × p, the formula can be simplified to n = 400(1 − p)/p. By reviewing the literature12, we obtained that the rate of stunting in Hainan infants was 13.4%, and taking a sampling error of 10%, we calculated the sample size as 2844. Using a finite population correction factor to adjust the calculation of sample size: ncorrection = n/[1 + (n − 1)/N], where N represents the overall population sample size, which is about 130,000 infants. Therefore, the corrected sample is calculated to be 2783. Removal of invalid data resulted in the inclusion of 2736 infants, including 1471 boys and 1265 girls. This study followed a prospective, multi-stage cross-sectional approach involving 2736 infants, selected through stratified sampling. The sampling method entailed the following steps: initially, stratification was carried out based on administrative grade, sequentially considering city or county, town or street, except for Haikou and Sanya. For the two prefecture-level cities, Haikou and Sanya, stratification was determined by district-town or street, and all districts were included. Towns or streets served as the Primary Sampling Units (PSUs) and were randomly chosen from the total of 18 cities, counties, or districts in the first stage. Subsequently, convenience sampling was employed, involving the consecutive recruitment of infants from each town's health clinics or community health service centers, based on the selected towns or streets.

Data inclusion and exclusion

Inclusion criteria encompassed infants born to long-term residents with local household registration in Hainan Province. Exclusion criteria were defined as follows: (a) multiple births; (b) preterm births (born between 28 and 37 weeks of gestation) or infants with low birth weight (birth weight < 2.5 kg); (c) infants with limb and head disabilities; (d) moderate-to-severe rickets or other malformations that hindered growth and development; (e) Infants with the following conditions: endocrine disorders such as hypothyroidism, hyperthyroidism (abnormal thyroid hormone secretion), and growth hormone deficiency impact their length, while neurological disorders such as hydrocephalus and abnormal cranial nerves affect their head circumference; and (f) infants experiencing diarrhea for more than 5 days, with more than five episodes daily. We confirmed that the research was performed in accordance with relevant guidelines/regulations. Informed consent forms were signed by all parents or guardians of the infants. This study was performed in accordance with the Declaration of Helsinki and approved by Medical Ethics Committee of Hainan Women and Children's Medical Centre (HNWCMC-2022-118).

Measurements and data collection

Infants' length and head circumference were consistently measured using a baby crib (seca Germany, Seca 416) and a soft, non-elastic fiber ruler (seca Germany, Seca 212). Prior to length measurement, infants were undressed, ensuring they remained comfortably warm. The infants were placed on their backs along the central line of the measuring board, and their heads were securely positioned against the headboard. The measurer positioned themselves to the right of the infant, using their left hand to support the infant's knees, straightening their legs and aligning them closely with the measuring board. Subsequently, the measurer moved the sliding board with their right hand towards the infant's heels and recorded the measurement when the scales on both sides aligned. For head circumference measurement, the infant's head covering was removed. The infant assumed a sitting or standing position, and the measurer stood in front of the infant. Using one thumb, the measurer anchored the zero point of the soft ruler, passed the ruler through the highest point of the occipital tuberosity, and aligned it with the upper edge of the eyebrow arch using their other hand, ensuring the ruler returned to the zero point. Throughout the measurement, the soft ruler was kept in close contact with the infant's skin, maintaining symmetry on both sides. Measurements of length and head circumference were recorded with a precision of 0.1 cm. Each measurement was repeated three times, and the average value was recorded. The personnel responsible for these measurements were trained and certified to ensure data accuracy.

Statistical analyses

Data compilation was performed using Excel 2019 software. Infant length and head circumference curves were constructed using LMS Chartmaker Light 2.54 software (Institute of Child Health, London, UK), and graphical representation was generated using OriginPro 2023b software (OriginLab Corporation, Massachusetts, USA). The curve fitting process utilized the LMS method, which stands for 'l' (skewness), 'm' (median), and 's' (coefficient of variation). This approach provides a comprehensive summary of data distribution across all age ranges by considering the median, coefficient of variation, and the Box-Cox transformation's power needed to normalize the data12. The L, M, and S curves were fitted using a triple spline function, with curve selection performed through multiplicative likelihood13. The values of L, M, and S vary with age, with the M curve representing the data's median curve. The LMS method involves data grouping based on the infant's age in months, and cubic spline functions are employed to fit smooth curves, denoted as L(t), M(t), and S(t), respectively. The required percentiles are determined using the formula: C100α (t) = M(t) [1 + L(t)S(t)Zα]1/L(t). Here, C100α (t) signifies the value on the 100α percentile curve for infants at the age of 't' months, 'Zα' represents the standard deviation for a given tail area of 1 − α (e.g., α = 0.03 corresponds to the 3rd percentile, with Z0.03 = −1.88), and L(t), M(t), and S(t) are the values specific to each curve corresponding to the infant's age in months at 't'. Before fitting the curves, calculate \({\overline{\text{x}}}\)  ± 4 s with SPSS 20.0 software (IBM Corporation, Chicago, USA)14, data outside or equal to the range were deleted.

Ethics approval

This study was performed in accordance with the Declaration of Helsinki. The study was approved by Medical Ethics Committee of Hainan Women and Children's Medical Centre (HNWCMC-2022-118).

Results

This study involved a total of 2736 infants from 18 cities and counties in Hainan, with approximately 54% being boys and 46% being girls. Baseline information concerning mothers and infants is presented in Table 1.

Table 1 Characteristics of the infant and maternal population.
Full size table

Percentile curves, including 3rd percentile (P3), 10th percentile (P10), 25th percentile (P25), 50th percentile (P50), 75th percentile (P75), 90th percentile (P90), and 97th percentile (P97), for length-for-age and head circumference-for-age among Hainan infants of both genders aged 0–12 months, were constructed using the LMS method. Reference curves can be observed in Fig. 1, length reference values are provided in Table 2, and head circumference reference values are listed in Table 3. Figure 1 illustrates that the length and head circumference of Hainan infants exhibit a consistent growth pattern, with more rapid growth observed in the first four months, followed by a gradual deceleration between months 5 and 12. Analyzing the median 50th value for different age months in Table 2 reveals that the length growth of Hainan infants in the first 3 months closely resembles that of the subsequent 9 months, with a growth of approximately 10.7–11.5 cm in the first 3 months and 12.5–13.4 cm from months 4 to 12. Furthermore, from the median 50th value for different age months in Table 3, it can be observed that head circumference growth during the first 3 months of life in Hainan infants is akin to the following 9 months, with head circumference increasing by about 4.9–5.4 cm in the first 3 months and by 4.0–5.1 cm from months 4 to 12.

Figure 1
figure 1

Length-for-age and head circumference-for-age reference curves for Hainan infants. P3 3rd percentile, P10 10th percentile, P25 25th percentile, P50 50th percentile, P75 75th percentile, P90 90th percentile, P97 97th percentile. (a) Boys' length. (b) Girls' length. (c) Boys' head circumference. (d) Girls' head circumference.

Full size image
Table 2 Infant length-for-age percentile reference values in Hainan (in cm).
Full size table
Table 3 Infant head circumference-for-age percentile reference values in Hainan (in cm).
Full size table

Compared with national standards

The P3, P50, and P97 growth curves are widely used for growth assessment1,15,16,17,18,19,20. In this study, we compared these three curves for length-for-age and head circumference-for-age in Hainan infants of both sexes with the curves derived from the latest child standard values released by the National Health Commission in 2022, as depicted in Fig. 2. The figure illustrates that, on the whole, the length and head circumference values of Hainan boys aged 0–12 months were lower than the new national standard values. As for girls aged 0–12 months, the length and head circumference curves of P3, P50, and P97 were lower than the new national standards, except for the P50 and P97 curves for 0–4 months, which closely matched the new national standards.

Figure 2
figure 2

Comparison of length-for-age and head circumference-for-age reference curves for Hainan infants with national standard curves. (a) Boys' length. (b) Girls' length. (c) Boys' head circumference. (d) Girls' head circumference.

Full size image

To further illustrate the disparities between the length and head circumference reference values of P3 and P50 for Hainan infants of both sexes and the new national standard values, we present the differences in Tables 4 and 5. Overall, the Hainan reference values at the P3 level are consistently lower than the new national standard values, with the maximum difference being 5.7 cm in length at 12 months for male infants and 1.9 cm in head circumference at 11 months. For female infants, the maximum difference is 4.4 cm in length at 11 months, 2.2 cm in head circumference at 11 months, and 1 year. Additionally, in comparison to the new national reference values, the P50 levels for length and head circumference in Hainan are predominantly lower. The only exceptions are the average head circumference values at birth, which are 0.4 cm and 0.5 cm larger than the national P50 values for Hainan male and female infants, respectively. Furthermore, the average head circumference reference value for female infants at 1 month matches the national standard.

Table 4 Difference between P3 reference values for infant length-for-age and head circumference-for-age in Hainan and new national standard values (in cm).
Full size table
Table 5 Difference between P50 reference values for infant length-for-age and head circumference-for-age in Hainan and new national standard values (in cm).
Full size table

Compared with WHO standards

The WHO standard curves offer a global perspective on children's growth levels, providing a broader assessment of the growth in length and head circumference among Hainan infants. Similarly, we created a chart to compare the three curves (P3, P50, P97) of length-for-age and head circumference-for-age in Hainan infants with WHO standard curves, as depicted in Fig. 3. The figure illustrates that the reference values for birth length and head circumference in Hainan infants closely resemble the WHO standard values. However, some variations become apparent as the infants continue to grow and develop. The P50 and P97 length curves are either similar to or slightly higher than the WHO curves, while the P3 length curve is lower than the WHO curve. Additionally, the three head circumference curves (P3, P50, and P97) for Hainan infants are, on the whole, lower than the WHO curves.

Figure 3
figure 3

Comparison of length-for-age and head circumference-for-age reference curves for Hainan infants with WHO standard curves. (a) Boys' length. (b) Girls' length. (c) Boys' head circumference. (d) Girls' head circumference.

Full size image

Discussion

This study is the first to establish reference curves and provide reference values for infant length and head circumference in Hainan for both genders. Furthermore, it is the inaugural effort to compare these reference values with the new national standards of 2022 and the WHO reference values from 2006/2007, thus offering a basis for evaluating infant length and head circumference in Hainan. The consistent pattern observed in the changes in length and head circumference of Hainan infants aligns with findings in the work of Caino21, suggesting that children share a similar growth pattern for both long bones and cranial bones22. Notably, the study revealed that the length and head circumference of Hainan infants were generally lower when compared to the new national standards. In contrast, when compared to the WHO standards, Hainan infants exhibited generally lower head circumference values, with length only falling below the WHO standards at the P3 level, while the P50 and P97 reference values were in line with the WHO standards. Clinicians typically opt for percentile reference values as they offer a straightforward and intuitive approach to assess children's growth levels. Therefore, this study employed percentiles to evaluate infant growth. P50 is generally used to gauge mean physical growth levels, whereas P3 and P97 are typically employed to assess abnormal growth levels16. In China, growth below < P3 is regarded as suboptimal, while growth equal to or exceeding ≥ P97 requires intervention5. Hui, in a scoping review, classified head circumference below < P3 and above > P97 as microcephaly and macrocephaly16.

On one hand, the lower infant length and head circumference in Hainan compared to the new national standards may be attributed to maternal factors, including breastfeeding practices, maternal age, and maternal height. This study revealed that only 40.2% of Hainan infants were exclusively breastfed up to 6 months, falling short of the target set by the "China National Program for Child Development (2021–2030)" issued by the State Council, which aimed for more than 50% exclusive breastfeeding23. It's worth noting that breastfeeding is known to promote both the length and head circumference growth of infants24,25,26. A study published in Pediatrics suggested that exclusive breastfeeding for the first 6 months, followed by complementary feeding until the age of 2, could reduce stunting in infants and toddlers by 67%27. In 2016, Huang developed growth reference values for breastfed infants in China28. Comparing the average length and head circumference of Hainan infants with the P50 reference values in Huang's study, it was evident that the average length and head circumference of Hainan infants from 1 to 12 months were smaller than the reference values for breastfed infants. Specifically, the average length of male and female infants was smaller by 0.4–2.3 cm and 0.7–1.9 cm, respectively, and the average head circumference was smaller by 0.5–1.4 cm and 0.4–1.4 cm, respectively. Therefore, promoting length and head circumference growth by increasing breastfeeding rates among Hainan infants is crucial. Additionally, a study conducted in five low- and middle-income countries29 noted that when mothers' childbearing ages were ≤ 19 or ≥ 35 years, their children were more likely to experience growth retardation and have poorer nutritional status. A Chinese study in Haikou City, Hainan, also identified maternal under or over-age as an influencing factor in stunting among children under 5 years old30. In this study, the proportion of mothers aged ≤ 19 or ≥ 35 years was 18.7%, with the youngest mother giving birth at 15 years and the oldest at 48 years. Furthermore, maternal height, as a genetic factor, may influence child length. Several studies have highlighted maternal short stature as a risk factor for child stunting31,32,33, and some studies suggest that children of taller mothers have higher head circumference-for-age Z scores2. In the present study, approximately 62.3% of mothers had a height of less than 160 cm.

Children's growth is the result of a complex interplay between genetic and environmental factors. In the case of Hainan, the lower length and head circumference of infants compared to the new national standards can be attributed to the unique geographical environment of this island province. Hainan Province is situated in the southernmost part of China and boasts the nation's only tropical monsoon climate, ensuring abundant sunshine year-round. The tropical location leads to high daytime temperatures and intense ultraviolet radiation, which, in turn, results in Hainan residents having active nightlife routines. A local tradition in Hainan, known as "Old Papa Tea," typically commences in the evening around 19:00–20:0034. These habits tend to lead Hainan parents to stay up late, affecting their children's sleep patterns due to the influence of their nightlife activities. Research has indicated that infants and children in predominantly-Asian countries tend to have significantly later bedtimes and shorter total sleep durations compared to those in predominantly-Caucasian countries35. A study by Ni36 delved into sleep duration and its influencing factors among infants and toddlers in Hainan, revealing that factors such as 2–3 h of outdoor activities and irregular vitamin D supplementation for infants aged 0–3 months were associated with sleep deprivation in Hainan. Furthermore, parental occupations, such as service and freelancing, were identified as negative factors for infants' sleep in Hainan, particularly for infants aged 4–12 months. Adequate sleep is crucial for the physical growth of infants. Sleep deprivation can lead to circadian rhythm disturbances, impacting brain structure and cortical functions37, and interfering with normal physical growth in infants and toddlers36. It is widely recognized that, after a certain period following birth, children gradually reduce their daytime sleep, increase nighttime sleep, and establish a circadian rhythm38,39,40. The primary hormone involved in establishing circadian rhythms in infants is growth hormone, which plays a critical role in promoting physical growth and is also influenced by the infant's sleep pattern and circadian rhythm41,42. The secretion of growth hormone steadily increases during sleep, reaching its peak shortly after sleep onset43. Consequently, nighttime sleep holds particular importance for infants' growth.

Moreover, when we compared the mean length and head circumference reference values for Hainan infants aged 1–12 months with the WHO standards, we observed that the mean length was similar, but the mean head circumference was smaller. This observation aligns with the findings of a Colombian study44. Hainan, located at 18°–20° N latitude in Asia, shares a tropical climate with Colombia, which is located at 4°–12° N latitude in South America. The common feature of both regions is their tropical location. Although tropical regions receive abundant sunshine, residents in these areas are also susceptible to vitamin D deficiency. Frost has pointed out that intense tropical sunlight can lead to tanned skin, which hinders the absorption of ultraviolet rays45. Vitamin D plays a crucial role in bone development, and sunlight exposure is a key source of vitamin D synthesis46. Since infant length and head circumference primarily reflect the growth of long bones and skulls, it is essential for infants in tropical regions to receive an appropriate amount of sunlight exposure to promote the growth of length and head circumference.

We have some suggestions for Hainan infants. This study highlights the significance of maternal education in promoting breastfeeding rates47,48. In this study, 55.9% of mothers had junior school education or lower (indicating they had not completed nine years of compulsory education). Therefore, efforts should be intensified to promote breastfeeding among mothers with lower educational levels. A systematic review published in JAMA 49 has shown that primary care interventions can support breastfeeding, including personalized education and peer support offered by perinatal healthcare professionals. The "Outline for Women's Development in China (2021–2030)"23, adopted by the State Council, has set the goal of increasing women's completion of nine years of compulsory education to over 96% and enhancing women's knowledge of prenatal and child-rearing practices to raise healthier children. Therefore, the Hainan government should focus on improving women's access to compulsory education and reducing the incidence of high-risk pregnancies among mothers aged ≤ 19 or ≥ 35.

To increase infants' sleep time, the American Academy of Sleep Medicine (AASM) recommends that infants should sleep for 14–17 h from 0 to 3 months, 12–15 h from 4 to 11 months, and 11–14 h at 1 year old50. Furthermore, guidelines in China suggest that infants should go to sleep no later than 21:00 from 3 to 5 months of age51. Hainan parents should adjust their work and leisure schedules to ensure that their infants have an appropriate bedtime. Evidence indicates that vitamin D deficiency can lead to sleep disorders52,53. A study by Ni36 identified irregular vitamin D supplementation as a risk factor for sleep deprivation in Hainan infants, underscoring the importance of emphasizing vitamin D supplementation. Health Canada54 also recommends that breastfed infants should receive 400 IU/days of vitamin D. Moreover, Hainan parents should increase their infants' outdoor activities during the day to promote vitamin D synthesis. However, it's essential to ensure that infants do not get sunburned by seeking shade when temperatures are too high, especially during the summer.

In order to explore the influence of tropical light on bone growth, this study provided the reference values and curves of body length and head circumference of China Hainan infants for the first time, which provided a tool for clinical evaluation. This study also had several limitations. We employed the percentile method to assess the length and head circumference levels of Hainan infants, but it did not offer a comprehensive evaluation of their growth. Future studies could focus on infants' weight levels, propose weight-for-age reference values, and combine length studies with the Kaup index to assess the nutritional status of infants.

Conclusion

In summary, this study has established reference values and curves for length-for-age and head circumference-for-age of Hainan infants of both sexes in China, providing a foundation for evaluating and monitoring the growth of long bones and skulls in Hainan infants. These reference values are instrumental in identifying, preventing, and managing stunting, microcephaly, macrocephaly, and other growth abnormalities in Hainan infants. Notably, the average head circumference of Hainan infants is below the new national standard and WHO reference values, while the average length is lower than the new national standard but similar to the WHO reference value. These findings can guide the Hainan Health Department in enhancing the growth of local infants concerning length and head circumference by promoting breastfeeding among perinatal women, ensuring infants' nighttime sleep, and reinforcing regular vitamin D supplementation.