Reassessing Mid-Parental Height in Thai Male Adolescents: Short-Term Growth and Modifiable Correlates of Positive Height Gap

Mid-parental height is widely used to estimate expected adult stature, although the predicted value represents a central estimate rather than a fixed biological limit. This study examined short-term height gain and factors associated with attaining a height above the mid-parental prediction in Thai male adolescents. Seventy-nine boys aged 12-17 years were followed over a three-month period. Baseline and follow-up height, parental height, weekly exercise frequency, and daily milk intake were recorded. Mid-parental predicted height was calculated as (father's height + mother's height + 13 cm)/2. The primary outcome was three-month height gain, while a positive height gap was defined as current height exceeding the predicted midpoint. Mean height gain was 1.76 +- 1.71 cm, and 57 participants (72.2%) showed an increase in height. Overall, 36 of 79 boys (45.6%) were taller than their mid-parental predicted midpoint. Among 17-year-olds, 20 of 37 (54.1%) exceeded the predicted value, with a mean height gap of +1.31 cm. Participants who exercised at least four days per week gained more height than those exercising three days or fewer (2.08 +- 1.75 vs. 0.90 +- 1.26 cm; mean difference 1.18 cm, 95% CI 0.46-1.90; p=0.0018). After adjustment for age, milk intake, and the baseline difference from predicted height, frequent exercise remained independently associated with greater height gain (adjusted ?=0.84 cm, 95% CI 0.11-1.57; p=0.024). Higher milk intake was associated with greater growth in unadjusted analysis, but the association was attenuated after adjustment (?=0.68 cm, 95% CI -0.22 to 1.58; p=0.141). A larger baseline gap below predicted height was also independently associated with subsequent growth (?=0.098 cm per 1-cm gap, 95% CI 0.047-0.148; p<0.001). These findings suggest that mid-parental height is more appropriately interpreted as an expected midpoint than as a strict ceiling. In this cohort, frequent exercise was associated with greater short-term height gain, whereas exceeding the parental midpoint appeared to reflect accumulated growth and developmental stage. Longer follow-up to near-final adult height is needed to determine whether conventional parental-height equations accurately reflect contemporary Thai adolescents.