This study aimed to elucidate the relationship of prenatal and/or postnatal factors, including acquired factors, with the calcaneus stiffness index as measured by quantitative ultrasound (QUS-SI) in adolescents. We recruited 1,143 adolescents with a mean age of 14.8 +/- 1.8 years (501 boys and 642 girls). The subjects' calcaneus QUS-SI was measured using an ultrasound bone densitometer. We also measured the subjects' height, weight, and grip strength. Data on prenatal and postnatal factors were obtained from maternal and child health handbooks. A self-reporting questionnaire was used to obtain information on subjects' secondary sexual characteristics and lifestyle factors. We found that maternal weight gain during pregnancy was independently associated with calcaneus QUS-SI in girls, and that grip strength was also significantly associated with calcaneus QUS-SI in both sexes. The present findings suggest that excessive restriction of maternal weight gain would have a negative effect on the calcaneus QUS-SI of girls, and that exercise and strength-building activities are likely to result in a higher calcaneus QUS-SI in both sexes of adolescents.