As shown in , 373 white girls (43.4%), 59 black girls (6.9%), 364 white boys (42.4%), and 63 black boys (7.3%) had valid pubertal measurements for at least 1 of the 7 assessments (taken annually from ages 9½ to 15½ years), thereby qualifying them for inclusion in the study. Girls had more valid data at all ages than did boys. Overall, 373 girls (86.3%) and 364 boys (85.2%) were white, 93 girls (23.4%) and 98 boys (24.5%) were classified as low income at age 9½ years, and mother’s mean (SD) years of education was 14.5 (2.4) for girls and 14.3 (2.4) for boys. (The low income percentages are based on numbers of participants who had income data at age 9½ years [ie, 397 of 432 girls and 400 of 427 boys].) Maternal age at the time the child was born was greater for the analysis group (mean, 28.9 years; 95% confidence interval, 28.4–29.4 years) than for girls not included in the analysis (27.7 years; 26.7–28.6 years). No significant differences existed for race, early family income-to-needs ratio, or years of maternal education. For boys, no significant differences emerged on any of these background factors for those included and not included in the analyses.
Descriptive Characteristics of the NICHD Study of Early Child Care and Youth Development Cohort
The eFigure shows the distribution of each sexual maturity stage, by age and race, for breast and pubic hair development for girls and for genital and pubic hair development for boys. At age 9½ years, most white girls showed no evidence of breast (56.2%) or pubic hair (71.5%) development; in contrast, most black girls showed some evidence of breast (69.0%) or pubic hair (59.2%) development. By age 15½ years, most white girls were in sexual maturity stage 5 for breast (79.5%) or pubic hair (81.9%) development; similarly, most black girls were in sexual maturity stage 5 for breast (84.1%) or pubic hair (92.5%) development. At age 9½years, more than two-thirds of white boys showed no evidence of puberty for genital (72.8%) or pubic hair (91.9%) development; less than half of the black boys showed some evidence of puberty for genital (47.9%) or pubic hair development (33.8%). At age 15½ years, most white boys were in sexual maturity stage 5 for genital (67.3%) or pubic hair (58.7%) development; similarly, most black boys were in sexual maturity stage 5 for genital (81.0%) or pubic hair (71.3%) development.
GIRLS’ PUBERTAL DEVELOPMENT
Overall, girls’ breast development occurred earlier than pubic hair development (F1,431=52.25; P <.001). This effect, however, was moderated by the stage being examined (F3,1293=53.29; P <.001). Girls were in stage 2 breast development a mean of 5.0 months earlier than stage 2 pubic hair development and were in stage 3 breast development a mean of 2.2 months earlier than stage 3 pubic hair development. There were no statistically significant differences between breast and pubic hair development in terms of the ages at which girls were in stages 4 and 5 ().
Mean Age at Which Girls Are in Each Sexual Maturity Stage
On average, girls took 1.5 years to develop from one sexual maturity stage of breast development to the next; girls took slightly less time (mean, 1.3 years) to develop from one sexual maturity stage of pubic hair to the next. Overall, girls took between 4 and 4½ years to go from sexual maturity stage 2 (beginning of puberty) to sexual maturity stage 5 (full maturity) ().
Mean Number of Years Between Sexual Maturity Stages Among Girls
When age of menarche was added to the repeated-measures analysis of variance as a between-subjects factor, we found a significant main effect for age of menarche (F1,407=435.72; P <.001), and significant interactions between age of menarche and sexual maturity stage (F3,1221=7.62; P <.001) and between age of menarche, characteristic (breast or pubic hair), and sexual maturity stage (F1,1221=4.54; P=.004). As expected, if girls were older when they experienced menarche they were also older when in all sexual maturity stages for breast and pubic hair development (B’s range, 0.41–0.48 for age at breast development stages 2–5 and 0.36–0.53 for age at pubic hair development stages 2–5; P <.001 for all). (B indicates unstandardized regression coefficient.) Age of menarche had a stronger positive relationship with age at developmental stages 3 and 4 (B’s range, 0.47–0.53) than it did with age of being in stage 2 (B=0.41 for breast development and 0.36 for pubic hair development). The relationship between age of menarche and age at breast development stage 5 declined to a B of 0.42, whereas the relationship between age of menarche and age at pubic hair development stage 5 remained relatively higher (B=0.48).
BOYS’ PUBERTAL DEVELOPMENT
Overall, boys were in each sexual maturity stage for genital development before they were in the same stage for pubic hair development (F1,426=821.26; P <.001); this effect was moderated by the stage being examined (F3,1278=411.40; P <.001). Stage 2 genital development was earlier than stage 2 pubic hair development by a mean of 1.1 years; stage 3, 3.9 months earlier; stage 4, 1.5 months earlier; and stage 5, 2.3 months earlier ().
Mean Age at Which Boys Are in Each Sexual Maturity Stage
On average, boys took 2 years to go from stage 2 to stage 3 genital development and 1.3 years to go from stage 2 to stage 3 pubic hair development. The change from stage 3 to stage 4 was shorter, taking 1.1 years for genital development and 10.8 months for pubic hair development. The change from stage 4 to stage 5 took approximately 1.4 years for both genital and pubic hair development. Overall, boys took approximately 4½ years and 3½ years to go from sexual maturity stage 2 (beginning of puberty) to sexual maturity stage 5 (full development) for genital and pubic hair, respectively ().
Mean Number of Years Between Sexual Maturity Stages Among Boys
At the onset of puberty, most girls (66.2%) were estimated to be at sexual maturity stage 2 for breast development at least 4 months before stage 2 for pubic hair development (ie, breast-first pattern) (). At later maturity stages, girls’ development tended to become more synchronous, with 51.4% estimated to be in sexual maturity stage 5 for breast and pubic hair development within approximately 4 months of each other.
Developmental Pattern by Sexual Maturity Stage
At the onset of puberty, most boys (91.1%) were estimated to be in sexual maturity stage 2 for genital development at least 4 months before stage 2 for pubic hair development (ie, genital-first pattern) (). At later maturity stages, boys’ development remained asynchronous, with 74.7% still showing the genital-first pattern for being in sexual maturity stage 5.
RACIAL DIFFERENCES IN PUBERTAL DEVELOPMENT
On average, black girls were in each sexual maturity stage for breast and for pubic hair development approximately 9 months earlier than white girls () (F1,430 = 72.05; P <.001). The other relationships described regarding differences in ages of being in each stage for breast and pubic hair development and the length of time between stages showed no significant relations with race. These findings remained the same when controlling for family income-to-needs ratio (measured at age 9½ years) and years of maternal education at the time of participants’ birth. Only one interaction between puberty characteristic and family income-to-needs ratio was significant (F1,393 = 8.33; P=.004) and indicated that girls from higher-income homes experienced later breast development than did girls from lower-income homes.
Similar to black and white girls, black boys were in each stage of genital and pubic hair development earlier than white boys () (F1,425=89.07; P <.001). The discrepancy between ages that black and white boys were in each stage was greater for earlier stages (range, 9.6 months-1.0 years earlier for stage 2) than it was for later stages (7.2 months earlier for stage 5) (F3,1275 = 7.36; P<.001). With demographic factors controlled, these results remained unchanged.