Figure 1.

Age reference curves. Age distribution of pubertal development in 6- to 16-year-old girls living in Norway. A: Stages of breast development with ultrasound (n = 696); B: Breast development stage by Tanner stage (n = 700); C: Pubic hair (PH) according to Tanner (n = 372); D: Menarche (n = 643). Black lines show the parametric probit model, assuming a Gaussian distribution of the age at transition; grey lines show the corresponding curves of the nonparametric generalized additive model with a probit link; connected dots show the cumulative proportion of girls who have reached a particular pubertal stage in overlapping one-year intervals.