New Norwegian reference for puberty development
The Bergen Growth Study 2 is the first study in Norway where the purpose is to establish references for puberty development. Almost 1200 healthy children and adolescents aged 6–16 were recruited from six different schools in the Bergen area in 2016. Female puberty was evaluated by breast development, which was assessed both clinically and with ultrasound, and through direct questions about the occurrence of menarche. For male puberty we used ultrasound to calculate testicular volume. Tanner staging of pubic hair was assessed for both sexes. In addition, several anthropometric measurements were recorded, and blood samples were taken.
Our results may indicate that puberty starts earlier in Norway now than 10–15 years ago
The Tanner assessment of breast development is usually performed by clinical palpation and inspection, and is especially challenging in girls who are overweight or obese. Ultrasound can distinguish adipose tissue from mammary gland tissue and may therefore be useful for assessing puberty in girls who are overweight (9). We identified six different ultrasound stages of breast development based on the breast morphology and composition of glandular, adipose and connective tissue (10). Puberty references from ultrasound examinations of the breast have never previously been published. For the boys, the use of ultrasound allows for a more direct measurement of testicular volume, without added volume from surrounding structures such as the epididymis, or from pathology such as a hydrocele. Testicular volume was calculated as an ellipsoid based on length, width and depth measurements (11).
Data from the Bergen Growth Study 2 showed that the average age for onset of puberty in girls was 10.4 years (Tanner stage B2), which is comparable with other northern European studies (12). With regards to menarche timing, we observed a significantly decreased mean age at occurrence for the entire cohort from 13.3 years in the first growth study (2003–06) to 12.9 years (2016). For the girls of Norwegian origin, we also found a small but significant decrease corresponding to 2.8 months. Our results thus indicate that puberty starts earlier in Norway now than 10–15 years ago. This finding should be followed with future studies. For the boys, modelling of testicular volume for age was done to construct the references (13). This is the first published growth curve for testicular volume from Norway and shows that the average age at onset of puberty is 11.7 years among healthy boys. Testosterone levels started to increase at the same age (14). This is comparable to the onset of puberty at 11.6 years observed in Dutch boys (measured using ultrasound in 2007–09) (15) and 11.7 years in Danish boys (measured using a Prader orchidometer in 2006) (4).
Early menarche is associated with a higher risk of breast cancer, cardiovascular disease, mental illness and increased mortality
Normal age for puberty onset is defined within ±2 standard deviation of the mean. Ages outside this range should be considered as early or late puberty. Based on our findings from the Bergen Growth Study 2, we recommend maintaining the current threshold values set out in the paediatrics manual of 8–13.5 years for girls and 9–14.5 years for boys (16). The current growth charts used in Norway contain a panel for puberty references from Denmark (1991–93) which are set to be replaced with our new Norwegian references. No major changes will incur for male puberty, but references for female puberty will reflect the current and somewhat earlier onset of puberty observed in Norway.