Pubertal development in Norwegian girls

Background: Evidence suggests that girls worldwide are experiencing a decrease in age at pubertal onset, but data from Norwegian girls have not been available. Early pubertal timing is associated with adverse health outcomes. Pubertal breast development is currently categorized using the Tanner B scale and relies on visual inspection and/or palpation. Breast ultrasound appears to be a good alternative to this Tanner B assessment because it can separate adipose tissue from breast tissue and facilitates both staging and volumetric measurements. Objectives: The objectives were to explore ultrasound as an alternative method to assess breast development throughout puberty, to investigate the timing of puberty in Norwegian girls and construct pubertal references, and to shed light on the relationship between anthropometric indicators of body composition and the timing of puberty. Methods: This thesis is based on data from the Bergen Growth Study 1 (BGS1; 2003–2006, n = 1485, aged 8–15.5 years) and the Bergen Growth Study 2 (BGS2; 2016 and 2017, n = 703, aged 6–16 years). Data on menarche were recorded in both studies, while breast development was assessed only in the BGS2. Six distinct ultrasound-based breast stages were described, and the performance of these were evaluated in terms of observer agreement and in relation to the standardized Tanner B scale. Descriptive pubertal references were estimated from the BGS2. The associations between indirect (body mass index, BMI) and direct (skinfolds and waist circumference (WC)) measurements of fat with the timing of menarche were explored based on data from the BGS1. The ages at menarche between the studies were compared. Results: The ultrasound staging system performed well in terms of the agreement between one and two observers; however, direct measurements of size and volume lacked the necessary precision. When comparing ultrasound staging and Tanner B staging, an overall good agreement was found. A modest decline in the age at menarche was observed during the last decade, although pubertal references suggest that pubertal timing in Norway is similar to that of neighbouring countries. After adjustment for age and sex, both high and low levels of BMI, WC, subscapular skinfold (SSF) and triceps skinfold were all associated with an earlier and later menarche. In a fully adjusted analysis of all the measurements together, only a high BMI was related to earlier menarche, while a low BMI and a low SSF were associated with later menarche. Conclusions: Ultrasound is a feasible method to determine the breast developmental stage. This research has clinical implications for the evaluation of puberty in paediatric and adolescent female patients in Norway by providing descriptive references based on a healthy sample of girls. The decline in age at menarche was not explained by BMI and therefore warrants further investigations in population-based studies. BMI was the strongest anthropometric predictor of early and late menarche