Prevalence of Premature Thelarche at 18 Months of Age: A Population- and Hospital-Based Study of Prevalence and Incidence in Girls Born at Northern Älvsborg County Hospital in Sweden

Objective: The aim of this work was to investigate the prevalence of premature thelarche (PT) in 18-month-old girls, and the incidence of clinically evaluated PT for girls aged 18–36 months. Methods: In the prevalence substudy, a prospective population-based cohort of 3,140 girls born at Northern Älvsborg county hospital (NÄL) in Trollhättan, Sweden, was followed for 2 years. Girls with breast development at the 18-month health check were referred to one pediatric center in NÄL for evaluation. All girls with PT were included and followed for clinical outcome and 17β-estradiol. The prospective incidence substudy covered 8 years in a 10-year period and included all girls aged 18–36 months born at NÄL who were clinically evaluated for PT. Results: The prevalence of PT at 18 months in our cohort was 1.6/1,000. The 5 girls with PT no longer showed symptoms at the follow-up 3–6 months later. The incidence was 1.1/1,000 for girls aged 18–36 months and 1.0/1,000 for girls aged 18–30 months who were clinically evaluated for their PT. Conclusion: This is the first prospective population-based study of PT and it shows a prevalence of PT at age 18 months of 1.6/1,000. The incidence of clinically evaluated PT was 1.1/1,000. Our result is in line with other studies reporting the incidence of PT from medical records (0.4–40/1,000). The outcome of PT in our study, as in the other studies, is that the great majority of girls show only benign symptoms.