A New Method for Estimating Menarcheal Status from the Fusion of the Spheno-Occipital Synchondrosis, and a Test of the Status Quo Approach for Estimating Median Age at Menarche in Archaeological Populations

Pubertal development is increasingly being investigated in bioarchaeology but estimating specific maturational events such as menarche remains challenging. This study addresses two related objectives: the first is developing an osteological method for estimating individual-level menarcheal status, and the second is adapting the status quo approach to estimate population-level median age at menarche. Three-dimensional cone-beam (3D-CBCT) data from 175 females aged 8–16 years with documented menarcheal status were used to model the relationship between spheno-occipital synchondrosis (SOS) fusion and the probability of being post-menarcheal. A modified 3-stage fusion SOS scheme, designed for dry bone application, achieved 84.0% overall classification accuracy, with 75.6% specificity and 86.6% sensitivity. The probability of being post-menarcheal increased from 0.367 for an unfused SOS to 0.914 once fusion was externally visible. The method was subsequently applied to the Lisbon documented skeletal collection, and SOS-estimated menarcheal status was combined with documented or estimated age and sex using logistic regression to estimate median age at menarche. Across six analytical combinations, estimated medians ranged from 13.8 to 14.4 years. Using documented age and sex produced a median of 13.8 years, closely corresponding to independent historical evidence of approximately 14 years for Lisbon females of comparable chronology. These findings indicate that SOS fusion provides a probabilistic indicator of individual menarcheal status and that individual classifications can be aggregated using the status quo approach to reconstruct population-level menarcheal timing. Further testing in independent populations is required to establish the broader applicability of both components of the approach.