Disclosure: A.V. Baaske: None. S. Shirin: None. C. Bos: None. D. Kalidasan: None. J.C. Prior: None. Background: Tanner Breast Stage 3 (TB3, at menarche) and Tanner 5 (TB5, once ovulatory cycles established), despite being similar with an “upstanding” nipple, are different. Evidence suggests that steady and high ovulatory-level progesterone exposure accounts for the larger areolar diameter in TB5. Progesterone levels indicate whether the menstrual cycle is ovulatory and have crucial implications for women’s reproductive, metabolic, cardiovascular, and bone health. However, few data show average areolar diameter in regularly cycling women in whom SES, body size, and reproductive variables have been documented. Objective: As a step towards better understanding the role of areolar diameter in reflecting estradiol-progesterone balance, this study aims to document the average mean bilateral lateral areolar diameter (AD) in a cohort of regularly menstruating, premenopausal women in whom ovulation was documented in one cycle. We also aim to describe relationships between AD and demographic, anthropometric, and reproductive/ovulation variables. Methods: Women were eligible if aged 19-35 years with about month-apart cycles and no exogenous hormone use in the last 3-months. Participants completed one cycle with a daily Menstrual Cycle Diary© (cycle length, CL) and first morning temperatures for Quantitative Basal Temperature© (QBT©) ovulation analysis (normal luteal phase length ≥10 days). Questionnaires gathered SES, ethnic, general health, and reproductive characteristics. Bilateral horizontal areolar diameters were measured by a woman researcher with the participant supine in a warm room. Results: From February 2020 to September 2022 (during the SARS-CoV-2 pandemic), 73 women from Metro Vancouver, British Columbia participated. Women were of median (min-max) age 30 (19-35) years, BMI 24.5 (17.1-41.4) and most identified as White (61.6%) with a median of 16 years of education. Median CL was 29 days (n=65) and QBT data showed 39.3% were normally ovulatory, 34.4% had short luteal phases and 26.2% were anovulatory. Median within-woman bilateral AD was 3.85 (2.35-8.00) cm. Left AD exceeded right (L 3.8 [2.3,8.2]; R 3.7 [ 2.4,7.8]; P=.003). Larger AD (≥3.85cm) was associated with greater body size (BMI r=.534), weight and waist circumference. However, earlier age at menarche (P=.02) was also significantly associated. No other reproductive variables (cycle length, luteal phase length, parity, ever use of CHC) were related to AD before body size adjustments. Conclusions: This study is the first to document, and describe demographic, anthropomorphic and reproductive associations with, AD in community dwelling, regularly cycling, premenopausal women. Future steps will determine the most influential of the anthropometric variables; adjustment for these will facilitate assessments of associations with ovulation related characteristics. Presentation: Monday, July 14, 2025