en Health Services, Quality of Life and Outcomes Original/Research Article <div style="text-align: justify;">An inverse association between fertility rates and breast cancer incidence has been observed at the national and global levels. However, few studies have examined this relationship using subnational data. We aimed to determine the correlation between total fertility rates (TFR) and breast cancer incidence across cities in Iran&#39;s Razavi Khorasan province between 2014 and 2019. This ecological study utilized age-standardized breast cancer incidence rates and TFRs for 28 regions of Razavi Khorasan province from 2014 to 2019. Pearson correlation coefficients assessed the TFR-incidence correlation annually. Annual percent changes (APCs) in incidence rates were estimated using linear mixed models, unadjusted and adjusted for region-level TFRs. TFR declined province-wide, with the steepest decreases in Bakharz (-45%), Torbat Jaam (-39%), and Torghab & Shandiz (-35%). Breast cancer incidence patterns were heterogeneous across cities, with significantly declining APCs in 7 regions, including Bajestan (-9.8%), Gonabad (-8.1%), and Torghab & Shandiz (-8.3%). Fertility adjustment had minimal impact on APCs. No significant correlation was observed between TFRs and incidence rates from 2014 to 2019 (correlation coefficients -0.193 to 0.193, p>0.05). In contrast to national and global trends, declining fertility rates did not correlate with increasing breast cancer incidence across Razavi Khorasan cities from 2014 to 2019. Regional differences may modify the impacts of changing fertility patterns on breast cancer occurrence. Further investigations into this relationship are warranted using robust analytical methods.</div> ,Breast cancer, Fertility rates, Ecological study 0 0 http://mcijournal.com/browse.php?a_code=A-10-690-1&slc_lang=en&sid=1 Ensieh Tavana TavanaE4012@mums.ac.ir 10031947532846004085 10031947532846004085 No Department of Epidemiology, School of Health, Mashhad University of Medical Sciences, Mashhad, Iran Masoumeh Andish AndishM4012@mums.ac.ir 10031947532846004086 10031947532846004086 No Department of Epidemiology, School of Health, Mashhad University of Medical Sciences, Mashhad, Iran Samaneh Eslami EslamiS4012@mums.ac.ir 10031947532846004087 10031947532846004087 No Department of Epidemiology, School of Health, Mashhad University of Medical Sciences, Mashhad, Iran Vajiheh Shayesteh ShayestehV1@mums.ac.ir 10031947532846004088 0009-0004-5111-0450 No Health Vice-Chancellor of Mashhad University of Medical Sciences, Population Youth Management, Family and School Health, Mashhad, Iran Toktam Parvish farkhanie@mums.ac.ir 10031947532846004089 10031947532846004089 No Economics and Development, Ferdowsi University of Mashhad, Khorasan, Mashhad, Iran Ehsan Mosa Farkhani emf1358@yahoo.com 10031947532846004090 10031947532846004090 Yes Department of Epidemiology, School of Health, Mashhad University of Medical Sciences, Mashhad, Iran