The article studies the processes of the birth rate of the Kazakh population in cities. The authors demonstrate the research capabilities of various methods and tools - the Pearson correlation coefficient, the clustering method based on a set of interrelated features and the method of analyzing the amplitude between values-extremes. As a result of the use of these tools and logical comparison of the results, a number of conclusions were made: a stable correlation between the specific weight of the Kazakh population in the cities and the total fertility rate of the republic was revealed; statistically, a high polarization of reproductive processes in the regions of Kazakhstan with a tendency of its increase was recorded. Based on the proven relationship between the ethnic structure of the urban population and total fertility rate, the regions were ranked and a four-cluster classification was compiled according to two demographic indicators. This allowed, on the one hand, to maintain a systematic view of the unified demographic system of Kazakhstan, on the other – to identify its internal imbalance. Using the method of extremes, a new demographic trend was revealed: an increase in regional differences in fertility rates (from a value of 2.64 in 2009 to the value of 2.71 in 2019) despite a gradual alignment of the specific weight of the city of the Kazakh population in different regions of the country (regional amplitude of the oscillations decreases 76.1% in 2009 to 64.1% in 2019). The polarization of reproductive trends and the choice of reproductive scenarios are determined not so much by ethnicity as by the value value that is invested in ethnic identity in different regions of the country.