Finding determinants of demographic processes is a highly topical issue in countries with negative demographic trends. Our research was aimed at studying the relationships between fertility and income indicators in Russia. The period under review was 2000 to 2016. To explore the correlation between the time series, we used the methodology of estimating trend deviation. We applied analytical smoothing to model trends, estimating regression models. To assess the strength of relationship between the time series, we analysed correlation between regressions' residuals. The results of our analysis showed no relationship between people's incomes and fertility rates. The research we carried out into time series dynamics did not confirm the results of other studies based on static data. Accordingly, this raises questions about the methodology for analyzing the relationship between dynamic processes with a high volatility of input data. Evidently to receive reliable and stable results, multidimensional analysis methods should be integrated into the study of relationships between dynamic time series, including preliminary multi-dimensional data classification. This will enable carrying out analysis on homogenous territorial or temporal segments, which would be more methodologically sound.