Abstract: The subject of this research is the illegitimate fertility of population, which represents one of the urgent demographic problems for modern world and is relevant for urban and rural areas of the Sakha Republic (Yakutia). The goal of this article lied in determining the scale and rate of spreading of illegitimate fertility, portion of various age groups therein, describing the objective and subjective factors that lead women to giving birth out of wedlock. Accent is made on the analysis of incentives towards making a decision to give birth to a child out of wedlock. The information base contains the data of the current statistics of birthrate considering the place of residence and age of a mother, two latest Russian censuses, as well as materials of sociological surveys conducted by the authors among the population of the republic in 2008 and 2017 in form of the focus group interview. The authors determine the direction and scale of the changes in birthrate, including birth out of wedlock by urban and rural areas from 2001 to 2016. There has been a decrease in the portion of young mothers under 25 and an increase in fertility out of wedlock in the older birth cohorts. It is revealed that women feel more independent with age in realization of their reproductive plans, as well as less committed to adhering to social disapproval even in rural areas with predominant traditional model of matrimonial-family relation. A conclusion is made that social outlook on the phenomenon of illegitimate fertility is tolerant regardless of the place of residence of the respondents. At the same time, it is recognized that the socio-demographic consequences of this phenomenon are multifaceted and cause substantiated worry for the future of the institution of family and marriage.