The transformation of reproductive behaviour in Slovakia after 1989 brought several historically unique and dynamically ongoing changes. One of them is the increase in the number and share of children born out of wedlock. In less than three decades, their representation has increased from less than a tenth to 40%. The historically valid model of reproduction realized almost exclusively in marriage received serious cracks, and Slovak society had to face this important pluralizing factor in reproductive and family behaviour. However, this is in contrasts with the almost minimal scientific interest in this issue. Therefore, the main goal of the paper is to try to analyse some selected aspects associated with the process of extramarital fertility and birth out of wedlock. On the one hand, we tried to point out some major developmental changes on the other hand we tried to identify some differences that could be associated with more frequent births of children of unmarried women, as well as to identify some internal demographic factors behind the increase in the share of illegitimate children since the early 90s. As our findings show, the youngest women, Roma women and people with low education have a higher chance of having a children out of wedlock in Slovakia. In terms of space, this phenomenon is more often present in women from medium-sized and small settlements, somewhat more often from the rural environment and districts of southern central Slovakia. The main demographic factor behind the increase in the proportion of children born out of wedlock is the growth in the number and proportion of unmarried women, and to a lesser extent, the growth of extramarital fertility.