The main purpose of this paper is to analyze the birth rates and nuptiality trends in Croatia and Slovenia in the period between 1985 and 2015. The paper primarily focuses on changes in total fertility rate, differences between the mean age of woman at first marriage and at first childbirth, births in marriage and outside of marriage. The causes of analyzed demographic trends have been explained, followed by statistical data with charts and tables, based on the Croatian and Slovenian bureau of statistical data. The results have shown negative demographic trends which are reflected in a decline in population, reduction in the number of marriages, and increase of more and more common late marriages. Also, both countries are facing a low birth rate that also shifts towards a later age. In the comparison of the differences between the mean age of woman at first marriage and the age at first childbirth, Slovenia shows a more liberal attitude to marriage then Croatia which has completely different features. The mentioned difference in nuptiality can also be proved by the proportion of children born inside and outside of marriage. When comparing the mentioned trends with the European Union, Slovenia has, in certain trends, already reached, if not crossed the level of the European Union. On the other hand, Croatia is lagging behind Slovenia and European Union. Nevertheless, according to the changes in the demographic trends in natality and nuptiality in the last observed 10 years, it can be sensed that Croatia will reach, or at least approach that level.