The last decade was the period of increase in number and proportion of extramarital births in Poland. At the end of 1980s only 6% and in 2001 13.1% of all births was ascribed to unmarried mothers. Analyses presented in the article indicate that this development should be explained using the framework of the economic fertility theories. Majority of unmarried mothers are young and low-educated. There are no evidence to confirm thesis that the increase in number of out-of-wedlock births in Poland is related to dissemination of cohabiting unions. Low wages, high level of unemployment encourage the young people living in the L A T unions to postpone their nuptial decisions well after the delivery of their first child. They save expenses related to religious or civil ceremony, and enjoy support given to them by their own parents. As result majority of extramarital births transform under advantageous conditions (stabile work, good wage) into pre-marital births.