As cohabitation becomes increasingly common and accepted, one might expect the meaning of this arrangement to change. In some countries in Europe (e.g., Sweden), as cohabitation became more prevalent, it moved from a deviant status to an acceptable alternative to marriage. Will the same thing happen in the United States? To investigate this question, I examine increases in the proportion of births occurring in cohabiting unions, using data from the 1987-1988 National Survey of Families and Households (NSFH) and the 1995 National Survey of Family Growth (NSFG). The standardization and decomposition procedure shows that most of the growth in the proportion of births to cohabitors is the result of increases in the proportion of women cohabiting, rather than changes in union formation behaviors surrounding pregnancies.