All couples who registered cohabitation in the year 2000 were selected, regardless of whether they first entered marriage. Same-sex partners were subsequently excluded (remaining number of couples = 180 thousand), because the demographic dynamics within the latter group requires a specific analysis. For example, the transition to parenthood may involve a third person (as the biological parent) among homosexual couples. Secondly, only those persons were selected who were most likely to have a first child in terms of their age. This applied to persons with a minimum age of 18 years and a maximum age of 40 (female partner) or 45 (male partner) when they started cohabiting (remaining number of couples = 146 thousand). Only couples were selected of whom both partners had never cohabited previously (whether married or unmarried) and were childless (remaining number of couples = 63 thousand). Relationships starting and ending within the same month were excluded from the analyses. Finally, couples who entered into marriage at least one year prior to the start of cohabitation were excluded (remaining number of couples = 59 thousand).