Objective: to analyze if social origin and certain sociodemographic characteristics such as: education level, having a job and being a mother, influence in two cohorts of women in Mexico to postpone their entry into their first union. Data source: Retrospective Demographic Survey of 2017. Methodology: descriptive and multivariate event history techniques were used. In multivariate analysis, two event history models were adjusted, one for women from the oldest cohort and one for the youngest. Results: Women born between 1978 and 1987 delayed their calendar compared to those born between 1962 and 1967. The proportion of occurrences of the first union at age 30 has changed according to the birth cohort, with younger women showing lower percentages of a first marriage at this age. Women from the younger cohort who are highly educated do not have children and are from a higher stratum, they are less likely to join. Conclusions: the calendar of the union in Mexico is heterogeneous and is accentuated by the new generations and the socioeconomic stratum to which they belong, in addition to factors such as educational level and pre-union fertility.