In Chile, as in most societies, women have historically taken on most parenting responsibilities, and not much research has been conducted into how couples manage the arrival of their first baby. The present study aimed to describe the main processes involved in the construction of co-parenting during the transition to parenthood in heterosexual couples. Through a purposive sampling procedure focused on key informants, 7 couples living together in Santiago de Chile were contacted and then interviewed during their first pregnancy and postpartum. The transcripts were analyzed following thematic analysis guidelines. Most couples had not planned their pregnancies, which caused them to experience the process with a high degree of ambivalence. Roles within the couple were distributed in a traditional way from the pregnancy onward. Conflict within the couple was evaded or naturalized, being attributed to hormonal changes and the baby's demands, with little space being available for reflection and discussion. It is concluded that co-parenting is built on the basis of gender differences between men and women, with parents establishing an asymmetric relationship that is neither made explicit nor questioned. [ABSTRACT FROM AUTHOR]