Objective. To describe the background characteristics of women who gave birth to their first child at an advanced and very advanced maternal age, including their sociodemographic background, social relationships, health behavior, physical and mental health, and reproductive history. Design. Cross-sectional data from the Norwegian Mother and Child Cohort Study (MoBa) conducted by the Norwegian Institute of Public Health. Setting. Norway. Sample. 41 236 Norwegian-speaking nulliparous women. Methods. Data were collected by the first questionnaire distributed in week 17 of pregnancy during the recruitment period 1999–2008. The distribution of descriptive variables in relation to age was investigated, by means of bivariate and multivariate logistic regression analyses. Main outcome measures. Advanced (33–37 years) and very advanced (≥38 years) maternal age. Results. Women who had their first baby at an advanced or very advanced age differed from the younger women with regard to a wide range of background characteristics, and this difference was most pronounced for the very advanced group. Problems related to physical aging were more common (infertility, physical health problems, sleep problems, depression and fatigue). Of the sociodemographic factors; high annual income and low level of education were most strongly correlated with high maternal age, followed by single status, unemployment, unsatisfactory relationship with partner and unplanned pregnancy. Conclusions. Besides having more age-related reproductive and physical health problems, women who had their first baby at an advanced or very advanced age constituted a heterogeneous group characterized by either socioeconomic prosperity or vulnerability.