As government-funded universal programs for new parents continue to expand, it is critical to investigate their short- and longer-term effects. The level of knowledge, especially on the effect of group-based interventions, is sparse. This study is the second report from a large trial of a widely implemented universal group-based parenting program in Denmark and includes outcomes on child socio-emotional problems and use of health services. A sample of 1701 unborn children from families representative within the area, were randomized to intervention or care as usual beginning November 2014. From these families, 1268 mothers and 999 partners (mean age 29.4 and 31.19, respectively; 6% migration background) filled in the Ages and Stages Questionnaire. Intention-to-treat-analyses showed fewer socio-emotional problems in the intervention group (between-group difference - 2.34, 95% CI [0.40, 4.30], d = - 0.13) as reported by mothers, but not partners, when children were 10 months old. This finding was transitory (no longer significant at 19 months). No effects were found on health care utilization. The previous report from this trial showed no effects on measures of parenting. In total, these findings serve the purpose of adjusting future expectations to the size and type of effect to be gained from a universal parent program in a resourceful setting. We conclude that even if the Family Startup Program (DK: Familieiværksætterne) 2 was liked, well implemented, and well attended by the parents, the positive effect on children's socio-emotional problems, was too modest to be the argument that can carry the weight of policy going forward. Public significance statementThis study examined the effects of the Family Startup Program (DK: Familieiværksætterne), a universal, group-based program to support parents during the transition to parenthood. Results showed a modest transitory preventive effect on children's socio-emotional problems and no effects on health care utilization. These findings adjust down previous expectations to the size and type of effects to be gained from universal group-based parent support in a well-resourced setting.