Play of younger siblings of children with autism spectrum disorder (ASD) (siblings; n = 44), very preterm children (preterms; n = 44), and children at typical likelihood for ASD (n = 36) was observed at 24 months. Children with ASD and atypical development engaged less in spontaneous (pre-)symbolic play than typically developing children. Total duration of spontaneous and elicited (pre-)symbolic play was associated with later ASD traits in siblings. However, no association between most play variables and ASD traits was found in preterms. This suggests possible different ASD-trajectories between siblings and preterms. Thus, spontaneous (pre-)symbolic play may be indicative of developmental challenges across several populations, and results highlight the need to move beyond studying only siblings in order to broaden our understanding of ASD. [This article was written with the Baby Study Team: Chloè Bontinck, Eva Bruyneel, Marjolein Muès and Liedewij Verhaeghe.]