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Background: Delays and loss of early‐emerging social‐communication skills are often discussed as unique to autism. However, most studies of regression have relied on retrospective recall and clinical samples. Here, we examine attainment and loss of social‐communication skills in the population‐based Norwegian Mother, Father and Child Cohort Study (MoBa). Methods: Mothers rated their child's attainment of 10 early‐emerging social‐communication skills at ages 18 and 36 months (N = 40,613, 50.9% male). Prospectively reported loss was defined as skill presence at 18 months but absence at 36 months. At 36 months, mothers also recalled whether the child had lost social‐communication skills. The Norwegian Patient Registry was used to capture diagnoses of Autism Spectrum Disorder (autism) and other neurodevelopmental disabilities (NDDs). Results: Delay in at least one skill was observed in 14% of the sample and loss in 5.4%. Recalled loss of social‐communication skills was rare (0.86%) and showed low convergence with prospectively reported loss. Delay and especially loss were associated with elevated odds of an autism diagnosis (n = 383) versus no autism diagnosis (n = 40,230; ≥3 skills delayed: OR = 7.09[4.15,12.11]; ≥3 skills lost: OR = 30.66[17.30,54.33]). They were also associated with an increased likelihood of autism compared to some other NDDs. Delay (relative risk [RR] = 4.16[2.08, 8.33]) and loss (RR = 10.00[3.70, 25.00]) associated with increased likelihood of autism versus ADHD, and loss (RR = 4.35[1.28,14.29]), but not delay (RR = 2.00[0.78,5.26]), associated with increased likelihood of autism compared to language disability. Conversely, delay conferred decreased likelihood of autism versus intellectual disability (RR = 0.11[0.06,0.21]), and loss was not reliably associated with likelihood of autism versus intellectual disability (RR = 1.89[0.44,8.33]). Conclusions: This population‐based study suggests that loss of early social communication skills is more common than studies using retrospective reports have indicated and is observed across several NDD diagnoses (not just autism). Nevertheless, most children with NDD diagnoses showed no reported delay or loss in these prospectively measured skills. [ABSTRACT FROM AUTHOR]