Autism spectrum disorder (ASD) diagnosis relies on parent-reported and clinician-observed instruments. Sometimes, results between these instruments disagree. The broader autism phenotype (BAP) in parent-reporters may be associated with discordance. Study to Explore Early Development data (N = 712) were used to address whether mothers with BAP and children with ASD or non-ASD developmental disabilities were more likely than mothers without BAP to 'over-' or 'under-report' child ASD on ASD screeners or interviews compared with clinician observation or overall impression. Maternal BAP was associated with a child meeting thresholds on a maternal-reported screener or maternal interview when clinician ASD instruments or impressions did not (risk ratios: 1.30 to 2.85). Evidence suggests acknowledging and accounting for reporting discordances may be important when diagnosing ASD.