This study considers how differences between adult-directed-speech (ADS) and child-directed speech (CDS) at 11 and 24 months affect child vocabulary development. The effects on child vocabulary development of 1) the size and stability of simplification in MLU and VOCD between ADS and CDS, 2) proportions of rare words, 3) one-word utterances and 4) nouns in CDS on toddler vocabularies are considered. Mothers' MLU and VOCD in CDS were stable, but did not relate to children's vocabulary growth, while other input factors were related to child outcomes, but were not stable. Results provide no evidence of an upper limit to beneficial complexity in CDS and do not support replacing time-lagged with concurrent measures in research.