Purpose: The aims of this study were to explore expressive vocabulary growth rates of children who are deaf or hard of hearing (DHH) during critical periods of brain development (birth to 3 years) as well as the factors that influence the trajectories of vocabulary growth in these early years of development. Of primary interest was the effect of intervention frequency on expressive vocabulary growth. Method: Hierarchical linear modeling was used to investigate trajectories of expressive vocabulary growth using multiple measures of longitudinal vocabulary scores. A total of 417 assessments across 105 participants were analyzed to determine the average rate of lexical growth in a young population of children who are DHH receiving early intervention before age 3 years. Expected growth trajectories were constructed based on varying frequencies of intervention during critical periods of brain development. Results: Results indicated average growth rates of 5.21 new words expressed per week. Increased intervention hours prior to age 3 years was significantly associated with higher degrees of expressive vocabulary growth. Conclusion: The findings of this study suggest that greater intervention hours received before age 3 years are associated with higher degrees of expressive vocabulary growth for children who are DHH.