When engaged in conversation, both parents and children tend to re-use words that their partner has just said. This study explored whether proportions of maternal and/or child utterances that overlapped in content with what their partner had just said contributed to growth in mean length of utterance (MLU), developmental sentence score, and vocabulary diversity over time. We analyzed the New England longitudinal corpus from the CHILDES database, comprising transcripts of mother-child conversations at 14, 20, and 32 months, using the CHIP command to compute proportions of utterances with overlapping content. Rates of maternal overlap, but not child overlap, at earlier time-points predicted child language outcomes at later time-points, after controlling for earlier child MLU. We suggest that maternal overlap plays a formative role in child language development by providing content that is immediately relevant to what the child has in mind.