This study examined the effectiveness of the "Thrive by Three" intervention for 1- to 3-year-old's language development. Data from 78 childcare centres, 187 toddler classrooms, and 1561 children (91.4% native Norwegian) were included. Results revealed that children in the intervention group had slightly steeper language development than those in the control group, but the difference was not statistically significant. Since previous studies find language stimulation in childcare to differ based on gender, we also examined if the Thrive by Three intervention affected boys and girls differently. We found that effects of the intervention were only present for girls' language development. Girls in the intervention group had an increase of 17 more words from baseline to post-intervention than those in the control group. There was no statistical difference in change of boys' language development between the intervention and control group. Results are discussed in light of theories and literature that may explain our findings.