There have been many studies examining the differences between infant-directed speech (IDS) and adult-directed speech (ADS). However, very few longitudinal studies exist that explore how patterns of maternal vowel articulation in IDS change as children get older, or whether these changes have any effect on a child's developing language skills. This study examines the vowel clarification of mothers' IDS at 10-11 months, 18 months, and 24 months, as compared to their vowel production in ADS. Relationships between vowel space, vowel duration, and vowel variability and child language outcomes at 2 years are also explored. Results show that vowel space and vowel duration tend to be greater in IDS than in ADS, and that a mother's vowel space at 18 months is significantly related to expressive and receptive child language outcomes at 2 years. Possible explanations are discussed.