Pregnancy is an important time in women’s lives that may place them at greater risk for decreased physical activity (USDHHS, 2000). For example, nearly 60% of women are sedentary during their pregnancy (Zhang & Savitz, 1996). Thus, to promote exercising during pregnancy, it is necessary to examine the psychosocial and cognitive determinants of women’s exercise behavior. However, the literature examining exercise determinants during pregnancy is scant, and it is limited by small samples, mostly cross-sectional designs, and a lack of theoretical research (Koniak-Griffin, 1994; Walker et al., 1999). Therefore, the general objective of this study was to prospectively examine pregnant women’s exercise intention and behavior from their second to their third trimester using the framework of the theory of planned behavior (TPB; Ajzen, 1991). Participants were 81 pregnant women (M age=29.93 years, SD=4.52) who completed a mail questionnaire during their second trimester assessing the TPB constructs (i.e., attitude, subjective norm, perceived behavioral control [PBC], intention, and behavior). Three months later, participants were mailed another questionnaire assessing their exercise behavior. To examine the predictive utility of the TPB, two hierarchical regression analyses were conducted and the variables were entered according to the theoretical tenets of the TPB. In the first regression, intention and PBC explained 43% of the variance in exercise behavior, F (2,72)=26.23, p < .001. Intention (b=.53, p < .001) significantly predicted exercise behavior. In the second regression, attitude and subjective norm (Block 1) explained 26% of the variance in intention, F (2,72)=12.47, p < .001. PBC (Block 2) explained an additional 5% of the variance, Fchange (1,71)=5.32, p < .05), with attitude (b=.26, p < .05) and PBC (b=.31, p < .05) maintaining their unique contributions, and PBC was the strongest predictor of intention. These findings are consistent with previous research, and they suggest that women’s intention to exercise is the strongest determinant of whether they will exercise during their pregnancy. In addition, women’s perception of their control over the factors obstructing their exercise (e.g., vomiting, nausea, fatigue) most strongly determined their intention. Researchers are encouraged to tailor their studies to meet the thoughts, needs, and beliefs of pregnant women in an attempt to increase their exercise behavior during this time.