Background: Engaging in physical activity (PA) and reducing sedentary behaviour (SB) are important for health during pregnancy, yet relatively few studies have examined these variables using objective measures and across trimesters during pregnancy.Objective: To determine the amount of objectively assessed PA and SB engaged in whether there was any weekday to weekend day variation in PA and SB during the first and second trimester of pregnancy. Method: PA and SB were determined using accelerometry worn over 7 consecutive days during each trimester in 137 Portuguese females (mean age ± SD = 29.6 ± 5.7). Results: In regard to the proportion of participants meeting the ACSM guidelines for PA, 37.5% of the participants in the first trimester and 29.6% of participants in the second trimester met the cut off of 30min or more of any type of moderate intensity activity on most (5) days of the week. Moderate intensity PA was significantly lower in trimester 2 compared to trimester 1 (P = 0.003). Moderate intensity PA was also significantly lower during weekends compared to weekdays irrespective of trimester (P = 0.003). SB, light and vigorous intensity PA were relatively stable from trimester 1 to trimester 2 and between weekdays and weekends (P < 0.05). Conclusion: The present study suggests that the majority of women do not meet PA guidelines for health during pregnancy and that moderate intensity PA declines form trimester 1 to 2 and is lower at weekends.