Background <p>Although physical activity during pregnancy is recommended, many women reduce their activity levels due to misconceptions, cultural beliefs, and inadequate guidance. In Sri Lanka, limited evidence exists on physical activity patterns among pregnant women, particularly primigravida mothers. Therefore, this study aimed to assess the level of physical activity during pregnancy among primigravida mothers attending an antenatal clinic at a tertiary care hospital in Sri Lanka.</p> Methods <p>A descriptive cross-sectional study was conducted in the ante-natal clinic from September’2019 to March’2020. Primigravida mothers who were in first (n₁=181), second (n₂=225) and third (n₃=178) trimesters were recruited (<i>N</i> = 584) using systematic random sampling. Data collection was done through a self-administered pregnancy physical activity Questionnaire (PPAQ) and analyzed using SPSS version 25.0. Pearson chi-square tests were adopted in data analysis to find associations between socio-demographic data and PA level. Outcome variables compared among trimesters by Kruskal Wallis H test.</p> Results <p>Mean age of participants was 25.88 ± 3.18 years and their response rate were 96.4%. Mothers who did not follow international recommendation of PA were 19.3%, 29.3% and 32.0% for first, second and third trimesters, respectively. Statistically significant association was found in between PA level and the trimester (<i>p</i> = 0.015). Also, significant associations were observed in between PA level and the family type (<i>p</i> = 0.001), and the living area (<i>p</i> = 0.031) and the pre-pregnancy BMI (<i>p</i> = 0.016) in first trimester. Further, no significant association was present in between level of education and level of PA within any trimester (<i>p</i> &gt; 0.05). Mean duration for total activity and energy expenditure of pregnant mothers were not significantly differ among the trimesters (<i>p</i> &gt; 0.05). Most favored modes of PA were household/care giving and light intensity activity in each trimester.</p> Conclusions <p>Nearly one-quarter of pregnant mothers did not meet the recommended level of physical activity during pregnancy. The proportion of inactive mothers increased progressively with advancing trimester. Therefore, structured education programmes demonstrating safe and appropriate physical activities according to gestational age are recommended. Particular attention should be given to inactive mothers to enhance knowledge and promote motivation to achieve recommended physical activity levels.</p>