- Background: Alteration of diastolic function is considered a sensitive means for detecting changes in the normal cardiac adaptation to pregnancy. Our aim was to evaluate volumetric and functional atrial parameters, using real time three-dimensional echocardiography (RT3DE) in women in early and late third trimester of pregnancy. Methods: We studied pregnant women in early third trimester (III-E = gestational age 26–33 weeks), in late third trimester (III-L = gestational age 34–40 weeks), and control nonpregnant women (C). Two-dimensional (2D-Echo) and RT3DE were used to study 3D left atrial (LA) systolic and diastolic stroke volumes and index (LASVI, LAEDVI), emptying fraction, left ventricular and LA cavities. Results: Although the LA end systolic volume index increased significantly (from 19.42 ± 0.1 to 24.7 ± 3.5 mm2, P < 0.01), the EF did not change significantly. This was mainly achieved by increasing atrial contraction (A-wave), maintaining cardiac output by increasing heart rate. A decrease in diastolic E-wave, increased atrial kick (A-wave) with reduced E/A ratio, was noted as the pregnancy progressed. Pulmonary pressure increased from 16.9 ± 6.6 to 20.5 ± 2.9 mmHg (P < 0.01), Using 2D-Echo revealed no change in LA diameter from control to III-E and III-L, respectively (from 17.1 ± 2.3 to 16.7 ± 2.6, 17.5 ± 2.2 mm) and area (from 11.7 ± 3.1 to 16.5 ± 2.3, 17.6 ± 1.6 cm2). However, using RT3DE, a significant increase in the LASVI, LAEDVI, and LA stroke volume index (from 12.02 ± 2.5 to 14.7 ± 3.2, and 15.1 ± 2.7 mL/m2) was detected. Conclusions: Enlargement of the LA volume with unchanged blood pressure values, as found using RT3DE, may be part of the adaptation to increased blood volume during pregnancy.