Objective Because fetal respiratory movements increase left ventricular compliance, we hypothesized that the left atrial shortening fraction increases during fetal respiratory motions. Methods A group of 26 normal fetuses with gestational ages between 28 and 38weeks were assessed in a prospective cross-sectional study. Left atrial telesystolic and presystolic diameters were measured during apnea and after five consecutive respiratory movements. Left atrial shortening fraction was obtained by the ratio: [maximal left atrium diameter (telesystolic)– minimal left atrium diameter (presystolic)]/maximal left atrium diameter (telesystolic). The mean of three measurements were considered. Two-tailed Student’st-test was used. Results Mean gestational age was (mean SD) 30.7 2.8weeks. Mean left atrial telesystolic diameter in apnea was 10.6 0.7mm and during respiratory movements it was 10.5 1.1mm (p=0.98). Presystolic left atrial diameter was 5.2 0.1mm in apnea and 4.4 1.3mm during respiratory movements (p<0.001). Left atrial shortening fraction was 0.50 0.05 in apnea and 0.58 0.13 during respiratory movements (p<0.001). Conclusion Left atrial shortening fraction is higher during respiratory movements as a result of increased left ventricular compliance and consequent optimization of left atrial functional status.