To evaluate changes in the amniotic fluid index (AFI) in low-risk pregnant women before and after physical activity in water. This was a prospective trial with a before-after approach. Women carried out moderate physical activity for 50 min, 3 times a week, while partially immersed in a swimming pool at 30 degrees C. Women with 2 or more previous cesarean sections, with a high-risk pregnancy or medical contra-indications for physical activity were excluded. They were evaluated weekly by ultrasonography, from 20 weeks of pregnancy until delivery, to evaluate AFI before and after physical activity in water. A second observer also performed the AFI measurements, in order to establish inter-observer variability. Analysis was performed using Student's t-test or Wilcoxon tests. The linear correlation coefficient was used to assess inter-observer variability. A total of 25 pregnant women, 19-36 years of age, participated in the study between May 2003 and December 2004. A total of 232 ultrasonographic evaluations of AFI were carried out, a mean of 9.28 examinations per woman. Statistically significant increases in AFI were found following immersion at almost every week of pregnancy. The increase in AFI post-immersion compared to pre-immersion values ranged from 8.8 to 21.5%. There was good inter-observer agreement. The correlation coefficient for the inter-observer variability was 0.78 for pre-immersion measurements, and 0.70 for post-immersion measurements. Physical activity in water appears to significantly increase AFI. Since this is a non-invasive therapy, we speculate that its clinical application may have significant value.86547-52