OBJECTIVE: To compare the quality of second trimester ultrasound images and their anatomical quality scores among obese women and those with a normal body mass index (BMI). MATERIALS AND METHODS: This prospective study, which took place from 2009 to 2011, included every obese pregnant woman (prepregnancy BMI greater than 30 kg/m(2)) who had an ultrasound examination at 20 to 24 weeks in our hospital and a control group with a normal BMI (20-24.9kg/m(2)) who had the same examination. A single operator evaluated the quality of all images, reviewing the standardized ultrasound planes - three biometric and six anatomical - required by French guidelines and scoring the quality of the six anatomical images. Each image was assessed according to 4-6 criteria, each worth one point. We sought excellent quality, defined as the frequency of maximum points for a given image. RESULTS: The obese group included 223 women and the control group 60. The completion rate for each image was at least 95 % in the control group and 90 % in the obese group, except for diaphragm and right outflow tract images. Overall, the excellence rate varied from 35 % to 92 % in the normal BMI group and 18 % to 58 % in the obese group and was significantly lower in the latter for all images except abdominal circumference (P=0.26) and the spine (P=0.06). Anatomical quality scores were also significantly lower in the obese group (22.3 vs. 27.2 ; P=0.001). CONCLUSION: Image quality and global anatomical scores in second trimester ultrasound scans were significantly lower among obese than normal-weight women.