Abstract： Objective To evaluate the role of ultrasound screening in fetus at 11-13+6 weeks and 14-17+6 weeks in unselected pregnant women. Methods From January 2002 to April 2008,first-time prenatal ultrasound routine examinations were performed for 3645 pregnant women at 11-17+6 gestationalweeks in the First Affiliated Hospital of Sun Yat-sen University,including 1556 cases of 11-13+6 and 2089 of 14-17+6 gestational weeks. Detection and types of structural anomalies were compared between these two groups. Results Primary detection identified fetal structural abnormalities in 2.3% (36/1556) and 3.3%(68/2089) of women at 11-13+6 and14-17+6 gestational weeks,respectively,with no statistically significant difference. However,constituents of structural anomalies varied (P=0.03),with the first three leading types being nuchal abnormalities[31.6% (18/57)](increased nuchal translucency thickness and cystic hydroma),fetal hydrops[15.8%(9/57)]and abdominal wall defects[12.3%(7/57)]in the 11-13+6 weeks group,and cardiovascular defects[16.7% (18/108)],central nervous system malformations[14.8% (16/108)]and abdominal wall defects[12.0%(13/108)]in the 14-17+6 weeks group. Conclusions The ultrasound examinations performed at 11-13+6 weeks can be useful in screening the soft markers for chromosomal abnormalities and certain fatal malformations,while those performed at 14-17+6 weeks may identify more details of fetal structure. The 11-13+6 weeks of gestation may be an optimal window for early ultrasound screening of the fetus.