Objective To investigate outcome and prognosis of isolated mild fetal ventriculomegaly (IMV) of fetus in uterus. Methods From Jan. 2006 to Dec. 2009, 18 200 singleton pregnancy women from 20 weeks gestation underwent prenatal ultrasonography examination in Department of Obstetrics and Gynecology, Second Affiliated Hospital of Wenzhou Medical College. One hundred and forty-eight women with IMV (transverse diameter of the atrium of the lateral ventricle measuring between 10 and 15 mm with no other abnormalities) were studied prospectively, which were divided into two groups: 99 women with transverse diameter of the lateral ventricle of 10 - 11 mm in group A and 49 women with transverse diameter lateral ventricle of 12 - 15 mm in group B. The changes of ventriculomegaly and the associated intracranial and extracranial anomalies were observed regularly every 2 or 4 weeks until delivery. The development of neurological system was also followed up. Results ( 1 ) The overall incidence of IMV was 0. 08% (148/18 200). The rate of bilateral ventriculomegaly were 20% (20/99) in group A and 51% (25/49) in group B, which reached statistical difference (P＜ 0. 05). (2) Prognosis of fetus: 139 cases with 2 or more ultrasonographic examinations, IMV resolved throughout pregnancy in 41. 7% (58/139) ,regressed in 7. 9% (11/139) ,remained stable in 36. 7% (51/139)and progressed in 13. 7% ( 19/139). Five cases in group A and 11 cases in group B present progress, which reached significantly difference (P ＜ 0. 05). (3) One hundred and eleven cases infant were followed up for 5-12 months,the rate of psycho-motor developmental delay was 5. 4% (6/111). The rate of neuro-developmental delay in progressed group (3/15) was higher than 2. 5% ( 1/40) in resolved group, 0 (0/8) in regressed group and 4. 2% (2/48) in remained stable group, which reached significantly difference (P＜0. 05). Conclusions About 85% of cases of IMV resolved, regressed or remained stable in utero would exhibited good prognosis. IMV with a transverse atrial size ≥ 12 mm or progression in utero was usually associated with a poor prognosis, which should be observed carefully.