A fetus affected by trisomy 18 was observed serially on ultrasound from 14 to 21 weeks of gestation. The earliest and sole dysmorphic feature noted at 14 weeks of gestation were non-septated nuchal cysts. At 17 weeks the ultrasound examination was considered normal. However, at 21 weeks gestation, bilateral choroid plexus cysts, clubfoot, growth retardation and overlapping fingers were observed. These findings suggest that the dysmorphology associated with trisomy 18 is dynamic in nature and may change along the course of pregnancy.