How early are fetal cerebral sulci visible at prenatal ultrasound and what is the normal pattern of early fetal sulcal development?

OBJECTIVES: To evaluate the time of appearance and pattern of development of fetal cerebral sulci at prenatal ultrasound.
METHODS: We studied 50 normal fetuses for visibility of cerebral sulci, especially sulci which appear early in anatomical studies, namely the parieto-occipital fissure, calcarine sulcus, cingulate sulcus, convexity sulci and insula/Sylvian fissure. The gestational ages of the fetuses studied ranged from 15.6 to 29.6 weeks.
RESULTS: Sulci could be seen by transabdominal ultrasound as early as 18.5 weeks. Medial hemispheric sulci and the insula were visible earlier and more confidently than convexity sulci. The earliest gestational ages at which specific sulci could be seen in any fetus were as follows: parieto-occipital fissure 18.5 weeks, calcarine sulcus 18.5 weeks, cingulate sulcus 23.2 weeks and convexity sulci 23.2 weeks. In the present series, the gestational ages at which these sulci were always visible were as follows: parieto-occipital fissure >20.5 weeks, calcarine sulcus >21.9 weeks, cingulate sulcus >24.3 weeks and convexity sulci >27.9 weeks. The insula and its margin (the circular sulcus) and the overgrowing opercula undergo characteristic maturation. The circular sulcus at the margin of the insula was initially smooth but started becoming angular after about 17 weeks as it started to be overgrown by the parietal and temporal lobe opercula. Initially the insula/operculum angle was obtuse. An acute angle was first evident at 23.2 weeks and in all fetuses older than 24.5 weeks. Our ultrasound data were consistent with anatomical studies and fetal magnetic resonance imaging findings.
CONCLUSIONS: Some cerebral sulci can be seen at prenatal ultrasound as early as 18.5 weeks. Familiarity with the normal pattern of sulcal development and the discriminating gestational ages for the appearance of different sulci may allow early suspicion of lissencephaly.