Aim: We aimed to establish normal reference ranges for insula, sylvian fissure (SF), parieto-occipital fissure (POF), and calcarine fissure (CF) measured by prenatal ultrasonography (USG) between 20–24 weeks of gestation in healthy fetuses. Method: A total of 186 fetuses in the second trimester were evaluated by transabdominal USG. All measurements were obtained by a single clinician. The study was divided into four subgroups (Group A: 20–20 weeks six days, Group B: 21–21 weeks six days, Group C: 22–22 weeks six days, Group D: 23–23 weeks six days). Results: Eight fetuses (4.23 %) between 20 and 21 weeks of gestation could not be included in the study because the sulcus borders could not be clearly evaluated. Measurements were obtained in all fetuses over 21 weeks of gestation. Reference ranges were obtained for insula, SF, POF, and CF in all fetuses and subgroups. At 20 and 23 weeks and six days gestation, mean insula depth was 14.96 ± 1.62 mm (min 11.0 mm − max 18.9 mm), mean SF depth was 6.96 ± 1.35 mm (min 3.6 mm − max 10.0 mm), mean POF depth was 2.05 ± 0.66 mm (min 1.1 mm − max 5.6 mm) and mean CF depth was 2.42 ± 0.68 mm (min 1.5 mm − 6.1 mm). There was a correlation between the cerebellum and cisterna magna and all fissure depths. Conclusion: Our nomograms of healthy fetuses may be helpful in the early detection of cortical maturation abnormalities.