This is a prospective study which was held in a single university hospital between November 2009 and April 2010. Detailed second-trimester US examinations were performed in 419 women between 18–24 weeks of gestation. All US examinations were performed by a radiologist who is experienced in fetal imaging. Fetuses with enlarged NFT measurements, major congenital anomalies, and patients with multiple gestations were excluded from the study. Pregnancy outcome was obtained from the hospital files. The fetuses with normal fetal US findings that have normal physical examinations right after birth were assumed to be normal.

The study group was divided into two groups, according to fetal gender (224 male, 195 female).  All US examinations were performed using a SSA 770A ultrasound system (Toshiba, Tokyo, Japan) with a 3.5-MHz curvilinear transducer. After obtaining an axial view of the cranium passing through the cerebellum and cavum septi pellucidum, NFT was measured from the outside of the external table of the occipital bone to the outer edge of outlying skin, using electronic calipers (Figure).

Statistical analysis was performed using SPSS statistics 15.0 version (SPSS Inc., Chicago, IL, USA). Mann Whitney test was used to compare the NFT measurements between male and female groups.