This study intends to evaluate different acoustic windows that can be used to obtain the midsagittal plane of fetal brain during abdominal ultrasound scanning and the application of acoustic windows in practice.

The midsagittal planes were obtained respectively through anterior fontanel, posterior fontanel, sagittal suture and forehead seam of 12 aborted fetuses, and feasible acoustic windows were used as standard acoustic windows to obtain midsagittal planes of second trimester screening fetuses, which were divided into two groups according to the position of fetal brain, cephalic presentation group (367 cases) and non-cephalic presentation group (384 cases). The success rates were compared between groups.

Of the second trimester fetuses, 497 midsagittal planes of 751 cases were successfully obtained. The success rate of the cephalic presentation fetuses (32.7%, 120/367) was statistically lower than that of the non-cephalic presentation fetuses (98.2%, 377/384, P<0.01). Of the cephalic presentation fetuses, the combined acoustic windows acquisition success rate of the forehead, anterior fontanelle, sagittal suture and posterior fontanel acoustic window was 32.7% (32.7%, 120/367), significantly higher than the single acquisition success rate of the forehead, anterior fontanelle, sagittal suture and posterior fontanel acoustic window (17.2%, 25.9%, 1.6% and 7.6%, all P<0.01). Of the non-cephalic presentation fetuses, the combined acoustic windows acquisition success rate of the forehead, sagittal suture and posterior fontanel acoustic window was 98.2% (98.2%, 377/384), significantly higher than the single acquisition success rate of the forehead, anterior fontanelle, sagittal suture and posterior fontanel acoustic window (31.8%, 61.2%, 64.8% and 62.0%, all P<0.01).

The midsagittal plane of fetal brain can be obtained through forehead seam, anterior fontanel, posterior fontanel and sagittal suture acoustic windows in transabdominal screening. The combination of four acoustic windows can increase the success rate of obtaining midsagittal planes.