The cavum septi pellucidi (CSP) is a fluid-filled cavity box situated on the midline between the medial walls of the two lateral ventricles and placed above the fornix and below the corpus callosum. The formation of the CSP begins at 14 weeks and is completed around 17 weeks.

A regular CSP indirectly indicates the correct development of the corpus callosum and the midline of the fetal brain. Therefore, its evaluation is mandatory during routine obstetric scans. The available guidelines do not report specific recommendations on the morphology or biometry of the fetal CSP, thus leaving to the experience of the operator and, thus, to a subjective evaluation, the identification of potential anomalies.

Our aim is to construct methodologically robust reference charts for the CSP’s width, length and length-to-width ratio in relation to gestational age and fetal biparietal diameter.

The REC-FAST study (Reference Charts for the Fetal cAvum SepTi pellucidi) is a prospective monocentric cross-sectional study on consecutively enrolled pregnant women accessing our Obstetric Unit at the Foundation IRCCS San Gerardo dei Tintori, Monza, Italy, for fetal ultrasound evaluation.

Women will be eligible if carrying an uncomplicated singleton pregnancy between 19^0/7 and 36^6/7 weeks’ gestation with a certain pregnancy dating by first trimester ultrasound with crown-rump length measurement, and if aged between 18 and 45 years.

After signing the informed consent, the ultrasound scan will be performed and the CSP’s width and length will be measured by means of the inner-to-inner technique and its morphology recorded.

In order to achieve the statistical power required for properly constructing reference charts, we will divide our population into six groups according to the gestational age when the ultrasound scan will be performed (each group will cover a 3-week interval starting at 19^0/7 until 36^6/7 weeks). A minimum sample size of 80 will be reached for each gestational age group. Before charting, the data will be checked for consistency to identify any outliers. Where possible, outliers will be corrected by comparing with the original values (computation errors); otherwise, such data will be excluded. The fetal charts will be traced using the Cole and Green-Lambda, Median, and Sigmamethod (CG-LMS); in addition, the use of alternative modelling approaches, such as parametric models derived from the Extended Mechanistic Growth Function method, will be explored.

Ethical approval for this study was obtained by the Lombardy Ethics Committee n.3 (15 December 2023) prior to the commencement of the research. Written informed consent will be obtained from all participants. Women will be free to decline participation or to withdraw at any time.

Findings will be presented at scientific meetings and published in peer-reviewed scientific journals in the field of obstetrics and fetal medicine. Also, they will be disseminated to study participants through dedicated online and in-person meetings and to the public through reach-out activities involving families and healthcare specialists.