Letter regarding “Updated criteria for the approval of subsequent pregnancy after cesarean section with a transverse uterine fundal incision based on 17 years of experience”: Our recommended measurement methodology of the scar thickness around the uterine fundus

We recently discussed the "wound thickness index," which we calculate using magnetic resonance imaging (MRI) to assess the suitability for subsequent pregnancy after cesarean section by transverse uterine fundal incision (TUFI).1 Here, we clarify our method of measuring uterine wall thickness, with details lacking in the original report. Our methodology comprises four main steps: 1. Identification of the TUFI scar on MRI: The TUFI scar is identified using T2-weighted MRI on a mid-sagittal section of the uterus. This section should be perpendicular to the uterine wall around the curved uterine fundus. The uterine wall thickness is then measured in this slice, opting for the slice showing the thinner scar when there are two options. 2. Measurement of the thinnest part of the scar: The thinnest part of the scar is identified on the outer contour of the uterine wall ("Point A") (Figure 1). The length of the vertical line drawn from Point A to the inner contour of the uterine wall is measured; it represents the thickness of the thinnest part of the scar ("length a") (Figure 1). 3. Measurement of both scar ends: Points B and C represent both ends of the scar on the outer contour of the uterine wall, where the wall begins to thin toward the thinnest part of the scar (clear scar ends, Figure 1). The perpendicular distances from Points B and C to the inner contour of the uterine wall are measured, they represent the thickness at each scar end ("length b" and "length c"). If the scar's boundaries are unclear, Points B and C can instead be set 1.5 cm from Point A (unclear scar ends, Figure 1). 4. Calculation of the "wound thickness index": The "wound thickness index" is calculated using the formula 2a/(b + c), where "a" is the thickness of the thinnest part of the scar and "b" and "c" represent the thicknesses of both scar ends. We consider the following conditions to be suitable for subsequent pregnancy after TUFI: a wound thickness index ≥70%, at least partially resumed blood flow on MRI, and no abnormalities detected on sonohysterography.1 Vertical measurement of the uterine wall is crucial, as oblique measurements can overestimate the "wound thickness index" by yielding longer than actual values. These methods facilitate accurate evaluation of TUFI scars, and potentially assessment of other uterine incision scars. Hiroki Kato: Methodology; project administration; writing – original draft; writing – review and editing. Takashi Shibata: Project administration; writing – original draft; writing – review and editing. Koji Nishijima: Project administration; supervision; writing – review and editing. The authors express our heartfelt gratitude to Dr. Sayoko Hosono and Yasunori Fukuoka at Takatsuki General Hospital for their invaluable assistance in developing the measurement method. Our sincere appreciation also goes to Dr. Satoshi Nakago and Dr. Fumikazu Kotsuji at Takatsuki General Hospital for providing the opportunity to pursue this research. The authors also thank Benjamin Phillis at the Clinical Study Support Center, Wakayama Medical University, for supporting English proofreading. This work was supported by JSPS KAKENHI Grant Number 21592092. The authors declare that they have no competing interests. The original retrospective study was conducted with the approval of the Takatsuki General Hospital Ethics Committee (permission number: 2021-40). Informed consent was obtained via an opt-out system on the website. Data sharing not applicable-no new data generated, or the article describes entirely theoretical research.