Single- vs double-layer uterine closure of the cesarean scar in niche development: the Nicest Study.

Ha Thi Thu Nguyen: Writing – review & editing, Supervision, Methodology, Formal analysis, Conceptualization. Giang Thi Tra Duong: Writing – original draft, Methodology, Formal analysis, Data curation, Conceptualization. Dat Tuan Do: Methodology, Formal analysis, Data curation, Conceptualization. Thuong Thi Huyen Phan: Writing – original draft, Methodology, Data curation, Conceptualization. Duc Anh Tran: Writing – original draft, Methodology, Data curation, Conceptualization. Toan Khac Nguyen: Writing – original draft, Methodology, Formal analysis, Conceptualization. Anh Duy Nguyen: Writing – review & editing, Supervision, Methodology, Conceptualization.

Between May 2022 and December 2024, 530 eligible women were enrolled in the study, randomly distributed in 2 groups: single-layer uterine closure (n=265) and double-layer uterine closure (n=265). After delivery, 429 participants attended the 6-month follow-up, and 381 attended the 12-month follow-up. Dropout reasons included intraoperative complications, loss of contact, withdrawal (because of long distances from home or difficulty arranging appointments), and subsequent pregnancies ( Figure ). Baseline demographic and obstetrical characteristics were comparable between the single-layer and double-layer groups at both follow-up visits ( Table 2 ). Figure Flowchart of the study Figure Nguyen. Niches in cesarean scar trial (Nicest Study). Am J Obstet Gynecol Glob Rep 2025. Table 2 Participants’ baseline characteristics at entry, 6-month follow-up visit, and 12-month follow-up visit Table 2 Baseline characteristics Entry (n=530) 6-mo visit (n=429) 12-mo visit (n=381) Single layer (n=265) Double layer (n=265) P value Single layer (n=216) Double layer (n=213) P value Single layer (n=194) Double layer (n=187) P value Maternal age (y) 27 (18–43) 27 (18–38) .589 27 (18–43) 27 (18–38) .796 27 (18–43) 27 (18–38) .713 Body mass index (kg/m 2 ) 26.0 (19.0–34.1) 26.0 (18.7–37.4) .929 26.0 (19.6–34.1) 26.0 (18.7–37.4) .930 26.0 (19.6–34.1) 26.0 (18.7–37.4) .878 Nulliparous 251 (94.7) 256 (96.6) .394 205 (94.9) 205 (96.2) .640 184 (94.8) 182 (97.3) .293 Previous abortion 49 (18.5) 53 (20.0) .741 41 (19.0) 42 (19.7) .903 34 (17.5) 36 (19.3) .693 Spontaneous conception 246 (92.8) 251 (94.7) .236 201 (93.1) 201 (93.4) .692 182 (93.8) 177 (94.7) .827 Maternal disease 30 (11.3) 27 (10.2) .779 21 (9.7) 25 (11.7) .535 18 (9.3) 23 (12.3) .409 Elective CD 50 (18.9) 46 (17.4) .735 41 (19.0) 37 (17.4) .708 38 (19.6) 32 (17.1) .312 Cervical dilation of >6 cm 60 (22.6) 68 (25.7) .478 44 (20.4) 49 (23.0) .558 38 (19.6) 40 (21.4) .704 PROM before CD 158 (59.6) 135 (50.9) .054 120 (55.6) 112 (52.6) .562 110 (56.7) 104 (55.6) .837 PROM >6 h 15 (5.7) 21 (7.9) .388 13 (6.0) 17 (8.0) .454 13 (6.7) 16 (8.6) .550 Intraoperative blood loss (mL) 300 (250–900) 300 (250–900) .854 300 (250–800) 300 (250–900) .853 300 (250–800) 300 (250–900) .803 Additional suture 72 (27.2) 86 (32.5) .217 57 (26.4) 70 (32.9) .169 55 (28.4) 61 (32.6) .375 Length of stay (d) 4 (2–9) 4 (2–9) .171 4 (2–9) 4 (2–9) .067 4 (2–9) 4 (2–9) .084 Gestational age at CD (wk) 39 (37–40) 39 (37–41) .223 39 (37–40) 39 (37–40) .297 39 (37–40) 39 (37–40) .473 CD , cesarean delivery; PROM , premature rupture of membranes. Data are presented as number (percentage) or median (interquartile range), unless otherwise indicated. Maternal disease includes hypertension disorder, diabetes mellitus, and anemia. Nguyen. Niches in cesarean scar trial (Nicest Study). Am J Obstet Gynecol Glob Rep 2025. Flowchart of the study Participants’ baseline characteristics at entry, 6-month follow-up visit, and 12-month follow-up visit CD , cesarean delivery; PROM , premature rupture of membranes. Data are presented as number (percentage) or median (interquartile range), unless otherwise indicated. Maternal disease includes hypertension disorder, diabetes mellitus, and anemia. The uterine position and most characteristics of the CD scars, including the relationship of the scar to the bladder, the distance to VV, the distance to external os, and the AMT, were found to be consistent between the single-layer and double-layer closure groups at both follow-up visits. At the 6-month follow-up, the RMT and the healing ratio (RMT-to-AMT) were significantly greater in the double-layer group than in the single-layer group (4.3 vs 4.0 mm and 60% vs 69%, respectively; P .05), the proportion of large niches was considerably higher in the single-layer group than in the double-layer group (19.4% vs 9.9%, respectively; P <.05). In addition, compared with niches observed in the double-layer group, the niches observed in the single-layer group exhibited significantly greater depth (3.0 vs 3.1 mm, respectively; P =.006), length (4.8 vs 5.2 mm, respectively; P =.005), and volume (39 vs 62 mm 3 , respectively; P =.001). Furthermore, when evaluating the severity of the niches, the mean VTS score in the single-layer group was higher than that in the double-layer group (1.09 vs 0.76, respectively; P <.05). At the second follow-up, the results mirrored those of the first visit, consistently showing better outcomes for the double-layer group in terms of RMT, healing ratio, the proportion of large niches, niche depth and length, niche volume, and VTS score ( Table 3 ). Table 3 Comparison of uterine ultrasound characteristics between the single-layer and double-layer uterine closure groups at 6- and 12-month follow-up visits Table 3 Ultrasound characteristics 6-mo visit (n=429) 12-mo visit (n=381) Single layer Double layer P value Single layer Double layer P value Uterine scar characteristics n=216 n=213 n=194 n=187 Retroverted uterine position 79 (36.6) 75 (35.2) .841 71 (36.7) 60 (32.1) .389 Bladder covering the CD scar 72 (33.3) 68 (31.9) .759 68 (35.1) 58 (31.0) .446 CD scar to VV fold distance (mm) 7.9 (1.8–29.7) 8.0 (1.3–27.5) .775 8.0 (2.0–27) 8.0 (2.0–28.8) .850 CD scar to external os distance (mm) 27.4 (10.0–41.4) 27.2 (8.5–43.8) .858 28.2 (9.8–41.2) 27.5 (7.7–44.0) .785 RMT (mm) 4.0 (1.9–9.7) 4.3 (2.0–8.6) .007 a 4.2 (1.9–9.7) 4.4 (2.0–8.7) .017 a AMT (mm) 6.9 (4.0–10.8) 7.0 (4.0–10.7) .673 6.9 (4.0–10.8) 7.0 (3.6–10.8) .623 RMT-to-AMT ratio (%) 60 (22–100) 69 (26–100) .011 a 60 (20–100) 69 (30–100) .048 a Niche 77 (35.6) 68 (31.9) .475 74 (38.1) 63 (33.7) .394 Large niche (RMT of <3 mm) 42 (19.4) 21 (9.9) .005 a 37 (19.1) 21 (11.2) .033 a Niche characteristics n=77 n=68 n=74 n=63 Depth (mm) 3.1 (2.0–7.5) 3.0 (2.0–8.9) .006 a 3.3 (2.0–7.5) 3.0 (2.0–8.8) .005 a Length (mm) 5.2 (2.0–12.2) 4.8 (2.0–9.4) .005 a 6.0 (2.5–12.0) 5.0 (2.0–10.4) .027 a Width (mm) 6.0 (2.7–16.0) 6.0 (2.8–18.1) .081 6.0 (2.0–18.2) 5.8 (3.0–17.0) .078 Volume (mm 3 ) 62 (10–690) 39 (9–179) .001 a 71 (11–735) 41 (9–181) .003 a VTS score 1.10±0.67 0.80±0.74 .006 a 1.20±0.73 0.80±0.76 .001 a 0–2 76 (98.7) 68 (100.0) 1.000 71 (95.9) 62 (98.4) .624 >2 1 (1.3) — 3 (4.1) 1 (1.6) Data are presented as number (percentage), median (interquartile range), or mean±standard deviation, unless otherwise indicated. AMT , adjacent myometrial thickness; CD , cesarean delivery; RMT , residual myometrial thickness; VTS , niche Volume; residual myometrial Thickness; Supplemental features; VV , vesicovaginal. a Statistically significant with p -value < 0.05. Nguyen. Niches in cesarean scar trial (Nicest Study). Am J Obstet Gynecol Glob Rep 2025. Comparison of uterine ultrasound characteristics between the single-layer and double-layer uterine closure groups at 6- and 12-month follow-up visits Data are presented as number (percentage), median (interquartile range), or mean±standard deviation, unless otherwise indicated. AMT , adjacent myometrial thickness; CD , cesarean delivery; RMT , residual myometrial thickness; VTS , niche Volume; residual myometrial Thickness; Supplemental features; VV , vesicovaginal. Statistically significant with p -value < 0.05. A total of 381 women underwent 2 follow-up ultrasounds. In the longitudinal assessment, the resolution of a niche was observed between the first and second scans in 2 of 131 women (1.5%), and 8 of 250 women (3.2%) developed a niche between the 2 ultrasound examinations. All 10 cases were confirmed at the diagnostic cutoff, with the niche depth measured at approximately 2 mm. However, the overall incidence of a niche was statistically consistent ( P =.109) between 6 (34.4%) and 12 (36.0%) months after delivery. In the long-term follow-up evaluation, the characteristics of the CD scar remained stable between 6 and 12 months after delivery. Among 129 women with a niche on ultrasound at both time points, the length of the scar exhibited a modest increase over time, from 5.0 to 5.5 mm, and the volume of the niche measured via 3D ultrasound showed a statistically significant increase, from 47 to 55 mm³ ( P <.05). These changes occurred in both groups, the single- and double-layer closures. Furthermore, the VTS score indicated a discernible upward trend, increasing from 0.94 to 1.04 ( P <.005), focusing on the single-layer group ( Table 4 ). Table 4 Change in uterine ultrasound characteristics between 6 and 12 months after delivery Table 4 Ultrasound characteristics 6-mo follow-up 12-mo follow-up P value Uterine scar characteristics (n=381) Retroverted uterine position 250 (65.6) 250 (64.6) 1.000  Single layer (n=194) 71 71 1.000  Double layer (n=187) 60 60 1.000 Urinary bladder covering the CD scar 126 (33.1) 126 (33.1) 1.000  Single layer (n=194) 68 68  Double layer (n=187) 58 58 CD scar to VV fold distance (mm) 8.0 (1.3–29.7) 8.0 (2.0–28.8) .951  Single layer (n=194) 7.9 (1.8–29.7) 8.0 (2.0–28.8) .793  Double layer (n=187) 8.0 (1.3–27.5) 8.0 (2.0–27.0) .850 CD scar to external os distance (mm) 27.20 (8.50–43.80) 27.60 (7.70–44.00) .274  Single layer (n=194) 27.15 (10.00–41.10) 28.15 (9.80–41.20) .319  Double layer (n=187) 27.20 (8.50–43.80) 27.50 (7.70–44.00) .652 RMT (mm) 4.3 (1.9–9.7) 4.2 (1.9–9.7) .471  Single layer (n=194) 4.1 (1.9–9.7) 4.2 (1.9–9.7) .364  Double layer (n=187) 4.4 (2.0–8.6) 4.4 (2.0–8.7) .904 AMT (mm) 6.9 (4.0–10.8) 7 (3.6–10.8) .135  Single layer (n=194) 6.85 (4.0–10.8) 6.9 (4.0–10.8) .123  Double layer (n=187) 6.9 (4.1–10.7) 7.0 (3.6–10.8) .541 RMT-to-AMT ratio (%) 65 (22–100) 63 (20–100) .178  Single layer (n=194) 60 (22–100) 60 (20–100) .207  Double layer (n=187) 69 (26–100) 69 (30–100) .480 Large niche (RMT of <3 mm) 57 (15.0) 54 (14.2) .581  Single layer (n=194) 36 34 .727  Double layer (n=187) 21 20 1.000 Niche characteristics (n=129) Depth (mm) 3.0 (2.0–8.9) 3.0 (2.0–8.8) .464  Single layer (n=69) 3.1 (2.0–7.5) 3.3 (2.3–7.5) .017  Double layer (n=60) 3.0 (2.2–8.9) 3.0 (2.0–8.8) .224 Length (mm) 5.0 (2.0–12.2) a 5.5 (2.0–12.0) a .000 a  Single layer (n=69) 5.2 (2.0–12.2) 6.0 (2.5–12.0) .000  Double layer (n=60) 4.7 (2.0–9.4) 5.0 (2.0–10.4) .000 Width (mm) 6.00 (2.70–18.10) 6.00 (2.00–18.20) .071  Single layer (n=69) 6.00 (2.70–16.00) 6.50 (2.00–18.20) .026  Double layer (n=60) 5.75 (2.80–18.10) 5.75 (3.00–17.00) .910 Volume (mm 3 ) 47.0 (9.0–690.0) a 55.0 (9.0–735.0) a .000 a  Single layer (n=69) 60.0 (10.0–690.0) 81.0 (11.0–735.0) .000  Double layer (n=60) 35.8 (9.0–179.0) 41.5 (9.0–181.0) .001 VTS score 0.900±0.720 a 1.000±0.780 a .029 a  Single layer (n=69) 1.070±0.671 1.230±0.731 .035  Double layer (n=60) 0.780±0.739 0.820±0.770 .727 Data are presented as number (percentage), median (interquartile range), or mean±standard deviation, unless otherwise indicated. AMT , adjacent myometrial thickness; CD , cesarean delivery; RMT , residual myometrial thickness; VTS , niche Volume; residual myometrial Thickness; Supplemental features; VV , vesicovaginal. a Statistically significant with p -value < 0.05. Nguyen. Niches in cesarean scar trial (Nicest Study). Am J Obstet Gynecol Glob Rep 2025. Change in uterine ultrasound characteristics between 6 and 12 months after delivery Data are presented as number (percentage), median (interquartile range), or mean±standard deviation, unless otherwise indicated. AMT , adjacent myometrial thickness; CD , cesarean delivery; RMT , residual myometrial thickness; VTS , niche Volume; residual myometrial Thickness; Supplemental features; VV , vesicovaginal. Statistically significant with p -value < 0.05.

This prospective randomized study was conducted from May 2022 to December 2024 at Hanoi Obstetrics and Gynecology Hospital. This study was approved by the Institutional Review Board for Ethics in Biomedical Research – Hanoi Medical University (IRB-VN01.001/IRB00003121/FWA 00004148). We enrolled all full-term (gestational age of ≥37 0/7 weeks) pregnant women aged ≥18 years who underwent a primary CD (emergency or elective). The exclusion criteria included previous major uterine surgery (CD or myomectomy) and abnormal placenta (placenta previa or placenta accreta spectrum) in the current pregnancy. Women were invited for 2 consecutive follow-up visits at 6 and 12 months. On admission, eligible pregnant women at term were provided with information about the study, and written consent was obtained from the women before delivery. Participants were randomly assigned in a 1:1 ratio to receive either single-layer or double-layer closure, using a computer-generated list prepared by an independent statistician. Although surgeons were not blinded to the closure technique, participants and physicians performing postoperative transvaginal ultrasounds were unaware of the assigned method. All patients were managed according to local protocols throughout the preoperative, intraoperative, and postoperative stages. CDs were performed by experienced obstetricians, following a standardized protocol. The intervention group used a double-layer closure technique with unlocked continuous running sutures, incorporating the endometrial layer in the first layer. The second layer is an unlocked continuous running suture that imbricates the first layer. The control group received a single-layer closure with unlocked continuous running sutures, including the endometrial layer. Vicryl 1.0 sutures will be used for uterine myometrium closure in both groups. Women with severe intraoperative complications, such as postpartum hemorrhage or the need for uterine artery ligation, were excluded from the outcome analysis. Data on maternal demographics, labor, surgical procedures, and postpartum complications were collected during hospital stays. Participants were contacted via phone to arrange follow-up visits for transvaginal ultrasounds, which were scheduled within 10 days of the onset of their menstrual cycle at 6 and 12 months after surgery. For women with amenorrhea, appointments were set approximately 2 weeks around each time point. A structural assessment of the uterine anatomy and CD scar was conducted at each visit using a 2-dimensional (2D) transvaginal ultrasound. This evaluation included uterine position, RMT, adjacent myometrial thickness (AMT; the thickness of a healthy myometrium located next to the uterine scar), and the distances from the CD scar to the vesicovaginal (VV) fold and external cervical os. In addition, the relationship between the CD scar and the urinary bladder was analyzed. Niches were identified as indentations at the CD scar with a depth of at least 2 mm, and its assessment included branching, depth, length in the sagittal plane, and width in the transverse plane. Of note, 3-dimensional (3D) transvaginal ultrasound was performed to measure niche volume using the Virtual Organ Computer-aided AnaLysis (VOCAL), preferentially using a rotation step of ≤15°. 12 A “large niche” was defined as an RMT of <3 mm. The VTS (niche Volume; residual myometrial Thickness; Supplemental features) score system, which was proposed by Ludwin et al 12 in 2019, was used to classify the severity of the niche ( Table 1 ). Table 1 Scoring system (VTS system) for classification of uterine niche according to presence of potentially clinically relevant features 12 Table 1 Feature Score 0 1 2 Volume of the niche a 1.0 cm 3 Thickness of the residual myometrium b >3 mm 1–3 mm <1 mm Supplemental features c Absent Present (−) VTS , niche Volume; residual myometrial Thickness; Supplemental features. a Estimated based on automatic calculation (SonoAVC), manual calculation using the Virtual Organ Computer-aided AnaLysis (preferentially using rotation step of ≤15°), or manual calculation based on 3 diameters (length × depth × width × 0.52) b Thickness of the residual myometrium above the main niche c Supplemental features, such as presence of branches, urinary bladder not covering the niche, and suspicion of deep infiltrating endometriosis in the niche. Nguyen. Niches in cesarean scar trial (Nicest Study). Am J Obstet Gynecol Glob Rep 2025. Scoring system (VTS system) for classification of uterine niche according to presence of potentially clinically relevant features 12 VTS , niche Volume; residual myometrial Thickness; Supplemental features. Estimated based on automatic calculation (SonoAVC), manual calculation using the Virtual Organ Computer-aided AnaLysis (preferentially using rotation step of ≤15°), or manual calculation based on 3 diameters (length × depth × width × 0.52) Thickness of the residual myometrium above the main niche Supplemental features, such as presence of branches, urinary bladder not covering the niche, and suspicion of deep infiltrating endometriosis in the niche. The GE Healthcare Voluson E8 system was used for 2D and 3D imaging, with practitioners trained following the 2019 International Society of Ultrasound in Obstetrics and Gynecology consensus guidelines. 2 Quantitative variables were recorded at least twice, and mean values were used for the final analysis. The primary outcome compared the RMT and niche characteristics between the single-layer and double-layer uterine closure techniques 1 year after primary CD. The secondary outcome evaluated the change in uterine scar characteristics from 6 to 12 months after CD. Based on tools, such as the “Sample Size Determination in Health Studies” (WHO,1991), and the expected dropout rate of 35%, the sample size required was calculated at 262 women for each group to detect an absolute difference of 30% in the primary outcome (proportions of niche being 50% in the double-layer group and 35% in the single-layer group, 13 80% power, and α error of 0.05). Statistical analyses were performed using SPSS (version 20.0; SPSS Inc, Chicago, IL). Categorical variables are presented as number (percentage), and continuous variables are presented as mean±standard deviation or median (interquartile range). For primary outcomes comparing ultrasound characteristics between single-layer and double-layer groups, the Fisher exact test and chi-square test were used for categorical variables, and the Mann-Whitney U test was used for continuous variables that were not normally distributed. The secondary outcomes across 2 follow-up visits were analyzed using complete-case analysis, with the McNemar test used for categorical variables and the Wilcoxon test used for continuous measures.

This randomized prospective study illustrated that although the niche incidence was comparable between 2 uterine closure techniques after 1 year of primary CD, the double-layer group yielded better outcomes in ultrasound evaluation. Specifically, the double-layer group exhibited a thicker RMT, greater healing ratio, lower prevalence of large niches, decreased niche volume, and lower mean VTS score. Longitudinal follow-up assessments revealed that although the uterine scar’s characteristics and the niche proportion were consistent between 6 and 12 months after delivery, the niche volume progressively increased over time. Following established local protocol, our experienced surgeons used a single- or double-layer unlocked continuous running suture technique to close a uterine incision in full-term pregnant women who underwent primary CD. Because excluding the decidua seems to result in a higher niche prevalence according to the findings of Stegwee et al 14 in a meta-analysis in 2018, both closure techniques in this study included the decidua layer. The quality of the uterine scar tissue after CD is mainly assessed through RMT and the healing ratio (RMT-to-AMT ratio). A reduced RMT was found to be associated with heavy spotting, infertility, 15 and obstetrical complications in the subsequent pregnancy. 16 Meta-analyses indicated that RMT is, on average, 1.15 mm thinner with single-layer closure than with double-layer closure, 4 along with a 7.7% decrease in the healing ratio. 14 Our study supports these findings, showing that RMT was approximately 4.4 mm in the double-layer group vs 4.2 mm in the single-layer group, with healing ratios of 69% and 60%, respectively, at 1 year after delivery ( P <.05). The use of a double-layer closure technique may offer better approximation of the uterine myometrium than the use of a single-layer closure technique, 17 which could explain these findings. Niches are common, but larger niches have more clinical relevance. 5 Currently, there is no consensus on the definition of a large niche. Here, we defined a large niche as one with an RMT of <3 mm, as this is related to management options, such as laparoscopy and hysteroscopy. 1 Our study found that at 12 months after CD, the incidence of the niche was similar between the 2 cohorts, 38.1% in the single-layer group and 33.7% in the double-layer group. However, the proportion of large niches was significantly higher in the single-layer group (approximately 19.1%) than in the double-layer group (11.2%). The study conducted by Hanacek et al 10 reported a similar finding, noting that the percentage of women with an RMT of <2.5 mm was higher in the single-layer group (12.2%) than in the double-layer group (6.8%). In contrast, the 2Close study (n=1961) reported no difference in large niche incidence between the 2 groups, with rates of 13.2% and 11.8%, respectively. 8 However, the 2Close study used sonohysterography to evaluate CD scars only when transvaginal ultrasound did not detect niches. This approach may lead to heterogeneous results, as RMT and niche sizes have been shown to differ significantly between the 2 procedures. 18 The study investigated the niche characteristics using 2D transvaginal ultrasound and found that the niche depth and length in the single-layer group were significantly greater than that in the double-layer group during both assessments. In addition, the niche volume measured using 3D transvaginal ultrasound, using the VOCAL method, 12 was greater in the single-layer group (approximately 71 mm³) than in the double-layer group (41 mm³) during the second visit ( P <.05). This finding may have clinical significance, as the niche volume has been shown to differ between women who experience spotting symptoms and those who do not, apart from the niche itself. 7 However, the 2Close study reported no difference in niche volume between the 2 groups 9 months after CD, using the formula: 1/3 × π × depth × (length/2)², based on 2D transvaginal ultrasound measurements. 8 Because the shape of the niche varied, 19 the formula may not accurately calculate the niche volume compared with the VOCAL method in 3D transvaginal ultrasound. This discrepancy could explain the differing results observed between our study and the study of Stegwee et al. 8 In addition, because of lower RMT and a higher volume, the mean VTS score in the single-layer group was higher than that in the double-layer group. However, the number of women with a VTS score of >2, which is considered clinically relevant, was small, at approximately 1% (4 of 381). This low percentage was primarily because many participants had just resumed menstruation after their CD. However, this scoring system is new and requires further research to confirm its effectiveness in niche evaluation. To date, the optimal time to diagnose a niche after CD is unknown. In previous studies, the niche detection did not change, 10 , 11 but the RMT and healing ratio were significantly decreased from 2 to 12 months after a CD. 11 In longitudinal follow-up assessments, our study demonstrated that the characteristics of uterine scars and the incidence of niches were consistent at both 6 and 12 months after delivery. This observation suggests that the uterus remains stable at 6 months after delivery, allowing for the evaluation of its characteristics and the presence of niches during this timeframe. In addition, the overall incidence of a niche at 1 year after CD was relatively high, in which approximately one-third of women underwent primary CD. This number is even higher when CD is performed by inexperienced surgeons according to the study by Hanacek et al 10 (approximately 75%) or when using sonohysterography ultrasound (approximately 80%). 8 However, only 30% to 50% of women with niche have clinical symptoms, and a few of them need treatment. 7 , 20 The risk of overdiagnosis was considered a threat in the management of a niche according to a SWOT (Strengths, Weaknesses, Opportunities, Threats) analysis. 21 Therefore, a cutoff diagnosis of a niche on ultrasound, which could be relevant to symptoms, should be made to avoid this threat. Regarding the alterations in niche characteristics, we observed that the overall niche volume demonstrated a significant increase, increasing approximately 8 mm 3 within 6 months, from 47 mm³ at 6 months after delivery to 55 mm³ at 12 months after delivery ( P <.05). These changes were evident in both the single-layer and double-layer groups. We hypothesized that the frequency of menstruation may be a contributing factor to this increase, as the accumulation of menstrual blood in the niche site could augment its dimensions, particularly its volume. Nonetheless, additional research is required to validate this hypothesis. Furthermore, it is noteworthy that the median volume of the niche 1 year after delivery remains relatively small, at 55 mm³. According to the findings of Bandry et al, 22 a scar volume exceeding 0.15 cm³ exhibited a sensitivity of 97% and a specificity of 100% in predicting postmenstrual spotting, achieving an overall accuracy of 98.4%. This insight may explain why many women experience symptoms long after undergoing CDs. 8 , 9 Long-term follow-up studies are essential to investigate how the duration after a primary CD may contribute to complications, particularly infertility, given the evolving changes in niche size over time. This research will help develop recommendations for the optimal timing of conception attempts after a CD, with the aim of reducing the risk of secondary infertility and enhancing our understanding of reproductive health after surgery. In addition, further research is needed to explore the reasons behind the increase in niche volume, which will help in creating effective strategies to prevent this increase and its associated consequences. The application of 3D ultrasound to assess niche volume is a notable strength of this study, providing crucial insights into niche characteristics that exceed the limitations of conventional estimation methods. In addition, we are aware that our study has some limitations. First, the reliance on 2D transvaginal ultrasound for diagnosis rather than sonohysterography may affect the proportion of niche detection. To counter this, participants were selected for evaluation within 10 days of menstruation to enhance visibility. Second, the follow-up period was intentionally limited to 1 year after delivery, with key ultrasound assessments at 6 months, when most participants had not yet resumed menstruation, and 12 months, when most participants had reestablished their cycles. This strategy provides valuable insights into the changes in the uterus and the niche related to menstrual cycles. Finally, although this article does not address gynecologic symptoms, we plan to publish a comprehensive analysis of the relationship between niches and gynecologic factors in the future, aiming to enrich the understanding of these important issues in women’s health. Although the niche incidence was similar between the 2 uterine closure techniques, the double-layer technique showed superior benefits, with greater RMT and healing ratio, lower large niche proportion, and smaller niche volume. In addition, the niche volume significantly increased over time. Future long-term follow-up research is necessary to elucidate the relationship between niche size and clinical symptoms and to investigate the factors contributing to the temporal evolution of niche volume.

The increasing prevalence of cesarean deliveries (CDs) and its long-term complications are significant concerns for obstetricians worldwide. In addition, patient-specific factors, improper uterine incision location, and inadequate closure can lead to the formation of a uterine niche or cesarean scar defect, 1 which is defined as an indentation deeper than 2 mm at the uterine scar site. 2 A niche can cause various gynecologic symptoms, infertility, and negative effects on subsequent pregnancies. Noninvasive imaging methods, such as transvaginal sonography and sonohysterography, are effective for diagnosing niches, 3 whereas 3-dimensional ultrasound facilitates volume measurement. 3 In recent decades, several studies have focused on identifying the optimal uterine closure techniques to minimize niche development, particularly comparing single-layer vs double-layer closure methods. The findings indicate no significant difference in the incidence of niches observed via ultrasound between the 2 closure techniques, except for a thicker residual myometrial thickness (RMT; the thickness of the remaining myometrium at the site of uterine scar) in the double-layer group. 4 It is important to note that a niche visible on ultrasound is only concerning if it negatively affects women’s health. Some studies have shown that small niches are common and typically not clinically significant. In contrast, larger niches are more likely to be associated with gynecologic symptoms, subfertility, or adverse outcomes in subsequent pregnancies. 5 The size of the niche, particularly its volume, has been found to correlate with the frequency of postmenstrual spotting. 6 , 7 However, most current research has focused on the prevalence of niches rather than their size. 4 In addition, a recent large randomized controlled trial indicated that most women with niches are asymptomatic at 9 months after delivery, 8 but approximately one-third of women may experience spotting by 3 years after delivery. 9 This suggests that niches can change over time, potentially leading to clinical effects when they become sufficiently large. Currently, there is a lack of studies that evaluate the changes in niche proportion and size over time. 10 , 11 Therefore, this study aimed to compare the RMT and niche characteristics between the single-layer and double-layer uterine closure techniques and to evaluate the change in uterine scar characteristics from 6 to 12 months after primary CD.