Abstract
Background/Objectives: First-trimester anatomical screening (FTAS) enables early detection of fetal structural anomalies. However, data on transvaginal ultrasound performance across different operator experience levels remains limited. The primary objective was to compare the performance of transvaginal FTAS between inexperienced and experienced operators by comparing complete scan rates and anatomical visualization scores. The secondary objectives were to compare scan duration and learning curves between the two operators and to identify factors associated with successful examinations. Methods: In this randomized controlled trial, participants underwent transvaginal FTAS performed either by a first-year maternal–fetal medicine (MFM) fellow (inexperienced operator) or an MFM specialist (experienced operator) according to International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) guidelines. Complete scan rates, anatomical visualization scores, scan duration, factors affecting visualization, and learning curves were compared between groups. Results: A total of 139 participants were included in the analysis, with 70 and 69 examinations performed by the inexperienced and experienced operators, respectively. Complete scan rates and visualization scores were not significantly different between groups (41.4% vs. 44.9% and 86.4% vs. 91.2%, respectively). On multivariate analysis, operator experience was independently associated with higher visualization scores (p = 0.010), but not complete scan rates. Scan duration was similar between groups (median (IQR): 20 (17) vs. 20 (15) min; p = 0.211). Fetal presentation and uterine position significantly influenced visualization outcomes. The inexperienced operator demonstrated a trend of gradual improvement over time, though not significantly, whereas the experienced operator maintained stable performance throughout the study period. Conclusions: After training, no statistically significant differences were observed between less experienced and experienced operators with respect to complete scan rate, scan duration, and learning curve, although visualization score was significantly higher in the experienced examiner on multivariate analysis. However, our findings should be interpreted with caution, as the conclusions were based on the performance of only one examiner in each group.
Keywords: first-trimester anatomical screening, transvaginal ultrasound, feasibility, learning curve, ISUOG protocol
1. Introduction
The traditional roles of early pregnancy ultrasound have been to establish accurate gestational age, determine chorionicity in multiple pregnancies, and measure nuchal translucency (NT) for aneuploidy screening. However, advances in transducer technology, improved image resolution, and a greater understanding of fetal anatomical development have greatly expanded the role of first-trimester ultrasound. When performed between 11 and 13+6 weeks of gestation, first-trimester anatomical screening (FTAS) now provides an important opportunity for the detection of major fetal structural anomalies early in pregnancy [1,2,3,4]. These scans have important clinical value. They provide early reassurance for parents when the fetus appears structurally normal. When abnormalities are suspected, they also enable timely counseling and facilitate early genetic diagnostic testing. Furthermore, in situations where pregnancy termination is considered, early assessment may reduce maternal morbidity by enabling the performance of a termination at an earlier gestational age.
To improve the detection rate, guidelines have recently been published by international professional societies, including the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) and the American Institute of Ultrasound in Medicine (AIUM) [5,6,7,8]. When performed according to guideline protocols, the combination of transabdominal and transvaginal ultrasound facilitates FTAS, with reported detection rates approaching 80% depending on the specific organ assessed [1,4,7,9,10,11].
Transvaginal ultrasound enables better visualization of fetal organs during early gestation, especially when the transabdominal examination is limited or inconclusive [5,6,12,13,14]. However, the technique can be technically challenging because of the limited maneuverability of the transvaginal probe. Previous studies using transvaginal ultrasound have shown that several factors, including gestational age and fetal position, influence the feasibility of completing a comprehensive first-trimester fetal anatomical assessment [12,15]. Previous studies confirmed the feasibility of using the transabdominal approach for FTAS and demonstrated a short learning curve for inexperienced operators [16,17]. However, studies evaluating transvaginal ultrasound, which is technically different from transabdominal ultrasound, among operators with varying levels of experience remain scarce.
The primary objective of this study was to compare the rate of complete transvaginal FTAS at 11 to 13+6 weeks of gestation and anatomical visualization scores between an inexperienced and an experienced operator. The secondary objectives were to compare scan duration and learning curves between the two operators and to identify factors associated with successful examinations.
2. Materials and Methods
Study design: This randomized controlled trial was conducted at the antenatal care clinic in the Department of Obstetrics and Gynaecology, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Bangkok, Thailand between August 2025 and February 2026. The study was approved by the Ramathibodi Hospital Institutional Review Board ethics committee (COA, MURA2025/221, date of approval: 10 March 2025) and registered at the Thai Clinical Trials Registry, number 20250309014.
Participants: Pregnant women attending our antenatal care clinic between August 2025 and February 2026 were enrolled in the study if they met the following inclusion criteria: (1) singleton pregnancy, (2) maternal age ≥ 18 years, and (3) gestational age between 11 and 13+6 weeks, determined by a reliable last menstrual period and crown–rump length (CRL) measurement on ultrasound. The exclusion criteria were as follows: (1) major fetal structural anomalies detected on either the first or second-trimester ultrasound scan, (2) prenatally diagnosed chromosomal abnormalities, (3) loss to follow-up or inability to complete the second-trimester scan, and (4) refusal to participate in the study.
After eligible pregnant women were enrolled by the research physician, written informed consent was obtained from all participants.
Randomization: The randomization sequence was generated by a statistician using block randomization with a block size of four and was concealed in sequentially numbered, sealed opaque envelopes. A perinatal research nurse was responsible for the allocation process and assigned participants to undergo ultrasound examination by one of two operators: an inexperienced operator (a first-year maternal-fetal medicine [MFM] fellow who, although inexperienced in first-trimester anatomical screening, had two years of experience in second-trimester anatomical screening as a general obstetrician), and an experienced operator (a certified MFM specialist with 10 years of experience in second-trimester anatomical screening and nuchal translucency measurement who had previously achieved competency in transabdominal first-trimester anatomical screening) [16]. Both operators were certified by the Fetal Medicine Foundation for 11- to 13+6-week scans and nuchal translucency measurement and had received standardized training in first-trimester transvaginal anatomical ultrasound according to the ISUOG guidelines [5]. Both operators were blinded to the randomization sequence.
Intervention: The operator performed TAS for evaluation of the general pregnancy status including fetal number, viability, crown-rump length (CRL), gestational age at the time of the scan, fetal presentation, placental location, uterine position, and the presence of uterine fibroid. Subsequently, a first-trimester fetal anatomical assessment was conducted transvaginally using a high-resolution two-dimensional 4–9 MHz convex endocavity transducer connected to a Voluson E10 ultrasound system (GE Healthcare, Zipf, Austria) with a first-trimester preset. All examinations followed the protocol outlined in the ISUOG guidelines [5]. The duration of each scan was limited to a maximum of 20 min. Both examiners recorded checklist images for all successfully visualized structures during the examination as evidence of visualization, including the NT image when an optimal plane for measurement was successfully obtained.
Outcome measures: Demographic and clinical data, including maternal age, nulliparity, maternal body mass index (BMI), and abdominal surgical scar, were recorded in the study record form. The visibility of each organ or anatomical structure was recorded immediately after the procedure using the checklist form. Each structure was assigned a score of 1 if it was clearly visualized and a score of 0 if it was not visualized or if the recorded image was inconclusive. For nuchal translucency measurement, a score of 1 was assigned when the structure was clearly visualized and the thickness could be measured; otherwise, a score of 0 was assigned. According to the study protocol, complete visualization of the mandatory, optional, and total fetal anatomical structures were defined as the successful assessment of 13, 27, and 40 anatomical views, respectively. The scan duration for each operator was recorded either upon completion of the full examination or after reaching the 20 min time limit. All cases in which fetal anomalies were detected during the first-trimester scan underwent repeat ultrasound examination by another MFM staff member on the same day. Prenatal diagnostic procedures were performed when indicated. The pregnancies with normal first-trimester anatomical findings that continued beyond the first trimester were confirmed to be structurally normal at the routine mid-trimester anomaly scan by other certified MFM staff members.
Sample size estimation: Based on a previous study by Rosati et al. [12], the complete screening capability of first-trimester transvaginal fetal anatomical scanning was approximately 79%. Accordingly, the complete screening capability was assumed to be 79% by the operator with greater experience, and a 25-percentage-rate difference was considered clinically relevant, corresponding to an assumed capability of 54% among the less-experienced operator. With a two-sided α of 0.05, 80% power, and a 1:1 allocation ratio, the required sample size was 55 pregnant women per group. Allowing for a potential 25% loss or exclusion of data, the final required sample size was 74 pregnant women per group, for a total of 148 pregnant women.
Statistical analysis: Continuous variables were expressed as mean ± standard deviation or median (IQR) and compared using Student’s t-test or Mann-Whitney_U test as appropriate. Categorical variables were expressed as number (percentage) and compared using the chi-square test. Logistic regression analysis and multivariable linear regression analysis were performed to evaluate factors associated with complete scan rate and visualization score, respectively. The primary exposure variable was operator experience. Models were adjusted for clinically relevant potential confounders, including maternal age, nulliparity, body mass index (BMI), abdominal surgical scar, gestational age at examination, fetal presentation, placental location, uterine position, and the presence of uterine fibroids, based on explanatory approach. Repeated-measures analysis was performed to evaluate changes in examiner performance over time as clinical experience accumulated. For each examiner, consecutive examinations were ordered chronologically and grouped into 14 sequential sets of 10 examinations (Sets 1–14). These sequential examination sets were treated as the time variable, representing increasing cumulative experience. For each examiner, the performance score was calculated for each 10-examination set, resulting in 14 repeated performance measurements per examiner across successive time points. Thus, the repeated observations represented longitudinal measurements of examiner performance. A repeated-measures model was used to account for the correlation among performance scores obtained from the same examiner across successive time points. The model included performance score as the continuous outcome variable, examination set (Sets 1–14) as the time variable, operator group (inexperienced versus experienced examiner) as the between-examiner factor, and the time × operator-group interaction to assess whether the pattern or rate of change in performance over successive examination sets differed between the two examiners.
Based on the final model, marginal prediction plots were generated to illustrate the estimated visualization scores from the first to the final examination set. A p-value < 0.05 was considered statistically significant. Finally, we performed a modified per-protocol analysis because the primary objective was to evaluate the effect of operator experience on visualization performance rather than diagnostic performance in fetuses with anomalies. Excluded cases were not omitted based on study outcomes but because the predefined study endpoint could not be validly assessed. Specifically, pregnancies complicated by miscarriage, severe fetal hydrops, or major fetal anomalies did not meet the protocol-defined conditions for evaluation. All analyses were performed using IBM SPSS Statistics for Windows, Version 26.0 (IBM Corp., Armonk, NY, USA).
3. Results
Among the 183 eligible pregnant women who were consecutively approached, 35 declined to participate, leaving 148 participants enrolled in the study. The participants were randomized for assessment by either the inexperienced operator (n = 74) or the experienced operator (n = 74). For the inexperienced operator, four participants were excluded due to spontaneous abortion (n = 3) and fetal Bart’s hydrops (n = 1). For the experienced operator, five participants were excluded due to spontaneous abortion (n = 1), fetal anomalies (n = 2; DiGeorge syndrome with right-sided aortic and ductal arch, and bilateral club feet), and gestational age greater than 13 + 6 weeks at the time of examination (n = 2). The final analysis included 70 participants for the inexperienced operator and 69 participants for the experienced operator. The CONSORT flow diagram of the study is presented in Figure 1.
Figure 1.
Flow chart of patient recruitment and allocation.
Maternal and fetal baseline characteristics, including maternal age, nulliparity, BMI, abdominal surgical scar, uterine position, presence of uterine fibroids, gestational age at the time of scan, crown–rump length, and fetal presentation, were not significantly different between the two operator groups (all p > 0.05). However, anterior placental location was significantly more common in the inexperienced operator group than in the experienced operator group (70.0% vs. 47.8%; p = 0.008) (Table 1).
Table 1.
Maternal and fetal baseline characteristics.
| Baseline Characteristics | Inexperienced Operator (N = 70) |
Experienced Operator (N = 69) |
p-Value |
|---|---|---|---|
| Maternal age (years), mean ± SD | 31.9 ± 4.9 | 32.3 ± 5.1 | 0.683 |
| Nulliparous, n (%) | 42 (60.0) | 41 (59.4) | 0.285 |
| Maternal BMI (kg/m2), mean ± SD | 25.9 + 6.1 | 24.6 + 4.7 | 0.170 |
| Abdominal surgical scar, n (%) | 0.451 | ||
|
10 (14.3) | 15 (21.7) | |
|
3 (4.3) | 4 (5.8) | |
| CRL (mm), mean ± SD | 66.6 ± 9.1 | 68.3 ± 7.9 | 0.239 |
| GA at first trimester scan (days), mean ± SD | 89.7 ± 4.3 | 90.9 ± 3.6 | 0.074 |
| Presentation | 0.267 | ||
|
10 (14.3) | 16 (23.2) | |
|
21 (30.0) | 23 (33.3) | |
|
39 (55.7) | 30 (43.5) | |
| Placenta location | 0.008 | ||
|
49 (70) | 33 (47.8) | |
|
21 (30) | 36 (52.2) | |
| Uterine position | 0.138 | ||
|
66 (94.3) | 60 (87.0) | |
|
4 (5.7) | 9 (13.0) | |
| Present of uterine fibroid, n (%) | 4 (5.7) | 6 (8.7) | 0.361 |
BMI, body mass index; CRL, crown-rump length; GA, gestational age.
The rate of complete first-trimester fetal anatomy scans was not significantly different between the inexperienced operator and experienced operator (41.4% vs. 44.9%; p = 0.677). No significant differences were detected in the mean percentage anatomical visualization score (86.4 ± 19.3 vs. 91.2 ± 11.9; p = 0.078) or scan duration; median (IQR): 20 (17) vs. 20 (15) min; p = 0.211 (Table 2). Notably, 83 of the 139 examinations (59.7%) reached the maximum time limit of 20 min, with nearly all cases failing to achieve the optimal plane for NT measurement within the allocated time.
Table 2.
Complete scan, score, time between two operators with different experience in routine second trimester anatomy scan.
| Inexperienced Operator (N = 70) |
Experienced Operator (N = 69) |
p-Value | |
|---|---|---|---|
| Complete scan, n (%) | 29 (41.4) | 31 (44.9) | 0.677 |
| % Scores, mean ± SD | 86.36 ± 19.29 | 91.20 ± 11.85 | 0.078 |
| Time (min), median (IQR) | 20 (17) | 20 (15) | 0.211 |
Visualization success rates for each fetal anatomical structure were not significantly different between the two operators, except for the thalami (inexperienced operator, 78.6% vs. experienced operator, 91.3%; p = 0.036), forehead (72.9% vs. 94.2%; p = 0.001), and upper lip (67.1% vs. 87.0%; p = 0.006), which were visualized more frequently by the experienced operator. Both operators were able to visualize most fetal structures, with visualization rates ranging from 70% to 100% for each structure, except for nuchal translucency (NT), which was visualized in approximately 60% of the cases (Table 3).
Table 3.
Visualization success rate of fetal organ/anatomical area or structures between the two operators.
| Total (%) |
Inexperienced Operator (N = 70) |
Experienced Operator (N = 69) |
p-Value | |
|---|---|---|---|---|
| Confirm singleton pregnancy * | 139 (100.0) | 70 (100.0) | 69 (100.0) | - |
| Overview of fetus, uterus and placenta | 139 (100.0) | 70 (100.0) | 69 (100.0) | - |
| Calcification of cranium * | 136 (97.8) | 67 (95.7) | 69 (100.0) | 0.082 |
| Contour/shape of cranium * | 135 (97.1) | 67 (95.7) | 68 (98.6) | 0.317 |
| Interhemispheric falx * | 133 (95.7) | 65 (92.9) | 68 (98.6) | 0.099 |
| Choroid plexus * | 133 (95.7) | 65 (92.9) | 68 (98.6) | 0.099 |
| Thalami | 118 (84.9) | 55 (78.6) | 63 (91.3) | 0.036 |
| Brainstem | 101 (72.7) | 49 (70.0) | 52 (75.4) | 0.478 |
| Cerebral peduncle with aqueduct of Sylvius | 118 (84.9) | 56 (80.0) | 62 (89.9) | 0.105 |
| Intracranial translucency | 120 (86.3) | 57 (81.4) | 63 (91.3) | 0.090 |
| Cisterna magna | 119 (85.6) | 56 (80.0) | 63 (91.3) | 0.058 |
| Forehead | 116 (83.5) | 51 (72.9) | 65 (94.2) | 0.001 |
| Bilateral orbits | 134 (96.4) | 66 (94.3) | 68 (98.6) | 0.177 |
| Nasal bone | 107 (77.0) | 50 (71.4) | 57 (82.6) | 0.117 |
| Maxilla | 120 (86.3) | 62 (88.6) | 58 (84.1) | 0.439 |
| Retronasal triangle | 128 (92.1) | 66 (94.3) | 62 (89.9) | 0.333 |
| Upper lip | 107 (77.0) | 47 (67.1) | 60 (87.0) | 0.006 |
| Mandible | 113 (81.3) | 55 (78.6) | 58 (84.1) | 0.407 |
| Nuchal translucency * | 81 (58.3) | 41 (58.6) | 40 (58.0) | 0.943 |
| No jugular cysts | 122 (87.8) | 59 (84.3) | 63 (91.3) | 0.207 |
| Shape of thoracic wall | 123 (88.5) | 61 (87.1) | 62 (89.9) | 0.616 |
| Lung fields | 122 (87.8) | 60 (85.7) | 62 (89.9) | 0.456 |
| Diaphragm | 133 (95.7) | 68 (97.1) | 65 (94.2) | 0.394 |
| Heart activity * | 138 (99.3) | 69 (98.6) | 69 (100.0) | 0.319 |
| Situs | 121 (87.1) | 59 (84.3) | 62 (89.9) | 0.328 |
| Position with axis | 119 (85.6) | 58 (82.9) | 61 (88.4) | 0.351 |
| Size | 120 (86.3) | 58 (82.9) | 62 (89.9) | 0.230 |
| Four-chamber view * | 121 (87.1) | 58 (82.9) | 63 (91.3) | 0.138 |
| Left ventricular outflow tract | 100 (71.9) | 46 (65.7) | 54 (78.3) | 0.100 |
| Three-vessel-and-trachea view | 113 (81.3) | 54 (77.1) | 59 (85.5) | 0.206 |
| No tricuspid regurgitation | 108 (77.7) | 51 (72.9) | 57 (82.6) | 0.167 |
| Positive a-wave of DV | 134 (96.4) | 67 (95.7) | 67 (97.1) | 0.661 |
| Stomach * | 133 (95.7) | 65 (92.9) | 68 (98.6) | 0.099 |
| Bladder * | 126 (90.6) | 65 (92.9) | 61 (88.4) | 0.367 |
| Intact abdominal wall * | 137 (98.6) | 68 (97.1) | 69 (100.0) | 0.157 |
| Two umbilical arteries | 126 (90.6) | 65 (92.9) | 61 (88.4) | 0.367 |
| Bilateral kidneys | 129 (92.8) | 65 (92.9) | 64 (92.8) | 0.981 |
| Spine | 135 (97.1) | 67 (95.7) | 68 (98.6) | 0.317 |
| Upper limb * | 139 (100.0) | 70 (100.0) | 69 (100.0) | - |
| Lower limbs * | 139 (100.0) | 70 (100.0) | 69 (100.0) | - |
| Success: Total (40 items) | 60 (43.2) | 29 (41.4) | 31 (44.9) | 0.677 |
| Success: Mandatory (13 items) | 75 (54.0) | 38 (54.3) | 37 (53.6) | 0.938 |
“*” indicates a mandatory item.
Both the univariate and multivariate analyses revealed no significant association between operator experience and the rate of complete scans (Table 4). However, operator experience was associated with a trend toward improved anatomical visualization scores on univariate analysis (Table 5) and was significantly associated with higher visualization scores on multivariate analysis, with an adjusted coefficient of 2.609 (p = 0.010) (Table 5).
Table 4.
Factors influencing complete first trimester fetal anatomy scan.
| Univariable Analysis | Multivariable Analysis | |||||
|---|---|---|---|---|---|---|
| Factors | Odd Ratio | 95%CI | p-Value | a Odd Ratio | 95%CI | p-Value |
| Operator experience | 1.04 | 0.88; 1.22 | 0.679 | 1.07 | 0.092; 1.24 | 0.355 |
| Maternal age | 0.99 | 0.97; 1.00 | 0.129 | - | - | - |
| Nulliparous | 1.04 | 0.88; 1.23 | 0.639 | - | - | - |
| Maternal BMI | 0.99 | 0.97; 1.00 | 0.075 | 0.99 | 0.97; 1.02 | 0.170 |
| Abdominal scar (Control = No scar) | ||||||
|
1.07 | 0.86; 1.33 | 0.535 | - | - | - |
|
1.17 | 0.80; 1.72 | 0.410 | - | - | - |
| GA at first trimester scan | 0.99 | 0.97; 1.01 | 0.390 | 1.00 | 0.98; 1.02 | 0.960 |
| Presentation (Control = Vertex) | ||||||
|
1.08 | 0.87; 1.33 | 0.501 | 1.15 | 0.95; 1.38 | 0.150 |
|
1.67 | 1.37; 2.04 | <0.001 | 1.70 | 1.42; 2.03 | <0.001 |
| Placental location (Control = anterior) | ||||||
|
1.14 | 0.96; 1.35 | 0.125 | 1.14 | 0.98; 1.34 | 0.092 |
| Uterine position (Control = anteversion) | ||||||
|
0.74 | 0.56; 0.97 | 0.032 | 0.82 | 0.69; 0.98 | 0.028 |
| Uterine fibroid (Control = No) | ||||||
|
0.85 | 0.58; 1.24 | 0.395 | 0.85 | 0.62; 1.16 | 0.306 |
|
0.64 | 0.24; 1.70 | 0.368 | 0.77 | 0.58; 1.02 | 0.066 |
|
0.64 | 0.32; 1.28 | 0.204 | 0.95 | 0.81; 1.12 | 0.530 |
BMI, body mass index; GA, gestational age.
Table 5.
Factors influencing scores of first trimester fetal anatomy scan.
| Univariable Analysis | Multivariable Analysis | |||||
|---|---|---|---|---|---|---|
| Factors | Coefficient | 95%CI | p-Value | a Coefficient | 95%CI | p-Value |
| Operator experience | 1.935 | −0.197; 4.068 | 0.075 | 2.609 | 0.619; 4.600 | 0.010 |
| Maternal age | −0.229 | −0.443; 0.015 | 0.056 | - | - | - |
| Nulliparous | −0.064 | −2.228; 2.099 | 0.953 | - | - | - |
| Maternal BMI | −0.199 | −0.396; −0.003 | 0.046 | −0.146 | −0.329; 0.0358 | 0.115 |
| Abdominal scar (Control = No scar) | ||||||
|
−1.365 | −4.182; 1.452 | 0.342 | - | - | - |
|
2.069 | −2.878; 7.017 | 0.412 | - | - | - |
| GA at first trimester scan | −0.018 | −0.287; 0.252 | 0.898 | 0.502 | −0.163; 0.332 | 0.502 |
| Presentation (Control = Vertex) | ||||||
|
1.135 | −1.727; 3.996 | 0.437 | 1.564 | −1.266; 4.395 | 0.279 |
|
6.102 | 3.440; 8.764 | <0.001 | 6.217 | 3.608; 8.825 | <0.001 |
| Placental location (Control = anterior) | ||||||
|
1.169 | −1.015; 3.353 | 0.294 | 0.709 | −1.305; 2.722 | 0.490 |
| Uterine position (Control = anteversion) | ||||||
|
−6.156 | −9.714; −0.598 | 0.001 | −5.139 | −8.471; −1.808 | 0.003 |
| Uterine fibroid (Control = No) | ||||||
|
−0.477 | −5.146; 4.462 | 0.850 | −0.136 | −4.508; 4.237 | 0.951 |
|
−0.620 | −13.396; 12.155 | 0.924 | 0.810 | −10.734; 12.355 | 0.891 |
|
−6.120 | −15.189; 2.948 | 0.186 | −1.650 | −9.858; 6.559 | 0.694 |
BMI, body mass index; GA, gestational age.
Notably, fetal presentation and uterine position significantly influenced the completeness of FTAS according to both the univariate and multivariate analyses (Table 4). Fetal transverse presentation was associated with a higher likelihood of successful examination, with an adjusted odds ratio of 1.70, whereas a retroverted uterus was associated with a slight reduction in the success rate (0.82). Similarly, when the anatomical visualization score was analyzed as the outcome, fetal transverse lie was significantly associated with higher scores, whereas a retroverted uterus was significantly associated with lower scores in both univariate and multivariate analyses (Table 5). Nevertheless, these findings should be interpreted with caution due to the wide confidence intervals, which likely reflect the limited sample size.
Learning curve analysis showed a trend toward improvement of visualization scores based on the parsimonious predictive model of repeated measurement (Table 6). Figure 2 illustrates the learning curves of each operator, expressed as the predicted mean anatomical visualization scores across sequential sets of 10 examinations. For the inexperienced operator, the predicted mean score demonstrated a progressive increase over time, from an initially lower level compared with the experienced operator with a trend to be the same by the end of the study, although the initial difference did not reach statistical significance (p = 0.061). In contrast, the learning curve of the experienced operator remained relatively stable throughout the study period. Nevertheless, the within-operator changes in the learning curves were not statistically significant for either the inexperienced operator (p = 0.082) or the experienced operator (p = 0.241).
Table 6.
Predictive model for visualization scores, using experienced operator as the reference base for comparison.
| Scores | Coef. | Std. Err. | z | p-Value | 95% Conf. Interval |
|---|---|---|---|---|---|
| Inexperienced operator | −4.240 | 2.267 | −1.87 | 0.061 | −8.683 to 0.202 |
| Set number of practices | 0.014 | 0.178 | 0.08 | 0.939 | −0.336 to 0.363 |
| Inexperienced operator × Set number of practices |
0.295 | 0.251 | 1.17 | 0.241 | −0.198 to 0.787 |
| Constant | 36.369 | 1.623 | 22.41 | 0.000 | 33.189 to 39.549 |
Figure 2.
Comparison of learning curve between the inexperienced and experienced operator based on the predictive model of visualization scores.
4. Discussion
Although the rates of complete FTAS were not significantly different between the two operators, the experienced operator showed a consistent trend toward higher complete scan rates, visualization scores, and superior performance during the initial phase of the learning curve. Although operator experience was not statistically significant in the univariate analysis, it remained independently associated with higher visualization scores after adjustment for potential confounding factors in the multivariable analysis. These findings suggest that prior experience has a modest yet statistically significant influence on examination performance following standardized training. Nevertheless, the magnitude of this effect appears limited and is unlikely to be clinically significant. Notably, both operators were able to visualize more than 80% of the fetal anatomical structures according to the ISUOG guidelines within a time limit of 20 min. Although the rates of complete visualization were relatively low (40–45%), both operators had acceptable scores for the mean percentage visualization [16,17,18]. The findings confirmed that FTAS is feasible for obstetricians following standardized training, with acceptable performance achieved even during the early phase of the learning curve.
Both operators were able to visualize almost all mandatory (90%) and optional (70–100%) anatomical structures recommended in the guidelines. Consequently, most fetal anomalies that are detectable in the first trimester can possibly be identified using the transvaginal approach [1,3,4]. However, NT, as one of the mandatory anatomical structures, was successfully visualized and measured in only 58% of cases. This relatively low visualization rate may be attributable to the stringent requirements for NT assessment [5]. Unlike other anatomical structures, NT measurement requires an exact mid-sagittal plane, which often necessitates additional examination time, particularly when the fetus is in an unfavorable position. In contrast, the remaining structures required only visualization without precise measurement. In routine clinical practice, obtaining an optimal NT measurement often requires additional time to allow the patient to rest, to encourage changes in activity, or to schedule a repeat examination. These findings suggest that, despite acceptable overall anatomical visualization following standardized training, a transvaginal FTAS alone may not be sufficient to achieve a complete examination, including NT evaluation, in all patients. Therefore, transvaginal FTAS should be considered a complementary imaging approach rather than a stand-alone examination and may need to be combined with transabdominal scanning or repeat assessment when complete visualization cannot be achieved.
The learning curve analysis demonstrated consistently stable scores for the experienced operator throughout the study period. In contrast, the inexperienced operator showed progressive improvement, with scores initially lower than those of the experienced operator with a trend to improve by the end of the study. Nevertheless, despite the observance of improved performance with increasing practice, the score differences across the study timeline were not statistically significant, even during the initial phase of training. These findings suggest that general obstetricians with prior experience in second-trimester anomaly screening appeared to demonstrate a short learning curve for FTAS and achieved performance levels to those of experienced operators when standardized guidelines are implemented.
A notable finding was that the learning curve of the experienced operator remained stable throughout the study period. This implies that successful anatomical visualization may be limited by several factors, particularly fetal presentation and uterine position, that cannot be modified during the examination, as suggested in previous studies [12]. Although transvaginal ultrasound generally provides superior image resolution compared with transabdominal ultrasound, our recommendation is that both approaches should be used complementarily to maximize anatomical visualization and enhance the opportunity for early detection of fetal anomalies [5,6,12,13,14]. Furthermore, additional examination time may be required in cases with suspected abnormalities to allow for more comprehensive evaluation and confirmation of the findings.
This study has several strengths. To the best of our knowledge, it is among the first studies to evaluate transvaginal ultrasound for FTAS performed by operators with different levels of experience while following the comprehensive ISUOG checklist protocol. The randomized controlled trial design and comparable baseline maternal and fetal characteristics between the two groups helped to reduce potential confounding factors. In addition, both operators underwent standardized training before study initiation, which may have reduced the variability related to the examination technique. Furthermore, the use of a structured 40-item checklist based on the ISUOG guidelines provided a standardized framework for fetal anatomical assessment.
The study also had several limitations. First, the inclusion of only two operators may limit the generalizability of the results. Second, the sample size was relatively small for multivariable analysis, particularly given the large number of potential confounding variables requiring adjustment. Third, interobserver variability in the same participants was not analyzed because of safety and ethical concerns related to prolonged transvaginal ultrasound examination during the first trimester. However, the random allocation of the participants to each study group and the comparable baseline maternal and fetal characteristics between the groups helped minimize potential selection bias and confounding factors.
From a clinical and educational perspective, this study supports the feasibility of FTAS and suggests that it can be incorporated into MFM training programs as either an optional or a routine component of screening practice. From a research perspective, the favorable findings of the present study should be interpreted in light of the aforementioned limitations. Further studies are warranted involving larger sample sizes and a greater number of operators with varying levels of experience to clarify the inconsistent findings and further validate the results. Finally, a dedicated equivalence or non-inferiority study is required to formally demonstrate comparable performance between operators with different levels of experience. Additionally, future research should focus on developing and validating objective quality-assurance tools to further improve the standardization and reproducibility of first-trimester transvaginal fetal anatomical ultrasound. Emerging artificial intelligence (AI)-based technologies, including AI-assisted plane identification and automated fetal anatomical measurements, have shown promise in reducing operator-dependent variability. However, further validation is required to establish their accuracy, reliability, and clinical applicability before routine implementation. Importantly, performance varies across anatomical structures. For example, SonoCNS™ demonstrates good reproducibility for fetal skull biometry (BPD, HC, and OFD) but lower accuracy for intracranial measurements, such as ventricular width, transcerebellar diameter, and cisterna magna [19]. Similarly, the reproducibility of Fetal HQ varies depending on the specific parameters assessed and may be influenced by operator experience and technical factors [20]. Overall, despite rapid advances, AI-based approaches remain insufficiently validated, particularly for routine fetal ultrasound screening.
5. Conclusions
Operator experience had a modest yet statistically significant impact on the performance of first-trimester anatomical scans. No statistically significant differences were observed between less experienced and experienced operators with respect to complete scan rate. Both operators achieved acceptable complete scan rates and anatomical visualization scores of approximately 80%, and similar scan durations. Fetal presentation and uterine version were significant factors associated with scan performance. The results of this study suggest that, in selected operators with previous second-trimester anomaly scan experience and FMF certification, standardized training may enable acceptable mean visualization scores for transvaginal FTAS according to the ISUOG guidelines within a 20-min examination. However, transvaginal FTAS alone did not achieve a high completion rate for the full examination protocol, including NT assessment, and independent validation is required.
Acknowledgments
We thank all the participants for taking part in the study and the Faculty of Medicine Ramathibodi Hospital, Mahidol University, for providing Research Career Development Awards to C.T.
Author Contributions
Conceptualization, P.L. and C.T.; Methodology, P.L., W.D., W.M., W.L., S.D., N.C. and C.T.; Formal Analysis, T.T.; Investigation, P.L. and C.T.; Resources, W.D., W.M., W.L., S.D. and N.C.; Data Curation, P.L.; Writing—Original Draft Preparation, P.L. and C.T.; Writing—Review and Editing, C.T. and T.T.; Supervision, C.T.; Project Administration, P.L.; Funding Acquisition, C.T. All authors have read and agreed to the published version of the manuscript.
Institutional Review Board Statement
The study was conducted in accordance with the Declaration of Helsinki, and approved by Ramathibodi Hospital Institutional Review Board ethics committee (COA, MURA2025/221, date of approval: 10 March 2025) and registered at the Thai Clinical Trials Registry, number 20250309014.
Informed Consent Statement
Informed consent was obtained from all subjects involved in the study.
Data Availability Statement
The datasets generated and/or analyzed during the current study are available from the corresponding author upon direct request.
Conflicts of Interest
The authors declare no conflicts of interest.
Funding Statement
This research was funded by the Faculty of Medicine Ramathibodi Hospital, Mahidol University, Bangkok, Thailand, Grant number R1106801023.
Footnotes
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets generated and/or analyzed during the current study are available from the corresponding author upon direct request.