The popularity of female genital plastic surgery is gradually rising, and so is labiaplasty (1). Enlarged labia minora could cause functional, cosmetic, and psychosocial issues (2). In 1984, the first article describing labiaplasty was published and edge trim techniques were used for three patients who felt the protrusion of the labia minora was aesthetically and functionally unsatisfactory (3). Indeed, the length, width, and color of the labia minora can vary widely. The first study regarding genital dimensions in healthy women was published by Lloyd et al. in 2005 (4). When the transverse distance from the interlabial groove to the widest part of the labia minora is greater than 40–50 mm, it could be diagnosed as labia minora hypertrophy (5). Many plastic surgeons have previously measured and described the female vulva and defined the hypertrophic labia minora (5–8). Nevertheless, there is currently no standard for an aesthetically perfect labia minora (9). Culturally projected interpretation biases toward one's genital appearance could cause concerns compelling patients to seek female genital plastic surgeries (10). The general shape of the labia minora has a far-reaching impact on the patient's willingness to undergo labiaplasty and choices of surgical strategies. Female genital parameters have been surveyed in the United Kingdom, China, the United States, Denmark, India, Australia, Israel, etc. (4, 9, 11–15).

Although plastic surgeons and gynecologists have taken measurements of the labia minora, such as the labial length and labial width (16). No scholars have quantitatively analyzed the morphological characteristics of the labia minora. This study is the first morphological study of the labia minora in a large cohort. To provide a better understanding of the morphological features of the labia minora, we present a descriptive study addressing labia minora measurement. Herein, the shape of the labia minora was converted to numerical values for an in-depth comparative analysis of 400 Chinese adult women.

The purpose of the study is to investigate the morphological features of the labia minora in adult women who had never had genital surgery before the vulvar measurement. After a graphic-to-digital and digital-to-graphic conversion, the average shape of the 400 included patients was constructed (Figure 5). Meanwhile, data were further analyzed to explore the association between the labia minora morphology and factors such as age, parity, and whether they planned to have labia minora surgery.

Female genital cosmetic surgery, particularly labiaplasty, is gaining in popularity (17). However, there has been no unified “aesthetic gold standard” for the shape of the labia minora to date. It is thought that the aesthetics of the female vulva is not determined by one or two structures but by the vulvar region as a whole. Therefore, an aesthetically pleasing labia minora must coordinate with adjacent structures (18). Most previous studies on the size of the labia minora have focused only on the width and length of the labia minora (8, 13, 18, 19). In this study, the shape characteristics of the labia minora were converted into measurable values which can be used for comparative analysis. The values of Ln, Rn, Ln/L(n + 1), and Rn/R(N + 1) reflected the contour of the labia minora and the labia minora characteristics.

The shape features of the labia minora in Chinese adult women could provide a useful supplement to the field of female genital plastic surgery. Simplifying the contours of a body part or organ into geometric figures is a common method for biomorphology (20, 21). In this study, the method used to measure labia minora draws on this strategy. The unit length was set as one-tenth of the longitudinal length of the vaginal vestibule in each patient, making it feasible to compare the shape of the labia minora between patients. The larger the Ln and Rn values, the wider the shape of the labia minora. In contrast, the smaller the Ln and Rn values, the thinner the shape of the labia minora. L1/L2, L2/L3, L3/L4, L4/L5, L5/L6, L7/L8, and L8/L9 represent the trend in the distance between the left labia minora edge and the median axis. Same for Rn/R(n + 1). When Ln/L(n + 1) or Rn/R(n + 1) value is 1, it means that the two adjacent line segments are equal. When Ln/L(n + 1) and Rn/R(n + 1) are greater than 1, the larger the value, the larger the gap between adjacent line segments. On the contrary, when the ratio is less than 1, the smaller the value, the larger the gap between adjacent line segments. Overall, Ln/L(n + 1) and Rn/R(n + 1) determined the protrusion degree of the labia minora when observing the vulva as a whole (Table 1). The average shape of the labia minora in the 400 included patients was constructed based on the mean values of L1–L9 and R1–R9 (Figure 5; Table 1). To some extent, it could represent the general shape of the labia minora in Chinese adult females. The simulated outline of the labia minora was close to a “butterfly wing shape” (Figure 5).

The golden rule is widely used in plastic surgery as a generally accepted aesthetic standard. The labia minora could also refer to the golden ratio to achieve visual beauty (22). For aesthetically pleasing labia minora, there is a golden ratio (1:1.618) between the anterior and posterior labial distances of the labia minora (23). In the present study, the ratio of the posterior labial distance (Line B) and the anterior labial distance (Line A) was 1.498 (7.183/4.794) on the left side and 1.335 (6.942/5.200) on the right side (Figure 5), with the ratio of left labia minora closer to the golden ratio.

The labia minora seemed to acquire a thinner shape with increasing age (Table 2). Nevertheless, no significant differences were found between most age groups when Ln and Rn values were compared, except for two subgroups (Table 2). As a result, it appears that there was no significant relationship between age and labia minora measurements, consistent with previous studies. For instance, Lloyd et al. reported no significant associations between the size of labia minora and age, parity, hormone use, or sexual history (4). In terms of parity, primiparous women (Group 2) had the largest Ln and Rn values (Table 3), implying that the labia minora seemed to be wider for this patient group. In comparison, the smallest value corresponded to Group 3 (number of births ≥2), which means that for females with two and more births, their labia minora appeared to be thinner (Table 3). However, the difference in mean values between the three groups was quite small, and no significant differences were found (P-value >0.05) (Supplementary File 2, Table 3). Patients who planned to undergo labiaplasty usually have a strong subjective desire since labiaplasty is a cosmetic procedure (24, 25). It was mentioned in certain studies that the size of the labia minora was associated with patients’ subjective perception (12, 14, 26). In this study, the association between whether labiaplasty was performed and the shape characteristics of the labia minora was also analyzed (Supplementary File 3). For patients who underwent unilateral or bilateral labia minora surgery, the values of Ln and Rn were larger (Table 4). Their labia minora appeared to be wider and more prominent than patients who did not undergo labiaplasty. However, all P-values were greater than 0.05 and there was no significant difference between Group I and Group II (Table 4). It seems to contradict other previous studies (12, 14, 26). While the exact cause is not known, we speculate that this may be due to uneven grouping and a small sample size in a single medical center. Moreover, the labia minora surgery is not only determined by the shape of the labia minora but also the associated physical symptoms and the patient's wishes. There are many patients seeking labiaplasty who have no physical symptoms, such as hygiene problems, tugging during intercourse, uncomfortable wearing tight clothing, pain during intercourse, erosions, etc. (1, 2, 27). They seek labiaplasty to achieve their “ideal labia minora”. Therefore, patients scheduled for labiaplasty may not have more hypertrophied labia minora than those who were not going to have labiaplasty.

The vaginal vestibule, as an inseparable structure from the labia minora, refers to the area between the two sides of the labia minora (11). The aesthetics of the labia minora is not only associated with the specific value of the width of the labia minora (28). Even if two patients have the same labia minora width, if their vaginal vestibule lengths differ, the shape and aesthetics of the labia minora will also differ. The labia minora and the adjacent vestibule should therefore be regarded as a whole (19, 29). Various factors such as weight and height may affect measurements of labia minora, so a direct comparison of the labia minora width could be uncomprehensive (15, 30, 31). The measurement strategy in the study is helpful to reveal the overall shape features. To our knowledge, no previous studies have quantified the shape features of the labia minora. Related studies usually focused on subjective perceptions, measurements of local structure, and rough generalizations. Herein, we further grouped and analyzed 400 patients at three levels (age, parity, and whether the labiaplasty going to perform). Independent samples t-tests were performed to explore whether the differences between the groups were significant.

Currently available classifications for labia protrusion are based on the distance between the lateral edge of the labia minora and the labia majora, or the distance between the base of the labia minora and the distalmost tip (19). Our finding could help plastic surgeons gain a direct visual cognition and understanding of the labia minora in Chinese adult women. It would be helpful to provide more guidance to plastic surgeons. Even if two patients have the same labia minora width, if the shape of the labia minora differs, the aesthetics of the labia minora and patients’ perceptions will also differ. We believe that the study based on quantifiable data and analysis is important for the further exploration of labia minora morphology.

Despite the fact that the study was only based on retrospective photographs, the combination of in-depth data analysis by image processing software gives this study an advantage over other studies. Although the photographs were not taken from the same distance all the time, the relative sizes of each part of the labia minora were not affected when calculating the values of Ln, Rn, Rn/R(n + 1), and Ln/L(n + 1). Another shortcoming of the study is that although we included patients with different clinical characteristics and surgical plans, bias due to population selection still exists. Since patients were obtained from a single center, the data presented here might be limited and caution is warranted when interpreting our findings. In the future, to correlate morphologic characteristics with surgical outcomes, clinical applications may be required to establish its utility. More large-scale studies from multi-centers are required to further validate our results. New criteria and references for labial morphology may develop subsequently.

The average shape of labia minora in 400 Chinese adult women could provide a visual reference to plastic surgeons. The shape of labia minora was not significantly associated with age, parity, or whether or not the patient was going to undergo labiaplasty. The shape characteristics of the labia minora were converted into numerical values and a new method was provided for measuring and evaluating the shape of the labia minora. Collectively, our findings could be useful for a better understanding of the morphology of labia minora.