Pelvic floor structural changes during the first singleton pregnancy and the risk factors of stress urinary incontinence

Abstract

Background

Pelvic floor dysfunction (PFD) usually occurs due to issues with the pelvic floor muscle and fascia tissue. Although many studies have shown that vaginal childbirth is a risk factor, little is known about the effects of pregnancy. Stress urinary incontinence (SUI) is a common symptom of PFD, characterized by involuntary urine discharge when abdominal pressure increase, such as sneezing, coughing, laughing or exercising. We aimed to observe pelvic floor ultrasound parameters during pregnancy and assess the association with SUI.

Methods

Pregnant participants were recruited from the Affiliated Obstetrics and Gynecology Hospital of Tongji University between March 2022 and December 2023. We assessed pelvic floor anatomy with three-dimensional ultrasound and administered standardized questionnaires to all participants. After conducting the assessments, we saved the volumetric data sets for standardized analysis.

Results

A total of 385 pregnant women were included, with 131 in the first trimester group; 122 in the second trimester group, and 132 in the third trimester group. SUI occurred 7 cases (5.34%) in the first trimester group, 26 cases (23.31%) in the second trimester group, and 63 cases (47.72%) in the third trimester group, the incidence of SUI in different groups with statistically significant (P < 0.001). As the pregnancy advanced, there were significant differences in HA, hiatal anteroposterior diameter, transverse diameter at rest, PFMC and VM (P<0.05). There were significant differences in hiatal transverse diameter at PFMC in the third trimester between women with SUI and those without (P = 0.048). However, no differences were observed in ultrasonic pelvic floor parameters in the first trimester between women with SUI and those without (P>0.05). We performed logistics regression analysis to evaluate the likelihood of SUI, the model was statistically significant (P < 0.001, AUC = 0.778, 95%cl: 0.729–0.827). Our analysis identified BMI, hiatal area at rest, hiatal anteroposterior diameter at rest, and hiatal anteroposterior diameter at VM as independent risk factors for SUI.

Conclusions

Pelvic floor structure begin to change during pregnancy, as the pregnancy advanced, the hiatal area increased gradually.

Background

Pelvic floor dysfunction (PFD) is a common disease, involving pelvic organs location abnormality and dysfunction resulting from abnormalities in the pelvic floor muscle and fascia tissue. It often presents with symptoms related to urination, defecation and sexual dysfunction, significantly affecting the quality of life [1,2,3].

Currently, the known risk factors for pelvic floor dysfunction diseases include pregnancy and vaginal delivery. Common symptoms during pregnancy include frequent urination and nocturia, with an incidence of about 75-77% [4]. Stress urinary incontinence (SUI) is the most prevalent type of urinary incontinence during pregnancy, characterized by involuntary urine discharge during abdominal pressure increase such as sneezing, coughing, laughing or exercising, with an incidence of about 18-75% [5–6]. Obesity before pregnancy and aging can increase the risk of urinary incontinence during pregnancy, and the incidence of SUI decreases to about 30% after the three months postpartum [7]. In particular, vaginal delivery increases the risk of pelvic floor dysfunction, which may be related to the pelvic nerve damage and pelvic floor muscle laceration.

Currently, the mechanism of urinary incontinence during pregnancy remains incompletely clear, however, the alterations in pelvic floor anatomy structure during pregnancy have attracted the attention from researchers. Three-dimensional transperineal ultrasound is capable of real-time assessment of morphological changes in pelvic floor anatomy, with the advantages of non-radiation, minimal invasiveness and high precision. Presently, three-dimensional transperineal ultrasound is the preferred examination method for evaluating pelvic floor function [8,9,10]. In recent years, several studies have investigated the predictive value of transperineal ultrasound in SUI, although the data remains limited. This study attempted to combine ultrasound parameters with clinical factors to explore the relationship between the changes of pelvic floor anatomy and clinical symptoms during pregnancy.

Methods

Three-dimensional transperineal ultrasound examination was performed for each participant, using a Voluson E8 (GE Healthcare) equipped with a RAB6-D probe operating at a frequency of 4-8 MHz. Participants were categorized into first trimester (10–13 weeks), second trimester (26–28 weeks), and third trimester (35–38 weeks) groups according to the time of receiving pelvic floor ultrasound examination. A questionnaire survey was conducted for each participant. Prior to the examination, participants were instructed to empty their bladder and rectum, assume the lithotomy position, and then placed the probe between the two labia majora to obtain the median sagittal section. The requirements of the median sagittal section of the pelvic floor: the posterior lower margin of the pubic symphysis is taken as the reference point, and it mainly includes the pubic symphysis, urethra, bladder neck, vagina, cervix, rectum, rectal ampulla and anal canal from front to back. The position of pelvic organs in centimetres at rest and at maximal Valsalva maneuver (VM) were measured by 2D ultrasound. The area, transverse diameter, anterposterior diameter of hiatal area (HA) were measured at rest, VM, and pelvic floor muscle contraction (PFMC) by 3D ultrasound.

During calm breathing, the positions of bladder neck, cervix, rectal ampulla and urethral inclination were measured by 2D ultrasound. Additionally, three-dimensional ultrasound was, employed to measure the HA, transverse diameter, and anterposterior diameter of the anal levator hiatus. Furthermore, when pregnant women performed PFMC, three-dimensional ultrasound was utilized to measure HA, transverse diameter and anterposterior diameter of anal levator hiatus. At VM, the position of bladder neck, cervical position, rectal ampulla position, posterior bladder Angle, urethral rotation Angle, and bladder neck movement were measured by two-dimensional ultrasound; three-dimensional ultrasound were used to measure the HA, transverse diameter and anteroposterior diameter of levator anal hiatus.

All the USG volume data were analyzed for the following parameters by an experienced sonographer, with the investigator blinded to the pelvic floor symptoms to ensure objectivity. A horizontal line was drawn using the postero-inferior edge of the pubic symphysis as the reference point and was used as the reference line. The vertical distances from the bladder neck, cervix (defined as the inferiormost part of the uterine cervix), and the anorectal junction (defined as the anorectal junction at anterior wall of the anorectum) to the level of the reference point at rest and VM. Movement of the pelvic organs away from the reference point (proximal displacement) was negative, and movement towards the reference point (caudal displacement) was positive. Moreover, the axial plane of the hiatal dimension was used for the measurement of hiatal area, anteroposterior (AP) and transverse.

Participants

Women with prenatal care at Obstetrics and Gynecology Hospital of Tongji University from March 2022 and December 2023 were recruited to the study. Informed consent was obtained from all participants. All participants were the first pregnancy with a singleton, each participant never had uterine operation history. Exclusion criteria: ① uterine malformation; ② history of gynecological surgery; (3) heart disease, high blood pressure and other organ or system diseases; ④ smoking history; ⑤ history of high-intensity exercise; ⑥ urinary system infection or abnormal of leucorrhea examination; ⑦BMI > 30 kg/m².

Statistical analysis

Statistical analysis was performed with SPSS version 22.0. Univariate analysis of variance was used for inter-group analysis, and unpaired two-tailed Student’s t–test was used for intra-group comparison, and categorical variable of subjects were compared using the Chi-square test. Fisher exact test was used to analysis when the number of cases was < 5. Logistic regression analysis was performed to assess the impact of pelvic floor biometry as factors on the likelihood of antenatal pelvic floor disorders, and P < 0.05 was considered to be statistically significant.

Result

General clinical data

In all, 385 women were enrolled in the study, with a mean age was 32.02 ± 3.79 years, including 131 in the first trimester, 122 in the second trimester, 132 in the third trimester. There was no significant difference in the age among the three groups (P = 0.122). The average BMI of the second trimester and third trimester group were higher than the first trimester group (P < 0.001), while there was no difference between the second trimester and third trimester (P > 0.05) (Table 1). There was no statistically significant difference in age between the SUI group and the non-SUI group in the first and second trimester groups (P > 0.05), however, in the third trimester, pregnant women with SUI were older than those without SUI (P = 0.046). There was no significant difference in BMI between the pregnant women with SUI and those without SUI in the third trimester (P > 0.05) (Table 2).

Table 1 Comparison of general clinical data

Table 2 Comparison of general clinical data between SUI and non-SUI

Pelvic floor ultrasonic parameters during pregnancy (Table 3) (Fig. 1)

Table 3 Pelvic floor biometry during pregnancy

Fig. 1

Measurement of hiatal (1 area; 2 transverse diameter; 3 anterposterior diameter)

At rest

At rest, there was statistically significant difference in HA (11.32 ± 1.98 versus 13.33 ± 2.19 versus 14.40 ± 2.49, P<0.001), AP diameter (47.38 ± 5.26 versus 51.59 ± 6.24 versus 54.65 ± 6.33, P<0.001)and transverse diameter (32.54 ± 4.49 versus 34.76 ± 3.28 versus 36.10 ± 4.59, P<0.001) among the first, second and third trimester. As the pregnancy progressed, these measurements gradually increase. But there was no significant difference in the angle of pubic arch (262.05 ± 12.71 versus 263.87 ± 10.42 versus 264.22 ± 10.90, P = 0.596).

At PFMC

The results were consistent with those at rest, as the pregnancy progressed, these measurements gradually increase, HA (9.01 ± 0.94 versus 10.80 ± 1.88 versus 11.26 ± 2.11, P<0.001), AP diameter (41.47 ± 5.92 versus 45.60 ± 6.19 versus 46.77 ± 5.62, P<0.001)and transverse diameter (29.66 ± 4.35 versus 31.65 ± 2.90 versus 32.99 ± 4.39, P<0.001). But there was also no significant difference in the angle of pubic arch (263.37 ± 15.16 versus 265.72 ± 11.25 versus 264.07 ± 12.63, P = 0.891 ).

At VM

At VM, the results were consistent with those at rest and at PFMC, HA (13.85 ± 3.61 versus 17.10 ± 4.22 versus 18.51 ± 4.52, P<0.001), AP diameter(51.30 ± 7.84 versus 59.14 ± 9.13 versus 60.50 ± 8.33, P<0.001), transverse diameter (36.16 ± 4.79 versus 38.20 ± 4.29 versus 40.19 ± 5.54, P<0.001), the angle of pubic arch (259.95 ± 24.14 versus 262.34 ± 12.38 versus 262.83 ± 11.46, P = 0.891). There findings confirmed again that there was a significant change in pelvic floor biometry as the pregnancy progressed.

Compare the position of organs

At rest, differences in the position of the bladder neck and rectal ampulla at different trimesters were observed, with P-values of 0.06 and 0.045, respectively. At VM, differences in the position of the bladder neck, cervix and rectal ampulla at different trimesters were observed, with P-values of less than 0.001, 0.008 and 0.045, respectively.

Pelvic floor ultrasonic parameters in SUI and non-SUI (Table 4)

In the third trimester, a statistically significant difference was found in the transverse diameter at PMFC between SUI and non-SUI (P = 0.048). However, there was no statistically significant difference in pelvic floor ultrasonic parameters between SUI and non-SUI in the first and second trimester (P > 0.05).

Table 4 Pelvic floor ultrasonic parameters in SUI and non-SUI

Compare the incidences of organs position abnormality and SUI during the pregnancy with Chi-square test (Table 5)

According to the guidelines [11], the posterior bladder Angle ≥ 140°, the urethral rotation Angle ≥ 45°, and the urethral inclination Angle ≥ 30° are abnormal. The results indicated significant differences in the incidences of SUI (P < 0.001), abnormal posterior bladder angle (P < 0.001), and abnormal urethral rotation angle (P = 0.019) among the three trimesters. Notably, the incidences in the second and third trimesters were higher than in the first trimester. (Fig. 2).

Table 5 Compare the incidences of organs position abnormality and SUI during the pregnancy

Fig. 2

Measurement of bladder/uterus/rectum position by transperineal ultrasound (1. Reference line; 2. Bladder neck position; 3. Cervix position; 4. rectal ampulla position)

Logistics Regression Analysis (Table 6)

Risk factors for SUI, such as age, BMI, and HA during the pregnancy, were utilized to create a predictive model with Logistics Regression Analysis. There was statistically significant difference of the model (P<0.001, AUC = 0.778, 95%cl = 0.729–0.827) (Fig. 3), and the BMI, HA at rest, AP diameter at rest and at VM were independent risk factors for SUI (P < 0.05).

Table 6 Logistics regression analysis

Fig. 3

Predictive value of ultrasound parameters combined with age and BMI in SUI

Discussion

SUI, a symptom of pelvic floor disorders, is characterized by uncontrolled urine discharge that occurs during activities that increase intra-abdominal pressure, such as sneezing, coughing, laughing, or exercising. Childbirth is a known risk factor for PFD, and pregnancy can induce structural changes in the pelvic floor due to the hormonal influence and the increased uterine weight [12]. However, the full impact of pregnancy on the pelvic floor structure has not been completely evaluated. Our study attempted to use 3D-ultrasound to evaluate the change in the pelvic floor structure at different stages of pregnancy. Consequently, the findings revealed a gradual increase in the incidence of SUI as pregnancy progressed. The incidence of SUI in the third trimester of pregnancy in our study was approximately 47.72%, which was similar to the 50.5% reported by Claudia PF et al. [13], but exceeded the 36.5% reported by Chan SS et al. [14]. The difference in SUI incidence among various studies can be attributed to the diversity of study population, including disparities in race, cultural backgrounds, education levels and age. Furthermore, the use of different assessment methods in various studies may also contribute to difference in results.

Some studies have indicated a positive correlation between pregnant women’s weight and various pelvic floor parameters, such as the AP diameter and HA at rest, PMFC and VM [15]. In our study, there was no significant difference in age among the three trimesters. The BMI and the incidence of SUI were both higher in the second and third trimester compared to the first trimester. However, no significant difference in BMI was observed between the second and third trimester. Even though there was no difference in age and BMI between the second and third trimester, the incidence of SUI between the two groups was still different. Hence, we can assume that pregnancy plays an important role in the occurrence of SUI.

A Norwegian study found that the HA was significantly enlarged at 37 weeks of gestation compared to 21 weeks, and there was an increase in the motion of the bladder neck [16]. In addition, in a recent study, 3D-ultrasound was used to compare pelvic floor parameters across various stages of pregnancy and postpartum, including the first trimester, second trimester, third trimester, 8weeks postpartum, 6 months postpartum and 12 months postpartum. The results indicated that the positions of bladder neck, anorectal junction and cervix at VM were significantly lower in 12 months after caesarean section compared to those in the first trimester [17]. Similar to the findings of previous study, our study also revealed a significant descent of the cervix at VM as pregnancy progressed, specifically, the cervix was lower in the second and third trimester compared to the first trimester. Moreover, the incidence of abnormal posterior bladder angle and urethral rotation angle in the second and third trimester were higher than those in first trimester. Those studies all shown that even without vaginal delivery, pregnancy can cause changes in pelvic floor anatomy.

When compared the SUI group with the non-SUI group across the three trimesters based on the timing of SUI onset, our study found that there was no significant difference between the groups in the first trimester. There was no significant change in pelvic floor structure during the first trimester, indicating that SUI maybe related to other factors beyond pelvic floor structure, or the appearance of clinical symptoms may precede the structural changes in the pelvic floor. The observation that some pregnant women experienced SUI despite no significant difference in pelvic floor anatomy suggests that SUI may also be related to pelvic floor muscle function, particularly factors such as muscle tissue compliance/elasticity. These properties of the muscle tissue could play a crucial role in the development of SUI, even when pelvic floor anatomical structures remain relatively unchanged. All subjects in our study were first pregnancy, and the other potential risk factors such as childbirth and surgery were excluded, which further supports the existence of individual differences in the development of SUI. As the progression of pregnancy, there was significant difference in the transverse diameter of the levator anal hiatus between SUI and non-SUI groups in the third trimester, and were associated with the increased incidence of SUI.

In addition, our study also collected pelvic floor ultrasound parameters and attempted to develop a predictive model for SUI. The result indicated that the BMI, HA at rest, AP diameter at rest and at VM were identified as independent risk factors for SUI.

Currently, most studies on SUI have predominantly focused on postpartum women, often neglected the changes in pelvic floor anatomy during pregnancy. Our study mainly analyzed the impact of pregnancy on the anatomical structure of the pelvic floor, which can help the clinicians to provide better guidance to pregnant women and potentially reduce the incidence of SUI postpartum. This approach not only highlighted the importance of considering pregnancy as a critical period for pelvic floor health but also emphasized the need for targeted interventions during this time. However, the subjects in our study were pregnant women in three trimesters respectively, rather than the same subjects at each trimester, which increased the variability introduced by individual differences. In the follow-up study, we will analyze the changes of pelvic floor structure at each trimester of the same pregnant woman to minimize the influence of individual differences on the results.

In conclusion, our study has confirmed the changes in pelvic floor structure during pregnancy and developed a predictive model for SUI. This model is anticipated to be a valuable tool for early prediction of SUI and help clinicians to design personalized interventions during pregnancy to mitigate the risk of SUI.

Limitation

Selection bias may be introduced due to differences in baseline characteristics among the groups, although unmeasured confounders cannot be completely excluded by statistical methods. The relatively small sample size may lead to insufficient statistical power and limited reliability of some subgroup analysis results.