Female pelvic floor dysfunction (PFD) is caused by a variety of factors, including structural relaxation of levator and ligament tissues that are responsible for pelvic floor support, as well as levator avulsion1,2, which disallows support for the normal anatomical positions of pelvic organs, leading to changes in the anatomical positions of the bladder, uterus and rectum, ultimately triggering abnormalities in the morphology, structure and function of other pelvic organs. Clinically, it manifests itself as stress urinary incontinence (SUI), pelvic organ prolapses (POP; including prolapses of anterior and posterior vaginal walls, uterine and rectal ampulla) and other diseases, which significantly affect the physical and mental health, as well as the quality of life of patients although not life-threatening3, and thus have become a public health issue of global concern. As suggested by extensive relevant epidemiological studies4,5, pregnancy and delivery are the major causes of PFD, and different methods of delivery lead to varying degrees of PFD. Pregnancy and delivery have also been considered as the independent risk factors for PFD6. With the improvement of medical level, ultrasound technology has gradually developed and matured, which has been widely applied in clinical practice. Owing to a high detection rate in the diagnosis of postpartum PFD, it seems necessary to take pelvic floor ultrasound in the early postpartum period, which facilitates the implementation of clinical treatment measures as early as possible to improve the prognosis. This study aims to observe the pelvic floor anatomical structure and various ultrasound indicators of primiparas by transperineal ultrasound, so as to evaluate the effects of different delivery methods on the pelvic floor function, with a view to providing more evidence and guidance for early clinical diagnosis and intervention of PFD, thereby lowering the risk of long-term pelvic floor injury.

Data and methods

General information

A total of 92 primiparas, who gave birth in the Maternal and Child Health Hospital Affiliated to Nantong University from August 2022 to August 2023 and underwent postpartum reexamination and transperineal pelvic floor ultrasound at the outpatient department 6–8 weeks after delivery, were enrolled. The modes of delivery include vaginal delivery and elective cesarean delivery. Selective cesarean section refers to the termination of pregnancy by cesarean section due to maternal or fetal factors before the start of labor. The inclusion criteria were as follows: (1) Primiparas with full-term single birth; (2) No history of genitourinary tract infection or pelvic surgery; (3) Thoroughly eliminate lochia; (4) Cooperative in completing effective Valsalva maneuver. Exclusion criteria: (1) Those complicated by urinary, reproductive infections and diagnosed with PFD before delivery; (2) Those with grade III or above perineal laceration during labor and those with protracted or prolonged second stage of labor7.(The total stage of labor exceeds 24 h or the second stage of labor for primiparous women exceeds 3 h and primiparous women with Painless delivery exceeds 4 h); (3) A previous history of pelvic surgery; (4) A history of chronic cough and constipation; (5) Those incapable of completing the effective Valsalva maneuver (lasting at least six seconds) and the anal constriction (lasting less than three seconds) after doctor’s guidance; (6) Those complicated by other systemic diseases. The 92 primiparas enrolled were divided into the vaginal delivery group (52 cases, 1 case of forceps assisted delivery and 3 cases of episiotomy) and the cesarean delivery group (40 cases) depending on the method of delivery. Basic information, including age, pre-pregnancy BMI, neonatal weight, delivery method and presence or absence of urinary incontinence during and after pregnancy, were recorded.

Methods

NuewaR9 T color Doppler ultrasound system (Mindray Bio-Medical Electronics, Shenzhen) was utilized for performing ultrasound examination. Transabdominal 3D volumetric SD8-1U probe was used for scanning over a frequency range of 1–8 MHz, and the maximum scanning angle was 85°. The primiparas were instructed to empty their bladders 10 min before the examination. During the examination, the primiparas were placed in the bladder lithotomy position, the transabdominal 3D volumetric probe, whose surface was applied with coupling agent and covered with a condom, was placed between the labia majora on both sides of the perineum, and the standard images for median sagittal plane of pelvic floor were obtained by using the lower margin of pubic symphysis as the reference line. The 2D, 3D and 4D volumetric images were collected, measured and stored separately in the resting, maximum Valsalva and constrictive anal states, and then the obtained images were transferred to the workstation for analysis of images and data information via the 4D Pelvic Floor software. All the operations were completed by practitioners who have passed the training for pelvic floor ultrasound, and all the data were measured in triplicate and averaged to serve as the final results.

Ultrasound measures

(1) The levator hiatus areas in resting, constrictive anal and maximum Valsalva states; (2) The posterior vesicourethral angles and the distances between vesical neck and posterior inferior margin of pubic symphysis in resting and maximum Valsalva states ; (3) The vesical neck mobility and urethral rotation angle in maximum Valsalva state; (4) Inter-group comparison of PFD was made, the funnel-like changes in internal urethral orifice were observed, and the presence or absence of SUI, POP were recorded.

Normal reference values

The normal ranges of posterior vesicourethral angle were set to be < 140° (Valsalva state) and < 110° (resting state), respectively. When the angle exceeded 140°, the posterior vesicourethral angle was considered open, which was often accompanied by the funnel-like changes in internal urethral orifice. The normal range of levator hiatus area was < 20 cm2 in maximum Valsalva state, and the levator hiatus expansion was ≥ 20 cm² during maximum Valsalva maneuver, which reflected the weakening of levator functional status and elasticity. The normal range of vesical neck mobility was <20 mm, and the mobility was considered slightly large when its range was 20–25 mm. A vesical neck within 10 mm of the marker line was defined as mild vesicocele. A vesical neck mobility of > 25 mm was considered large and was defined as evident vesicocele. The normal range of urethral rotation angle was < 45°. The normal uterus, bladder and rectum were all located above the reference line of the posterior inferior margin of pubic symphysis8.

Statistical processing

Data were statistically analyzed via the SPSS 22.0 software. Measurement data tested by P-P chart and conforms to a normal distribution were presented as \(\:\stackrel{-}{x}\)± s, and their inter-group comparison was made by independent t-test. Count data were expressed as n (%), and their inter-group comparison was made by χ2 test. Differences were considered statistically significant when P < 0.05.

Results

No significant differences in the age, pre-pregnancy BMI or neonatal weight were found between the primiparas in the two groups (P > 0.05). Details are listed in Table 1.

Table 1 Inter-group comparison of general information.
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Based on whether the levator ani muscle is damaged, the incidence of levator ani muscle damage primarily occurs in the vaginal delivery group, with a rate of 15.38%. There is a close correlation between newborn weight and levator ani muscle damage, where a higher newborn weight increases the risk of developing levator ani muscle damage.Details are listed in Table 2.

Table 2 Comparison of general conditions between group with intact levator ani muscle and group with damaged levator ani muscle.
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The vaginal delivery group exhibited significantly larger levator hiatus areas in the resting, constrictive anal and maximum Valsalva states compared to the cesarean delivery group (P < 0.05). Details are listed in Table 3.

Table 3 Inter-group comparison of levator hiatus areas in resting, constrictive anal and maximum Valsalva states (\(\:\stackrel{-}{x}\) ± s).
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Compared to the cesarean delivery group, the vaginal delivery group exhibited significantly greater posterior vesicourethral angle and distance between vesical neck and posterior inferior margin of pubic symphysis at both resting and maximum Valsalva (P < 0.05). The distances between vesical neck and posterior inferior margin of pubic symphysis at both resting and maximum Valsalva were greater in the cesarean delivery group than in the vaginal delivery group, with that at maximum Valsalva showing significant inter-group difference (P < 0.05). Details are listed in Tables 4 and 5.

Table 4 Inter-group comparison of posterior vesicourethral angle in resting and maximum Valsalva states (\(\:\stackrel{-}{x}\) ± s).
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Table 5 Inter-group comparison of distance between vesical neck and posterior inferior margin of pubic symphysis in resting and maximum Valsalva states (\(\:\stackrel{-}{x}\) ± s).
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The vaginal delivery group exhibited greater vesical neck mobility and urethral rotation angle at maximum Valsalva compared to the cesarean delivery group, showing statistically significant differences (P < 0.05). Details are listed in Table 6.

Table 6 Inter-group comparison of vesical neck mobility and urethral rotation angle at maximum Valsalva (\(\:\stackrel{-}{x}\) ± s).
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Comparison of pelvic floor functional injury in parturient women under Valsalva condition, the incidence of pressure incontinence/urethral infundibulation, bladder prolapse and uterine prolapse in the vaginal delivery group were higher than those in the cesarean section group, but there was no statistical significance in the incidence of urethral infundibulation and uterine prolapse between the two groups (P > 0.05). There were significant differences in the incidence of stress incontinence and bladder prolapse between the two groups (P < 0.05). Details are listed in Table 7.

Table 7 Inter-group comparison of PFD (\(\:\stackrel{-}{x}\) ± s).
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Discussion

Female pelvic floor is an integral structure comprising pelvic muscle group, bone, connective tissue, nerves and organs. Damage to any of these tissues and structures will affect the pelvic floor function to cause PFD. During pregnancy, the weights of fetus and fetal appendage gradually increase over time, leading to increased weight of the uterus, the pelvic floor tissue will be compressed, leading to its stretching to result in the relaxation and gradual weakening of connective tissue ligament. During delivery, the fetal pressure on the pelvic floor supporting tissue increases, and the pelvic floor tissue expands continuously, resulting in mechanical injury. All the above factors are risk factors for PFD. Pregnancy and delivery, as the major causes of structural and functional damage to the maternal pelvic floor support, may further lead to the occurrence of PFD, resulting in a series of symptoms like SUI, POP, sexual dysfunction and fecal incontinence9,10,11,12,13. During pregnancy14,15 and delivery, structural and functional damage to the pelvic floor is inevitable regardless of what kind of pregnancy method is adopted. However, there is no unified conclusion in the clinical setting regarding the degrees of influences of these two methods. In this study, transperineal ultrasound was employed to quantitatively analyze the pelvic floor function of primiparas with different delivery methods, and to find the factors associated with PFD, with a view to achieving early diagnosis and intervention, delaying or avoiding PFD progression, and providing guidance for subsequent pregnancies to avoid the occurrence of severer pelvic floor injury.

Levator anal muscle is the most important muscle group in the pelvic floor supporting system, which supports pelvic organs and maintains their normal positions. The hiatus of levator anal muscle is formed by the bilateral levator anal muscles and the anterior pubic ramus. It is the largest portal in the peritoneum and the main path of pelvic organ descent. The integrity of anal levator muscle and the morphology of its hiatus can reflect the ___location and structural changes of pelvic organs. Meanwhile, the size of hiatus of levator anal muscle can reflect the compliance and elasticity of pelvic floor. In this study, it was discovered that the risk of levator ani muscle injury during vaginal delivery is significantly elevated, particularly for mothers giving birth to larger infants. This is attributed to the stretching of pelvic floor muscles during delivery, which enlarges the levator ani hiatus, potentially causing tearing or even rupture of the muscle during the birth of the fetus. Levator hiatus is composed of the pubic symphysis, the left and right pubic rami and the levator group. As a vital supporting structure of the pelvic floor, it not only supports the pelvic and abdominal organs, but also participates in the physiological functions like the excretion of pelvic organs and the coordinated control of urination. Levator hiatus is a relatively weak site in the pelvic floor supporting tissue. During labor, the pelvic floor muscles extend, and the levator hiatus enlarges to allow the delivery of the fetus. Measurement of its area can effectively reflect the levator function and the degree of pelvic floor structure relaxation. The smaller the measured value, the milder the pelvic floor relaxation and the lower the risk of PFD16. This study found that the levator hiatus area increased regardless of the delivery method chosen, which was more significant in the vaginal delivery group than in the cesarean delivery group. The inter-group differences in levator hiatus area at resting and maximum Valsalva were statistically significant, indicating that the delivery method would affect the hiatus histomorphology. Suggestively, delivery causes great positional changes in the pelvic organs, and cesarean delivery has a certain protective effect on pelvic structure and function compared to the vaginal delivery, which agree with the previous findings17,18. The mechanism whereby pregnancy and delivery cause increase in the levator hiatus area may involve the increasing pressure of the enlarging fetus on the pelvic floor tissue (affecting the blood supply of the pelvic floor connective tissue), along with the indirect effect of increasing hormones during pregnancy19, which lead to the relaxation of the pelvic floor tissue to ultimately enlarge the area of levator hiatus formed by the levator group and fascia. Another study20 found that during vaginal delivery, the fetus was delivered through the levator hiatus, and the pelvic floor muscle stretching and expansion were 1.47 times that in the non-delivery state. The levator group was overstretched during vaginal delivery, even exceeding the physiological limit, and levator tearing was detected in some women after vaginal delivery21, which subsequently developed into PFD.

In this study, statistically significant differences were found in the posterior vesicourethral angle measured at rest and in the posterior vesicourethral angle, the distance between vesical neck and posterior inferior margin of pubic symphysis, the vesical neck mobility and the urethral rotation angle measured after maximum Valsalva maneuver between the two delivery groups (P < 0.05). Women who underwent vaginal delivery exhibited lower positions of vesical neck and internal cervical orifice than those in the cesarean delivery group, as well as greater posterior vesicourethral angle, vesical neck mobility and urethral rotation angle, showing significant differences (P < 0.05). This might be associated with the greater degrees of decline in the vesical and uterine positions after vaginal delivery, which further verifies alteration in the pelvic floor muscle during the fetal passage through the birth canal due to excessive extension of its elastic function.

It was found that when the lowest point of vesical neck was closer to the horizontal line of the posterior inferior margin of pubic symphysis, the bladder descension was greater and the bladder prolapse was severer. This might be attributed to the passive stretching of pelvic muscle group supporting the bladder position by external forces due to the downward pressure of the fetal head during delivery, which significantly affected the bladder (located in the anterior pelvic cavity) alteration in the pelvic position. Vesical neck mobility is a crucial functional indicator for the supporting structures around the bladder and urethra. The smaller its value, the milder the injury of surrounding supporting tissues. This study revealed that the posterior vesicourethral angle, vesical neck mobility and urethral rotation angle at maximum Valsalva of women in the vaginal delivery group were all greater than those in the cesarean delivery group, indicating that compared to the cesarean delivery, the vaginal delivery caused severer damage to the vesical neck of primiparas. This was due to the following three reasons: Firstly, during vaginal delivery, the fetus descended through the birth canal, directly squeezing the bladder located in front of the uterus, leading to the fetal displacement and tilt to affect the pelvic floor function. Secondly, during labor, the fetal head directly acted on the pelvic floor muscle group such as the levator muscle, so that the entire group was stretched to cause mechanical injury. Thirdly, the prolonged compression of fetus and fetal appendage during labor might reduce the blood supply to the pelvic floor support structure, leading to its hypoxia and ischemia to affect the pelvic floor muscle group and fascia, thereby impacting the pelvic floor structure and function. During cesarean delivery, the fetus was delivered through the abdominal incision, which greatly reduced the impact on the pelvic floor tissue compared to the fetal head descension, perineal laceration and forceps traction during vaginal delivery. Thus, vaginal delivery caused greater damage to the pelvic floor than cesarean delivery. However, at the present stage, it remains controversial whether cesarean delivery can be adopted for protecting the pelvic floor function to lower the occurrence of long-term PFD.

This study also found that the incidences of SUI, urethral funnel formation, bladder prolapse and uterine prolapse in the vaginal delivery group were all higher than those in the cesarean delivery group. Among them, the inter-group differences in SUI and bladder prolapse were statistically significant (P < 0.05), and the vaginal delivery group exhibited higher incidence of urethral funnel formation during Valsalva maneuver compared to the cesarean delivery group. Some studies have considered the urethral funnel formation as a marker of urethral sphincter dysfunction, which could evaluate and predict the occurrence of SUI22,23, showing consistency with the present results as well. In this study, the incidence of PFD was higher in the vaginal delivery group than in the cesarean delivery group. Nonetheless, when choosing the specific delivery method, the clinicians need to combine the actual situation of primiparas and various factors to choose the most appropriate delivery method, so as to minimize the delivery risk.

The limitations of this study include small sample size, Non- pregnant ultrasound indicators were not included and short follow-up duration. They are directions that need improvement and effort in later research. A larger sample size, more complete and comprehensive grouping, and longer follow-up datas will make the experimental results more credible.

Conclusion

Pelvic floor ultrasound, as a convenient, reproducible and non-radiative examination technique, is applicable for evaluating the function and anatomical structure of female pelvic floor24. By quantifying the positional changes in pelvic floor organs, it can achieve early detection of asymptomatic PFD, which has high application value in evaluating the pelvic floor function of women with different delivery methods. Transperineal pelvic floor ultrasound enables early diagnosis of diseases and thus provides a scientific basis for clinical practice.

To follow up the pelvic floor status in the later stage, we will further explore the effects of different delivery methods on the postpartum rehabilitation by enlarging the sample size, incorporating the ultrasound indicators for non-pregnant women, extending the follow-up duration and including the postpartum rehabilitation information.