Female pelvic floor dysfunction (PFD) is a complex syndrome involving impairment of the pelvic floor muscles (PFM) and tissues. Currently, PFD is one of the five most common chronic diseases that seriously affect women’s quality of life. Reports show that the prevalence of PFD ranges from 23.7% to 46.5% [1, 2, 3]. Pregnancy and childbirth are recognized as the major risk factors for PFD. Prolonged elevated hormone levels during pregnancy can lead to altered metabolism of PFM collagen fibers, resulting in abnormal pelvic floor support structures [4]. In addition, excessive stretching during childbirth causes damage to the PFM, connective tissue, and nerves. Pregnant and postpartum women are more likely to have PFD, up to approximately 49% [5].

PFD seriously affects the physical and mental health of women. Therefore, early diagnosis and timely treatment are extremely important. The early stages of PFD are changes in the biochemistry and electrophysiology of the pelvic floor, which may progress to symptomatic PFD under further damage. Electromyography (EMG) records the electrical potentials generated by the depolarization of muscle fibers. When the electrodes are placed on the skin of the perineum or inside the urethra, vagina, or rectum, it is called surface EMG (sEMG) [6]. Pelvic floor sEMG could be used for early diagnosis of PFD, which is an objective and non-invasive method by recording the change in voltage across the PFM fiber membrane. Several reports have confirmed that sEMG is reliable for measuring PFM in different populations [7, 8]. sEMG assessment based on the Glazer protocol is widely used for the evaluation of PFD in postpartum women [9, 10]. The Glazer protocol consists of a series of muscle relaxations and contractions, including pre-baseline rest, phasic contractions, tonic contractions, endurance contractions, and post-baseline rest.

As well known, the delivery mode is a crucial risk factor for PFD [11]. It is necessary to predict and/or diagnose the early stage of PFD in postpartum women, since they could be offered timely interventions to prohibit progression. However, the effect of delivery mode on early pelvic floor function remains controversial [12, 13]. Most previous studies applied qualitative assessments of the different delivery modes on PFD. This study aimed to quantify the impact of different delivery modes on PFM function at 6–8 weeks postpartum by sEMG based on the Glazer protocol. This study also tried to distinguish the detail types and grade of PFM impairment, in order to provide the individual therapeutic strategy to patients at the early stage of postpartum.

This study successfully screened a total of 3385 subjects (Fig. 1), including 1286 cases in the CD group and 2099 cases in the VD group. There were no significant differences in age, body mass index (BMI), gestational age at delivery, and neonatal weight between CD and VD (p $>$ 0.05, Table 1). Our analysis showed that the mean peak amplitude of phasic (flick) contractions and the mean amplitude of tonic contractions were both significantly higher in CD than in VD (p 0.01, Tables 2, 3). In contrast, the mean amplitude variability of tonic contractions was lower in CD than in VD (p 0.05, Tables 2, 3). The results showed that the strength of fast muscle (class II fibers) and slow muscle (class I fibers) was higher in CD than in VD, and the variation of slow muscle (class I fibers) was significantly higher in VD. When comparing the value at rest before and after baseline, VD had a lower mean amplitude compared to CD (p 0.01, Tables 2, 3). This study showed that abdominal muscle involvement was significantly higher in VD than in CD (p 0.01, Table 2).

Fig. 1.

Flowchart of the protocol used to select the study population.

This study further differentiated into 4 subgroups of vaginal delivery (A: intact perineum without laceration and first-degree laceration; B: second-degree laceration; C: mediolateral episiotomy; D: forceps delivery). There was no statistical difference in maternal age, BMI, gestational age at delivery and neonatal weight among the four subgroups of vaginal delivery (p $>$ 0.05, Table 4). Significant differences were identified among the four subgroups of vaginal delivery groups (Table 5). The average mean amplitude of resting before baseline was significantly higher in group C compared to group D. The mean peak amplitude of phasic (flick) contractions and the mean amplitude of tonic contractions were statistically lower in group D than in the other vaginal delivery groups (p 0.01). The mean amplitude variability of tonic contractions was greater in group D than in groups A, B and C (p 0.05). The mean resting amplitude after baseline was shorter in group D than in groups A, B and C (p 0.01) (Table 3). There was no significant difference in the engagement of the abdominal muscles between the 4 subgroups of vaginal delivery (p $>$ 0.05, Table 5).

In this study, analysis of pelvic floor sEMG data at 6–8 weeks postpartum in primiparous women showed that fast muscle strength was slightly but significantly weaker in VD compared to CD, and for the slow muscle, muscle strength and stability of contractile control were both significantly weaker in VD compared to CD. The function of the pelvic floor muscles decreases significantly after pregnancy, and the supporting force becomes weaker. The high pressure exerted on the pelvic floor by pregnancy and childbirth causes damage to the PFM, connective tissue and nerves, ultimately leading to PFD [15]. Multiple previous studies have shown that the incidence of PFD, such as pelvic organ prolapse (POP), stress urinary incontinence (SUI), etc., is significantly higher in women with vaginal delivery compared with cesarean section [16, 17]. Blomquist et al. [18] found that the cumulative incidence of POP, SUI and overactive bladder (OB) after vaginal delivery was associated with reduced PFM strength. A meta-analysis [19] of a total of nine studies also showed that PFM strength was significantly lower in the VD group than in the CD group. This study suggests that vaginal delivery, as a major risk factor for impaired postpartum PFM strength, may affect postpartum PFM function by decreasing the strength of the fast and slow muscles and the stability of the slow muscles.

Several studies [19, 20] have suggested that elective cesarean section may protect the pelvic floor muscles. This study found that the mean values of pre- and post-baseline sEMG were higher in the CD group than in the VD group in early postpartum. Guo et al. [21] found that the pre-baseline resting pressure was higher in the CD group in early postpartum compared with the VD group, which is consistent with our findings. The resting baseline amplitude reflects the resting activity of the PFM, which is essential to maintain the pelvic organs in optimal position [22, 23]. The lower resting baseline amplitude in the VD group indicated the greater resting underactive status of the PFM [9].

Numerous studies have shown that assisted vaginal delivery, especially forceps delivery, significantly increases the risk of PFD. The study reported that the risk of fecal incontinence and POP was significantly higher in those who had an assisted vaginal delivery compared to a spontaneous vaginal delivery [24]. Meyer et al. [25] reported a higher incidence of PFM weakness (20% vs. 6%) in women with forceps compared to spontaneous delivery at 10 months postpartum. Of all types of vaginal delivery, forceps delivery is associated with the highest risk of pelvic floor damage, mainly due to the potential destruction of pelvic floor muscles, nerves and connective tissue. Weakened PFM strength can be caused by levator avulsion injuries and extensive levator hiatus [26]. Our results suggest that forceps delivery has the worst effect on PFM function of all vaginal deliveries in the early postpartum period, mainly by reducing the strength of the fast and slow muscles and the stability of the slow muscles. And for those who require forceps delivery as a high-risk PFD population, pelvic floor function assessment should be performed in the early postpartum period, and a precise and effective strategy for postpartum PFM recovery should be initiated as early as possible.

To date, there are no reports comparing abdominal muscle engagement between different modes of delivery. In our clinical routine, the EMG signal representing the engagement of the abdominal muscles was captured by an additional channel through the patch attached to the abdomen. Our results showed that abdominal muscle engagement was significantly higher in VD compared to CD. The possible reason for this could be the compensatory use of the abdominal muscles in the vaginal delivery group, as they had weakened pelvic floor muscles. The discordance between the pelvic and abdominal muscles was more pronounced in women who delivered vaginally, so their pelvic floor muscles need more attention and follow-up.

This study recruited a large sample size in the final analysis, which can reduce the bias and provide reasonable comparisons in different study groups. And this study applied the modified Glazer protocol to evaluate the signal data of pelvic floor sEMG, which demonstrated the precise extent and detailed type of PFM in dysfunction based on a quantitative approach. It is powerful to support the comprehensive and accurate analysis of the effect of different deliveries on PFM function in the early postpartum period.

This study also has several limitations. First, this study only focused on the detection of EMG signals on the vaginal surface, and lacked further diagnosis of PFD. Second, due to the small sample size of third-degree perineal lacerations, this study did not have sufficient power to analyze third-degree perineal lacerations in vaginal delivery. Third, we educate all women delivering at our hospital about pelvic floor muscle screening 6–8 weeks postpartum, but about 20% of women still do not get screened as promised. Although we were not biased in our counseling and most of these women were out of town and returned home after delivery, it is possible that this may have had an impact on the results of the study. Finally, this study only assessed pelvic floor sEMG at 6–8 weeks postpartum, which may not be sufficient to access final pelvic muscle function. Therefore, a long-term epidemiologic follow-up study is suggested.

Routine mediolateral episiotomy in low-risk pregnant women is controversial. Some believe that routine mediolateral episiotomy not only protects the anal sphincter [27], but also protects against the defects of central support from the anterior vaginal wall [28]. In contrast, others believe that routine mediolateral episiotomy increases the incidence of episiotomy infection and pain, postpartum hemorrhage, and urinary tract dysfunction [29]. A systematic review [30] showed that mediolateral episiotomy was not beneficial in preventing urinary and fecal incontinence and pelvic floor relaxation and, ironically, increased the incidence of dyspareunia. In this study, there was no statistical difference in sEMG testing between the mediolateral episiotomy group, the vaginal delivery with hard protected perineal integrity group, and the 1st and 2nd degree lacertation groups. This study found that episiotomy did not significantly protect pelvic floor muscle function compared with the perineal integrity group and the 1st and 2nd degree perineal laceration groups. Therefore, episiotomy should be performed after careful evaluation.

In this study, the analysis showed that the strength of the fast and slow muscles, the baseline resting amplitude and the stability of the slow muscles were weaker in the vaginal delivery group, especially in the forceps delivery subgroup, than in the cesarean delivery group at 6–8 weeks postpartum.

The datasets used and/or analyzed during the current study are available from the corresponding author on rea- sonable request.

XC, LW and HZ designed the research study. LC, LW and HZ performed the research. LC analyzed the data. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

All subjects gave their informed consent for inclusion before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of the International Peace Maternity and Child Health Hospital, School of Medicine, Shanghai Jiao Tong University (No. GKLW 2023-024-01).

We sincerely thank all women who participated in the study, and the medical staff at the Pelvic Floor Screening and Rehabilitation Center.

The study is supported by the Chinese Academy of Medical Sciences Research Unit (No 2019RU056), Shanghai Jiao Tong University, CAMS Innovation Fund for Medical Sciences (CIFMS) (No 2019-I2M-5-064) and Shanghai Municipal Key Clinical Specialty, Shanghai, China (No. GFY1808004).

The authors declare no conflict of interest.