Pelvic floor dysfunctions (PFDs) refer to a group of conditions with diverse etiologies that damage the pelvic floor muscle (PFM) function. This leads to the weakening of PFM reaction and impairment of pelvic floor support functions, including stress urinary incontinence (SUI), anal incontinence (AI), pelvic organ prolapse, and postpartum sexual dysfunction [1]. Presently, PFDs are regarded as a hidden epidemic that affect approximately 21%–26% of women, with urinary incontinence (UI), having the highest incidence rates worldwide [2]. Pregnancy and delivery are independent risk factors for impaired pelvic floor function [3, 4], with operative vaginal delivery (OVD) being closely associated with PFDs [5]. OVD refers to a key means of delivery in which forceps or vacuum extraction is used to apply direct traction on the fetal head during the second stage of labor to accelerate or achieve vaginal delivery [6]. It currently serves as a crucial operating method for dealing with obstructed labor. However, there are relatively few reports regarding the effects of different indications for forceps delivery on postpartum pelvic floor function. The present study utilized Glazer PFM surface electromyography (sEMG) and International Consultation on Incontinence Questionnaire-Urinary Incontinence Short Form (ICIQ-UI-SF) for the assessment of postpartum PFM function to determine the effects of different forceps delivery indications on early postpartum pelvic floor function in primiparas. Our results contribute to a better understanding of the effects of forceps delivery on the incidence of PFD and provide a theoretical basis for the early screening of PFDs.

Between September 2019 and December 2021, postpartum pelvic floor function screening was performed on 24,637 puerperas at the Pelvic Floor Screening and Rehabilitation Center of our hospital. Four hundred puerperas were eventually included. The participants were divided into three groups: fetal distress (FD) (n = 260), prolonged second stage of labor (PSSL) (n = 30), and intrapartum fever combined with fetal distress (IFFD) (n = 110) (Fig. 1). Age, educational attainment, pre-gestational body weight, gestational BMI gain, method of conception, gestational complications (gestational hypertension, gestational diabetes, gestation anemia), gestational age at delivery, neonatal birth weight, and method of placental delivery were not significantly different among the three groups (p $>$ 0.05) (Table 1).

Fig. 1.

Flowchart of the subject screening process.

The overall scores of the PSSL and IFFD groups were significantly lower than that of the FD group, with the IFFD and FD groups being significantly different (p 0.05) (Fig. 2). The peak EMG values during the fast-twitch (type II muscle fibers) stage was significantly different between the IFFD and the FD groups (p 0.05). Parameters of the pre-testing resting stage, slow-twitch (type I muscle fibers) stage, and post-testing resting stage did not differ significantly between the three groups (p $>$ 0.05) (Table 2).

Fig. 2.

Total Glazer score for the three groups.

The incidence of postpartum SUI was significantly higher in the IFFD and PSSL groups compared with the FD group, with the IFFD and FD groups being significantly different (p 0.05). Incidences of OAB and constipation did not differ significantly between the three groups (p $>$ 0.05) (Table 3).

Studies have proposed the following reasons for the increased incidence of PFDs with OVD: (1) long-term elevation of hormone levels during gestation causes changes in the metabolism of PFM collagen fibers, thereby leading to abnormalities in pelvic floor support structures [14], and (2) OVD increases the risk of PFM injury [5]. This study assessed the effects of different forceps delivery indications on early postpartum pelvic floor function in primiparas using the Glazer protocol.

Despite the inability of sEMG to directly measure muscle strength, it has been demonstrated that the levels of rapid and sustained contractions measured by sEMG are closely associated with the motor and resting functions of muscles [13].

Our results revealed that the participants who underwent forceps delivery owing to IFFD had a significantly lower overall Glazer assessment score at 42–60 days postpartum compared with those who underwent forceps delivery solely because of FD. The overall score of participants who underwent forceps delivery owing to PSSL was not significantly different from that of the IFFD group but was significantly lower than that of the FD group. Moreover, studies have reported that prolongation of the second stage is an independent risk factor for the occurrence of PFDs. This may be related to the persistent exertion of pressure by the fetal head on the nervous tissues and PFMs during the second stage of labor, which results in tissue edema, hyperemia, and muscle relaxation [15]. The fetus exerts the strongest effects on the birth canal, and the pelvic floor soft tissues are subjected to great forces during the second stage of labor [16]. Caudwell-Hall et al. [17] has reported that a prolonged second stage causes an increase in the time that the PFMs are subjected to pressure. Under such persistent and strong action, the probability of the occurrence of tears and ruptures in muscle fibers (especially type I muscle fibers) is significantly increased, and patients who sustain perineal tears are more prone to developing severe PFDs [17]. Research has further shown that PSSL is associated with an increased possibility of postpartum incontinence in women and increases the long-term risk of UI [18]. In the present study, the incidence of early postpartum UI in the PSSL group was higher than that in the FD group, with the incidence of SUI being twice as high as that of the FD group. This is consistent with the results of a study on an Australian population by Brown et al. [19], who reported that the probability of SUI occurrence in puerperas who experienced PSSL was 1.9 times that of puerperas with a normal second stage of labor.

Additionally, we found that the IFFD group had the lowest overall Glazer assessment score and was prone to decreased strength in the pelvic floor fast-twitch muscles (type II muscle fibers). The maximum EMG values of the three groups during the fast-twitch (type II muscle fibers) stage were 32.4 $\pm$ 17.7 $\mu$V (FD group), 31.7 $\pm$ 26.1 $\mu$V (PSSL group), and 26.5 $\pm$ 12.2 $\mu$V (IFFD group), and the decreases in fast-twitch muscle strength were 73.1%, 76.7%, and 88.2%, respectively, with the differences being statistically significant. PFMs constitute a part of the support system for female pelvic floor elasticity. Therefore, functional impairment of PFMs or injury to the innervating nerves is the direct cause of the occurrence of PFDs [20]. Both type I and II muscle fibers are the main components of PFMs, with type I (slow-twitch) muscle fibers being responsible for the static support function of the pelvic floor and type II (fast-twitch) muscle fibers being responsible for the dynamic movements of the pelvic floor.

The main pathophysiological mechanism of PFD onset is the occurrence of pathological changes in muscle fibers, which induces abnormalities in overall muscle tone, or contraction function of muscles, resulting in the transposition, or dysfunction, of pelvic organs such as the uterus, bladder, and rectum [21]. The incidence of intrapartum fever is 1.6–14.6% [9]. A case-control study has shown fever was associated with epidural analgesia, nulliparity, and a long duration of labor. Multiple regression analysis showed that all three variables were independently associated with maternal temperature [22]. First-time delivery, a higher rate of epidural analgesia and a longer second stage of labor are associated with an increased incidence of intrapartum fever. Current research indicates that intrapartum fever is not entirely attributable to maternal infection, as most cases of intrapartum fever are caused by non-infective inflammation [23]. Possible mechanisms of intrapartum fever occurrence include endogenous heat generated by skeletal muscle contractions, infective inflammation following amnion rupture, and epidural analgesia [24]. In the present study, the white blood cell count and C-reactive protein (CRP) inflammation indicator values of all the participants in the IFFD group were higher than the normal values.

Furthermore, the inflammatory response may aggravate muscle injury [25, 26]. Since anaerobic metabolism does not favor the absorption and excretion of inflammatory cytokines [27, 28], the intrapartum fever-induced decrease in pelvic floor fast-twitch (type II muscle fibers) muscle strength may be related to the fact that metabolism is predominantly anaerobic in the fast-twitch muscles (type II muscle fibers). Impairment of the rapid contraction function of PFMs is mainly manifested as decreased muscle strength, rhythm disturbances, and weakening of intentionality in fast-twitch muscles; impairment of the persistent contraction function is mainly manifested as a reduction in the supportive and immobilization abilities of slow-twitch muscles, which causes the weakening of support strength.

The fast-twitch (type II muscle fibers) stage is mainly aimed at testing dynamic fast-twitch muscle (type II muscle fibers) strength and reaction speed. Individuals with inadequate fast-twitch muscle strength are prone to UI, AI, sexual indifference, and decline in sexual experience [29]. Our results further indicated that the IFFD group had a higher incidence of UI compared with the PSSL and FD groups, with the incidence of SUI being 2.18 times that of the FD group. This may be related to the decline in fast-twitch muscle strength caused by intrapartum fever. Further research is required on the mechanism of the decreased pelvic floor fast-twitch muscle strength caused by intrapartum fever. Pelvic floor muscle exercises decrease urinary incontinence in pregnancy, postpartum period, and later in life. A study has shown that there is a lack of knowledge about the relationship between pelvic floor muscle exercises and pelvic floor disorders [30]. The present study faces the same problem, and that is what we are going to work on in the future.

The main limitation of this study was that we only focused on vaginal sEMG tests in different indications for forceps delivery; sEMG values were not obtained for prenatal and non-pregnant women. The sEMG evaluation at 6–8 weeks after delivery may be insufficient. Moreover, the long-term pelvic floor function outcome of puerpera warrants further clinical follow-up study.

Intrapartum fever affects the early postpartum pelvic floor function of primiparas who underwent forceps delivery, which mainly manifests in the short term as reduced fast-twitch muscle (type II muscle fibers) strength and the occurrence of SUI. Therefore, vaginal examinations should be minimized as much as possible prior to the administration of labor analgesics to avoid the occurrence of intrapartum fever and consequent adverse effects of forceps delivery on the pelvic floor function of primiparas. Furthermore, the screening of pelvic floor function should be performed during the early postpartum period in primiparas indicated for forceps delivery due to intrapartum fever combined with fetal distress. Future studies should use larger sample sizes and longer follow-up times to assess whether intrapartum fever and a prolonged second stage of labor increase the risk of pelvic floor diseases.

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

XLC—conception and design, data analysis and writing original draft; SSY—conception and design, data analysis and writing original draft; KZ—Data analysis and questionnaires; CCX—data acquisition and questionnaires; XHS—data acquisition and questionnaires; XLC—conception and design, funding acquisition, and writing review & editing. All authors read and approved the final manuscript.

This study was approved by the ethics committee of the International Peace Maternity and Child Health Hospital, School of Medicine, Shanghai Jiao Tong University (No. GKLW 2021–55). All participants gave their informed consent for inclusion before they participated in the study.

We sincerely thank all women who participated in the study, and the medical staff at the Pelvic Floor Screening and Rehabilitation Center. And we would like to express our appreciation to the peer reviewers for the contributions they made to the study.

This research was funded by the Project of National Natural Science Foundation of China (CR2018WX01) and the Shanghai Municipal Key Clinical Specialty, Shanghai, China (GFY1808004).

The authors declare no conflict of interest.