Discussion
Main findings
In this study, the analysis of pelvic floor sEMG data at 6-8 weeks postpartum in primiparous women represented that the fast muscle strength was significantly weaker in VD compared with CD, as well as for the slow muscle, muscle strength and stability of contractile control were both significantly weaker in VD compared with CD. The vitality of the pelvic floor muscles decreases significantly after pregnancy, and the supporting force becomes weaker. The high pressure generated by pregnancy and delivery to the pelvic floor results in the impairment of PFM, connective tissue and nerves, eventually leading to PFD. Numerous previous studies have shown that compared with cesarean section, the incidence of PFD, like pelvic organ prolapse (POP), stress urinary incontinence (SUI) and so on in women with vaginal delivery is significantly higher [10-12]. Blomquist[13] found that the cumulative incidence of POP, SUI and overactive bladder (OB) after vaginal delivery was associated with decreased PFM strength. A meta-analysis [14], reviewed in total nine studies, also demonstrated the PFM strength in the VD group was significantly lower than that in the CD group. Our study aligned with other reports, suggested that vaginal delivery, as the main risk factors for impairment of postpartum PFM strength, can affect postpartum PFM function via decreasing the muscle strength of the fast and slow muscles and the stability of the slow muscles.
Several studies[14,15] suggest that elective cesarean delivery may protect the pelvic floor muscles. But other reports[16] showed that this protection from cesarean delivery could be ignored with the long-term postpartum follow-up and its effects in this regard remain controversial. Our study found that the mean values of the pre-baseline and post-baseline rest in sEMG were higher at early postpartum of the CD group than those in the VD group. Guo et al [17] found the higher pressure at pre-baseline rest in CD in the early postpartum compared with VD group, which is consistent with our findings. The pre-baseline and post-baseline resting phases is correlated to the magnitude of muscle tension in a relaxed state. Increased resting tension of pelvic floor muscles can easily lead to pelvic floor muscle ischemia, present as clinical symptoms such as dyspareunia, urinary retention, and constipation. A study on quality of life with 6 years follow up postpartum[18], revealed that the incidence of lower abdominal, genital pain, and pain related to sexual life were significantly more frequent after cesarean delivery than that with vaginal deliveries. The causes of increased muscle tone could be neurogenic and non-neurogenic hypertonicity, which are both associated with muscle contraction and/or passive stiffness[19]. Our findings suggest that cesarean section may increase pelvic floor muscle tension and impact the PFM function in the early postpartum period. The relevant mechanism needs further investigation.
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 had assisted vaginal delivery compared with natural vaginal delivery [20]. Meyer et al[21] reported a higher incidence of PFM weakness (20% vs 6%) in women with forceps delivery than spontaneous delivery at 10 months postpartum. Among all kinds of vaginal deliveries, forceps delivery brings the highest risk of impairment of pelvic floor structure, mainly due to its potential destruction on the pelvic floor muscles, nerves, and connective tissue. Weakened PFM strength may be caused by levator avulsion injuries and extensive levator hiatus[22]. Our results suggest that the forceps delivery has the worst impact on the PFM function among all vaginal delivery in the early postpartum period, mainly by reducing the muscle strength of fast and slow muscles as well as the stability of slow muscles. And for those with necessary forceps delivery as high-risk PFD population, pelvic floor function assessment should be performed at the early postpartum stage, and a precise and effective strategy for postpartum PFM recovery should be initiated as early as possible.
So far there are no reports on comparison of abdominal muscle engagement among the different modes of delivery. In our clinical routine, EMG signal representing the abdominal muscle engagement, was captured by an additional channel via the patch attaching the abdomen. Our results showed that the engagement of the abdominal muscles was significantly higher in VD compared with CD. The potential reason behind could be the compensative utilization of abdominal muscle in vaginal delivery group as they had weakened pelvic floor muscle. The discordance of pelvic-abdominal muscle was more pronounced in women who delivered vaginally, thus their pelvic floor muscle requires the professional rehabilitation therapy.