Three-dimensional transperineal ultrasound (3D-TPUS) assists in obtaining images of pelvic structures related to the support of pelvic organs [1, 2, 3, 4, 5, 6, 7, 8]. Magnetic resonance imaging (MRI) has also been used [9], but 3D-TPUS is generally quicker and easier to perform in everyday clinical practice. The axial plane has been used to represent the levator ani muscle (LAM), an anatomical-functional unit that plays a pivotal role in normal physiology of the pelvis. The sonographic images obtained with 3D-TPUS focus mainly on (a) the presence of any LAM trauma, defined as a sonographic defect in the muscle, and (b) the dimensions of the LAM. Both LAM trauma and increased dimensions have been consistently associated with the presence of symptomatic pelvic organ prolapse (POP) [1, 2, 3, 10].

The origins of LAM trauma are primarily associated with vaginal delivery (VD), as this is when damage to the pelvic floor is most likely to occur due to the descending presenting part of the fetus. Immediately postpartum, LAM trauma can be depicted sonographically [11], and its incidence as assessed using 3D-TPUS ranges from 10 to 35% [12, 13, 14, 15, 16]. In some cases, lesions initially diagnosed may not be confirmed in later scans [17, 18]. Nevertheless, the majority of LAM injuries can still be recognized sonographically at anytime after the puerperium [17, 18, 19, 20, 21].

Recently, changes in LAM dimensions throughout gestation and the postpartum period have been studied [16, 22, 23, 24, 25, 26, 27, 28]. Several prospective studies in nulliparous womenhave shown that LAM dimensions expand throughout pregnancy, continuing to increase until the late third-trimester [22, 23, 24, 25, 26, 27]. All these studies have attempted to describe the changes in LAM dimensions both in nulliparous women with levator trauma and those without. They concluded that trauma is the major cause of the expansion of LAM dimensions after delivery [22, 23, 24, 25, 26, 27]. Only a few studies have examined the impact of subsequent deliveries on levator hiatus morphology [29, 30]. Moreover, no study has described the changes in LAM dimensions throughout gestation and the postpartum period in multiparous women. The rationale for such a study would be to define the role of multiple vaginal deliveries in the physiology of the pelvic floor support. As such, the aim of this study was to investigate the changes in LAM dimensions (1) during pregnancy and (2) after labor in a cohort of multiparous women.

In this prospective study, pregnant women with a history of VD were compared with those with a history of CS. We have shown that during pregnancy, US measurements of the levator hiatus in multiparous women significantly increased from the first- to the second- and third-trimesters. Furthermore, this study demonstrates that in multiparous women, many dimensions of the levator hiatus decrease significantly postpartum, whether following a non-traumatic vaginal birth or a CS. Finally, this study indicates that the incidence of new levator trauma during VD is reduced in women with a history of previous vaginal births.

In the present study, there was only one new LAM trauma in the women of the VD subgroup (1/27, 3.7%). This finding confirms the already low published rates of new LAM avulsion after a second VD. Chan et al. [21] evaluated 151 multiparous women after their second delivery and found a 0.9% rate of new LAM avulsion. Another important issue is the possibility of LAM injury following VBAC. Epidemiologically, CS appears to offer partial protection against POP. This could be explained because of the very low rates of LAM trauma following CS. In the literature, Horak et al. [30] reported a new case of avulsion following VBAC in 6 women who had a successful VBAC (1/6, 16.7%). In the present study, 3 women delivered via VBAC, and 1 suffered a LAM injury (1/3, 33.3%). These findings highlight a potential risk for LAM injury in multiparous women attempting VBAC, which may have implications for future symptoms of POP.

Changes in the dimensions of the levator hiatus in nulliparous women during pregnancy and after delivery have been investigated in several studies. These studies have shown that the dimensions of the hiatal opening increase as pregnancy progresses [22, 23, 24, 25, 26, 27]. Following delivery, parameters influencing hiatal dimensions include (i) the mode of delivery, and (ii) the presence of pelvic floor trauma [22, 23, 24, 25, 26, 27]. In multiparous women, hiatal dimensions are expected to exhibit trends quite similar to those observed in primiparous during pregnancy and after delivery. Antenatal measurements appear to support this expectation. Both VD and CS subgroups of multiparous womenexhibit a similar trend, with an increase in hiatal dimensions as pregnancy progresses towards term. Paradoxically, postnatal measurements do not enterally align with the observations from nulliparous cohorts. On the one hand, as expected, the CS subgroup exhibits a significant reduction in hiataldimensions postpartum. On the other hand, the VD subgroup also demonstrates a significant reduction in the hiatal dimensions following delivery. Despite differing initial measurements in the first-trimester, the two subgroups exhibit nearly identical trends in the alterations to the leavator hiatus (LH) area sustain during pregnancy and after delivery. The most likely explanation for these findings is the absence of new levator trauma in women with previous VD. Apparently, the effects of childbirth primarily impact the pelvic floor muscles during the first VD, with subsequent vaginal births not significantly affecting their integrity.

According to our results, all hiatal dimensions are significantly increased in the VD subgroup compared with the CS delivery subgroup. This finding is consistent with the published literature. In detail, Kamisan Atan et al. [33] conducted a retrospective study using 3D-TPUS to examine women with POP symptoms. They found that multiparous women who had delivered vaginally exhibited a significantly larger LH area during Valsalva compared to those who had undergone CS [33].

Changes in hiatal dimensions and pelvic floor trauma during pregnancy and delivery are key factors associated with the developmentof POP later in life. A third influential epidemiological parameter contributing to the development of POP is the number of pregnancies, particularly the number of VD. Epidemiologically, the incidence of POP appears to increase in direct proportion to parity [34, 35]. Our study results cast doubt on the precise role of multiparity in the pathophysiology of POP. Apparently, pregnancy alone does not appear to be traumatic enough to induce the prerequisites for POP. If it were, one might expect that multiparous women who delivered by CS would also have POP symptoms. However, this has not been confirmed in epidemiologically. Therefore, we could presume that LAM trauma sustained during VD is the single most important peripartum factor leading to POP in multiparous women with a history of VD.

Our results are in agreement with recent literature and challenge the significance of subsequent VDs in the development of POP later in life. Jundt et al. [29], in a small study, concluded that a second VD did not further compromise the pelvic floor. Kamisan Atan et al. [33] retrospectively studied a large cohort of urogynecology patients and concluded that subsequent deliveries do not appear to have any effect on hiatal dimensions. Horak et al. [30] concluded that a second pregnancy and delivery do not appear to have a major effect on levator function and bladder support. Overall, it appears that subsequent vaginal deliveries do not negatively impact the pathophysiology of POP in women with a history of VD, especially in the absence of new LAM trauma.

The findings of this study indicate that (i) the likelihood of LAM trauma during subsequent vaginal births is low and (ii) there is a peripartum reduction in hiatal area in multiparous women, regardless of their previous delivery mode. These findings differ from data on nulliparous women in the literature and may imply that most pelvic floor injuries to the occur duringthe first VD. Such a conclusion could only be reached through detailed prospective studies where multiparous women with history of traumatic VD are compared with those multiparous women with history of non-traumatic VD, and both groups are followed for a long period of time. As such, other important perinatal, genetic or behavioral factors could be identified and further investigated. However, postpartum identification of LAM trauma could potentially serve as a screening tool for clinical surveillance of women at high risk of developing symptomatic POP later in life.

To the best of our knowledge, this is the first study that prospectively followed multiparous women from the first-trimester through to the postpartum period using 3D-TPUS to investigate ongoing LAM changes. Multiparous women appear to exhibit a different trend in the alterations of the levator hiatus after delivery compared to the nulliparous women. Studies involving nulliparous women can serve as a basis for comparison with our findings and vice versa.

Limitations of our study include: (i) the relatively small sample size and the increased number of missing follow-up examinations; (ii) the high rates of CSs in tertiary referral centers in Greece; (iii) the lack of analysis of different grades of multiparity, which may reduce the statistical power of our sample; (iv) the scheduling of the third-trimester US scan at 32 weeks, which may miss any additional changes before birth and hinders direct comparisons with findings from other published studies.

This study found that in multiparous women who deliver vaginally, the dimensions of levator hiatus increase during pregnancy and decrease after delivery. A similar trend is identified in multiparous women who deliver with CS. The incidence of new LAM trauma is reduced in multiparous women with a history of previous vaginal births who deliver vaginally. Further studies involving multiparous women who deliver vaginally, both with and without levator hiatus trauma, are warranted in order to elucidate the influence of multiple deliveries (traumatic or otherwise) on the pelvic floor.

All data points generated or analyzed during this study are included in this article and there are no further underlying data necessary to reproduce the results.

AS clinical and ultrasound examinations of patients, data collection and analysis, drafting of the manuscript; TM study conception, design and supervision; APA and EA study conception and design; MZ and GG analysis and interpretation of data, revision of the manuscript for important intellectual content; PP and DT analysis and interpretation of 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 Medical School of the Aristotle University of Thessaloniki, Greece (No. 3767/2011).

We would like to thank all those from the medical and nursing staff of the 1st Department of Obstetrics and Gynecology of the Aristotle University of Thessaloniki who helped us conduct this study. We would like to express our gratitude to all the peer reviewers for their opinions and suggestions.

This research received no external funding.

The authors declare no conflict of interest.