Objective
The objectives of the investigation were to study the association between levator hiatus dimensions in late pregnancy and both the length of second stage of labor and also the delivery mode in women delivering their first child.
Study Design
In this cohort study, 231 nulliparous women were examined with 3- and 4-dimensional transperineal ultrasonography at 37 weeks of gestation. The anteroposterior, transverse diameter, and the area of levator hiatus were measured at rest, during levator ani muscle contraction, and during Valsalva maneuver. The second stage of labor was divided into passive and active second stage and delivery modes into normal vaginal or instrumental deliveries. Spearman correlation coefficient, independent-sample t test, and standard logistic regression were used for analysis.
Results
Larger levator hiatus dimensions at rest and during contraction at 37 weeks of gestation correlated with a shorter duration of the active second stage in women with normal vaginal delivery (Spearman correlation coefficient, –0.13 to –0.35, P ≤ .08). Women having normal vaginal deliveries had significantly larger transverse diameter at rest, during contraction, and during Valsalva maneuver compared with women having instrumental deliveries (mean difference, 0.29; 95% confidence interval (CI), 0.16–0.41; mean difference, 0.33; 95% CI, 0.21–0.44 and mean difference, 0.24; 95% CI, 0.06–0.42; P < .05). The same was true for the levator hiatus area at rest and during contraction (mean difference, 1.22; 95% CI, 0.37–2.07 and mean difference, 0.84; 95% CI, 0.22–1.46; P < .01). These estimates were unchanged by adjustments in the logistic regression analysis.
Conclusion
Larger levator hiatus dimensions in late pregnancy had a significant association with a shorter active second stage of labor and normal vaginal delivery.
Lack of progression of labor can be caused by many factors such as insufficient uterine contractions, size and presentation of the fetus, and the size of the pelvis. When passing the birth canal, the fetus meets resistance not only from the bony pelvis but also from soft tissue, in particular the levator ani muscle. The most medial part of the levator ani muscle, the symphysis, and inferior ramus pubis border the area of levator hiatus. Hence, if the levator hiatus is of limited size, this may have the potential to negatively influence the progress of labor, especially the second stage. This might eventually cause fetal distress and exhausted mothers, in turn necessitating instrumental intervention for delivery.
Complicated deliveries can cause adverse outcomes for the child and also serious injuries to the women’s pelvic floor and anal sphincter muscle, accompanied by loss of function and quality of life. To date, reliable identification of women at risk is not available, and there is a need for an increased understanding of the mechanisms that may lead to difficult vaginal or abdominal operative deliveries. Only a few studies have assessed the influence of levator ani muscle anatomy in late pregnancy on delivery outcome, and results were inconclusive. Thus, the aim of the present study was to study the associations between levator hiatus dimensions measured with 3- and 4-dimensional transperineal ultrasonography at 37 weeks of gestation and both the length of the passive and active second stage of labor and also the delivery mode in 231 women delivering their first child.
Materials and Methods
In this cohort study, 300 pregnant nulliparous women were recruited between December 2009 and April 2011 at Akershus University Hospital (Norway). During this period, all nulliparous women attending the hospital for routine prenatal ultrasound examinations at 18 weeks of gestation were invited to participate.
Inclusion criteria were a singleton pregnancy and being able to understand one of the Scandinavian languages. Women with a previous pregnancy of more than 16 weeks of gestation were not included. In this study, ongoing exclusion criteria were stillbirth, serious illness of the mother or child, missing delivery data, prelabor cesarean section, and cesarean section before full cervical dilatation. The Regional Ethics Committee (REK Sør-Øst D 2009/170) and the Norwegian Social Science Data Service (2799026) approved the study, and all women gave informed written consent to participate.
The women were examined at 22 and 37 weeks of gestation using 3- and 4-dimensional transperineal ultrasonography. The ultrasound examination of levator ani muscle was performed by 2 trained investigators, using a GE Voluson E 8 system (GE Medical Systems, Zipf, Austria) with a 4-8 MHz curved array volume transducer (RAB4-8l/obstetric). The ultrasound volumes were acquired with the women in the lithotomy position, with empty bladder at rest, during maximum levator ani muscle contraction, and maximum Valsalva maneuver. Care was taken to avoid cocontraction of the most medial part of the levator ani muscle during the Valsalva maneuver.
The ultrasound images were stored offline using anonymous code numbers and were analyzed using 4-dimensional software (version 10.0; GE Healthcare, Fairfield, CT). Render mode around the plane of minimal hiatal dimensions was used. Measurements of the levator hiatus were of anteroposterior and transverse diameter and the levator hiatus area itself ( Figure ). Volume rendering is a technique used to display a 2-dimensional projection of a 3-dimensional structure. In this case, ultrasonography of the levator ani muscle in the midsagittal, the axial, and the coronal 2-dimensional planes produce a semitransparent rendered representation of the pelvic floor. It provides a semitransparent representation of all gray-scale pixels within a render box, which contains the plane of minimal hiatal dimension also the region of interest. The render box was approximately 1-2 cm thick. Hence, it covers the entire craniocaudal space by the non-Euclidean shape of the levator hiatus. This results in an axial image that corresponds to an observation of the patient’s pelvic floor as seen from below.
In the present study, 4 trained investigators analyzed the images. Intraclass correlation coefficient (ICC) values for interobserver reliability of the 4 investigators were good to excellent (ICC >0.80) for all measurements, except for the transverse diameter of the levator hiatus at rest and during Valsalva maneuver (ICC >0.62). The assessors were blinded to the women’s demographic data and obstetric history, information that was retrieved from the hospital’s electronic birth records.
Obstetric data such as induction of labor were coded yes or no. Epidural analgesia (yes/no) was given as continuous infusion with the possibility of top-ups. The labor augmentation (yes/no) included amniotomy and oxytocin administration and breast stimulation.
The second stage of labor was divided into passive and active second stage. Passive second stage was defined as the interval between full cervical dilatation and the commencement of active pushing. Active second stage was defined as the time of active pushing until the delivery of the infant. To determine the impact of levator hiatus dimensions on delivery mode, women were stratified into having a normal or an instrumental delivery. The instrumental delivery group included all women having vacuum, forceps, or cesarean section after full cervical dilatation.
Statistical analysis was performed using SPSS version 20 (SPSS Inc, Chicago, IL). Demographic and obstetric data were presented as frequencies with percentages, means with SDs, or medians with ranges for normally or nonnormally distributed data. The correlation between levator hiatus dimensions and active and passive second stage of labor was estimated using the Spearman correlation coefficient. Differences in levator hiatus measurements in women with normal vaginal and instrumental deliveries were given as the mean differences with 95% confidence intervals (CIs) and were analyzed using an independent-sample Student t test.
To control for possible covariates when analyzing the influence of levator hiatus measurements on delivery mode, a standard logistic regression analysis was used. The selection of covariates was based on existing literature and clinical reasoning. Covariates found to have a significant association ( P < .05) with delivery mode or ultrasound measurements were taken into the regression model. Logistic regression analysis was performed for each levator hiatus measurement individually along with the covariates. Crude and adjusted odds ratio (OR) with 95% CI were reported. Values of P < .05 were considered significant.
Results
Of the 300 women recruited, 7 women did not attend the ultrasound examination at 37 weeks of gestation, 16 women were not examined owing to delivery before ultrasound examination, and 3 women were excluded because of stillbirths. Seventeen women had a prelabor cesarean section, 16 women had a cesarean section before full cervical dilatation, and delivery data were missing for 10 women. This left 231 women in the study.
The age, prepregnancy body mass index, fetal birthweight, and delivery mode of the 300 women recruited was comparable with the total population of pregnant nulliparous women scheduled to deliver at Akershus University Hospital during the inclusion period (n = 2547).
Table 1 presents demographic and obstetric data for the participants. One hundred eighty- four women (79.7%) had a normal and 47 (20.3%) had an instrumental delivery (38 vacuum, 2 forceps, 2 vacuum and forceps, and 5 cesarean section after full cervical dilatation). Indications for instrumental intervention were failure to progress (n = 27), fetal distress (n = 18), abnormal fetal presentation (n = 1), and exhausted mother (n = 1). A comparison of women who had normal vaginal deliveries with those who had instrumental deliveries showed significantly lower fetal birthweight and shorter duration of the second stage of labor. Labor augmentation and epidural analgesia were more frequently used in instrumental deliveries than in normal vaginal deliveries ( Table 1 ).
| Demographic | All (n = 231) | Normal vaginal delivery (n = 184) | Instrumental delivery (n = 47) | P value |
|---|---|---|---|---|
| Age, y a | 29.3 (4.1) | 29.3 (4.3) | 29.4 (3.6) | .84 |
| Prepregnancy body mass index, kg/m 2 a | 23.9 (4.0) | 23.9 (4.1) | 23.8 (3.8) | .96 |
| Weeks of gestation at ultrasound examination a | 36.8 (0.7) | 36.8 (0.7) | 36.8 (0.7) | .81 |
| Total gestational length, d a | 282.5 (8.4) | 282.0 (8.4) | 284.6 (8.0) | .051 |
| Fetal birthweight, g a | 3527.7 (444.0) | 3478.4 (434.5) | 3720.8 (431.7) | .001 |
| Length of passive second stage, min b | 16.0 (0–325.0) | 15.0 (0–180.0) | 40.0 (0–325.0) | < .01 |
| Length of active second stage, min b | 34.0 (6.0–107.0) | 32.0 (6.0–102.0) | 51.0 (8.0–107.0) | < .001 |
| Induction of labor | 16.0% | 15.8% | 17.0% | .83 |
| Epidural analgesia | 41.6% | 35.9% | 63.8% | .001 |
| Labor augmentation | 58.9% | 52.2% | 85.1% | < .001 |
a Values are given as mean with SD
No correlation between levator hiatus dimensions and the length of the passive second stage of labor was found in the 2 delivery groups (data not shown). For women with normal vaginal delivery, there was a significant weak to moderate inverse correlation between anteroposterior diameter and levator hiatus area at rest and all levator hiatus dimensions during contraction, and the length of active second stage. In deliveries with instrumental intervention, no significant correlation between levator hiatus dimensions and length of active second stage was found ( Table 2 ).
| Variable | LHap R | LHrl R | LHarea R | LHap C | LHrl C | LHarea C | LHap V | LHrl V | LHarea V | |
|---|---|---|---|---|---|---|---|---|---|---|
| Normal vaginal delivery (n = 183) | r P value | –0.23 < .005 | –0.13 .08 | –0.29 < .001 | –0.29 < .001 | –0.21 < .05 | –0.35 < .001 | –0.05 .52 | –0.10 .17 | –0.09 .24 |
| Instrumental delivery (n = 47) | r P value | –0.18 .23 | –0.13 .38 | –0.11 .47 | –0.06 .97 | –0.05 .73 | –0.10 .51 | –0.12 .43 | –0.16 .28 | –0.16 .30 |
Women having normal vaginal deliveries had significantly wider transverse diameter of levator hiatus at rest, during contraction, and during the Valsalva maneuver compared with women who required instrumental interventions. The same was true for levator hiatus area at rest and during contraction. The lengths of the anteroposterior diameter of the levator hiatus did not differ between delivery groups ( Table 3 ).
| Variable | Normal vaginal delivery (n = 184) | Instrumental delivery (n = 47) | Mean difference | P value |
|---|---|---|---|---|
| LHapR, cm | 5.13 (0.69) | 5.05 (0.63) | 0.08 (–0.13 to 0.30) | .45 |
| LHrlR, cm | 3.82 (0.38) | 3.53 (0.35) | 0.29 (0.16–0.41) | < .001 |
| LHareaR, cm 2 | 13.92 (2.74) | 12.70 (2.16) | 1.22 (0.37–2.07) | < .01 |
| LHapC, cm | 4.15 (0.59) | 4.15 (0.58) | 0.00 (–0.19 to 0.19) | .99 |
| LHrlC, cm | 3.53 (0.35) | 3.20 (0.36) | 0.33 (0.21–0.44) | < .001 |
| LHareaC, cm 2 | 10.61 (1.96) | 9.77 (1.77) | 0.84 (0.22–1.46) | < .01 |
| LHapV, cm | 5.90 (1.07) | 5.84 (1.12) | 0.06 (–0.29 to 0.42) | .71 |
| LHrlV, cm | 4.21 (0.59) | 3.97 (0.51) | 0.24 (0.06–0.42) | < .05 |
| LHareaV, cm 2 | 18.91 (6.20) | 17.74 (5.44) | 1.17 (–0.65 to 3.00) | .24 |
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