Background
Fetal head “attitude” (relationship of fetal head to spine) in the first stage of labor may have a substantial impact on labor outcome. The diagnosis of fetal head deflexion traditionally is based on digital examination in labor, although the use of ultrasound to support clinical diagnosis has been recently reported.
Objectives
The aims of this study were: (1) to quantify the degree of fetal head deflection via the use of sonography during the first stage of labor; and (2) to determine whether a parameter derived from ultrasound examination (the occiput-spine angle) has a relationship with the course and outcome of labor.
Study Design
This was a prospective multicentric, cross-sectional study conducted at the Maternity Unit of the University of Bologna and Parma from January 2014 to April 2015. A nonconsecutive series of women with uncomplicated singleton pregnancies at term gestation (37 weeks or more) were submitted to transabdominal ultrasound during the first stage of labor. If fetal position was occiput anterior or transverse, the angle between the fetal occiput and the cervical spine (the occiput-spine angle) was sonographically obtained on the sagittal plane. The measurements of the occiput spine-angle were performed offline by 2 operators who were blinded to the labor outcome. The intra- and interobserver reproducibility and the correlation between the occiput-spine angle and the mode of delivery were evaluated.
Results
A total of 108 pregnant women were recruited, 79 of which underwent a spontaneous vaginal delivery and 29 were submitted to obstetric intervention (19 cesarean delivery and 10 instrumental vaginal deliveries). The mean value of the occiput-spine angle measured in the active phase of the first stage was 126° ± 9.8° (SD). The occiput-spine angle measurement showed a very good intraobserver ( r = 0.86; 95% confidence interval [95% CI] 0.80–0.90) and a fair-to-good interobserver ( r = 0.64; 95% CI 0.51–0.74) agreement. The occiput-spine angle was significantly narrower in women who underwent obstetric intervention (cesarean or vacuum delivery) due to labor arrest (121° ± 10.5° vs 127° ± 9.4°, P = .03). Multivariable logistic regression analysis showed that narrow occiput-spine angle values (OR 1.08; 95% CI 1.00−1.16; P = .04) and nulliparity (OR 16.06; 95% CI 1.71−150.65; P = .02) were independent risk factors for operative delivery. A larger occiput-spine angle width (i.e., >125°) showed to be significantly associated with a shorter duration of labor (hazard ratio = 1.62; 95% CI 1.07−2.45; P = .02).
Conclusion
We described herein the “occiput-spine angle,” a new sonographic parameter to assess fetal head deflection during labor. Fetuses with smaller occiput-spine angle (<125°) are at increased risk for operative delivery.
The arrest of labor progression is the leading cause of obstetric interventions, including cesarean delivery and instrumental vaginal delivery. In the attempt to decrease the incidence of primary cesarean delivery, the classical definition of abnormal labor course has been revised recently, and a longer duration of the second stage has been declared as acceptable before diagnosing a labor arrest (up to 4 hours or more in nulliparous and to 3 hours or more in parous with epidural). Some authors, however, have challenged this new statement claiming that, based on the available evidence, a second stage of labor beyond 3 hours is unsafe for the unborn infant.
Deflexed cephalic presentations are an important cause of obstructed labor and account for one third of cesarean deliveries as the result of labor arrest. Three varieties of deflexed cephalic malpresentations traditionally are described according to the degree of head extension, including sinciput, brow, and face. In some of these cases, such as brow presentation, the achievement of vaginal delivery is not possible because the mean fetal head presenting diameter (mento-occipital diameter) is 13 cm, which is larger than the widest diameter of the birth canal (obstetric conjugate = 11 cm). The diagnosis traditionally is based on digital examination during labor, although the use of ultrasound to support clinical diagnosis has been reported recently.
Apart from these 3 varieties, minor degrees of fetal head deflexion in respect of the trunk but not clinically detectable may be sonographically documented at suprapubic ultrasound. It has never been established whether minor degrees of fetal head deflection are associated with disorders of labor progression. The aims of this study were: (1) to quantify the degree of fetal head deflection by the use of sonography during the first stage of labor; and (2) to determine whether a parameter derived from ultrasound examination (the occiput-spine angle) has a relationship with the course and outcome of labor.
Materials and Methods
A prospective, multicentric observational study with a sample of convenience was carried out at the Maternity Unit of the University Hospital of Bologna and Parma. The study was approved by the Institutional Review Board of the 2 istitutions. From January 2014 to April 2015 a nonconsecutive series of low-risk pregnant women in the first active stage of labor at or beyond 37 weeks of gestation were enrolled in this study. Patients were considered eligible for the study if 1 of the main investigators (T.G., F.B., or J.M.K.) was available in labor ward and if cervical dilatation was between 3 and 6 cm, the fetal head station was above the ischial spine (level 0), and regular uterine contractions were present. The patients provided a written informed consent to participate the study.
In these cases, the fetal head position was ascertained by mean of transabdominal sonography and described as on a clock face, as elsewhere reported Cases in which fetal occiput was posterior (between the 4- and 8-clock position) were excluded. Additional exclusion criteria were prelabor rupture of membranes lasting more than 24 hours, obvious signs of deflexed presentation or asynclitism at digital examination, or abnormal cardiotocography at enrollment. In fetuses with anterior (right or left) or transverse (right or left) position, a 2-dimensional sagittal picture of the fetal head and upper spine was acquired ( Figure 1 ) and stored in the ultrasound machine. On this image, the offline measurement of the angle formed by a line tangential to the occipital bone and a line tangential to the first vertebral body of the cervical spine (occiput-spine angle) was performed to quantify the degree of fetal head deflexion in respect of the trunk ( Figures 2 and 3 ). For each case, the angle was calculated twice and independently by the 3 main investigators (J.M.K., F.B., and T.G.) to evaluate the intra- and interobserver agreement of this measurement.
Ultrasound did not alter labor management because the examiner was not involved in the patient’s care. Furthermore, the results of ultrasound were not made available to the clinicians managing the patient. For each patient of the study group, the labor outcome and the mode of delivery was assessed retrospectively. Women submitted to obstetric interventions only due to nonreassuring fetal heart rate were eventually excluded because we sought to assess the relationship of the ultrasound findings with the risk of operative delivery due to prolonged or arrested labor. Prolonged first stage of labor was defined as cervical dilatation <1.2 cm/h in nullipara and 1.5 cm/h in multipara; arrest of the first stage was defined as nonprogression of cervical dilatation for >4 hours despite adequate uterine activity (3−5 contractions every 10 minutes) and rupture of membranes; prolonged second stage of labor was defined as fetal head descent <1 cm/h in nullipara and <2 cm/h in multipara; arrest of the second stage was defined as lack of fetal head descent after 2 or 3 hours of active pushing in nullipara (respectively without or with epidural) and after 1 or 2 hours in multipara (respectively without or with epidural).
Statistics
Intraobserver agreement in occiput-spine angle measurements was determined with the use of the Pearson correlation coefficient (Pearson r ); in addition, the repeatability coefficient was calculated as described by Bland and Altman —this defines the range within which 2 measurements by the same observer will fall for 95% of subjects. Interobserver agreement in occiput-spine angle measurements was expressed as the Pearson correlation coefficient (Pearson r ) and the 2-way mixed-effects intraclass correlation coefficient, with variance components being estimated by analysis of variance of replicate measurements. Agreement between the 2 observers also was assessed by calculating the limits of agreement as described by Bland and Altman ; the limits of agreement define the range within which 95% of the differences between 2 observers are likely to fall.
The first measurement by the expert operator was used for all subsequent analyses. The association between occiput-spine angle values and fetal head station was assessed graphically with the use of box plots ( Figure 4 ). The χ 2 test, Fisher exact test, and Mann−Whitney U test were used, where appropriate, to compare the distribution of demographic and clinical characteristics of women who underwent spontaneous vaginal delivery with those of women submitted to an operative delivery (defined as caesarean delivery or instrumental vaginal delivery).
A multivariable logistic regression model was used to assess whether the occiput-spine angle affected the mode of delivery (spontaneous vaginal delivery vs operative delivery) after we accounted for possible confounding variables. The area under the receiver operating characteristic curve was calculated to assess the discriminatory power of occiput-spine angle. Lastly, we investigated in a secondary analysis the association of occiput-spine angle with overall labor duration using a multivariable Cox proportional hazards model. We tested proportionality of the hazards using the method of Schoenfeld.
Results
Overall, 108 patients were included in the study group. Among these, spontaneous vaginal delivery occurred in 79, whereas cesarean delivery or vacuum were performed in 19 and 10, respectively. The indications for obstetric intervention were labor arrest in 19 patients and nonreassuring fetal heart rate in 10 patients. The mean value of the occiput-spine angle was 126° ± 9.8° with comparable measurements among the different subtypes of fetal occiput position: anterior 9.3%; right-transverse 14.8%; right-anterior 7.4%; anterior-left 31.5%; and anterior-left transverse 37.0%. There was no significant difference in the occiput-spine angle among these groups (analysis of variance F-test = 0.78, P = .5). The occiput-spine angle measurement yielded very good intraobserver agreement (Pearson = 0.86, 95% confidence interval [95% CI] 0.80–0.90; repeatability coefficient ±10.1°) and fair-to-good interobserver agreement (Pearson = 0.64, 95% CI 0.51–0.74; intraclass correlation coefficient 0.63, 95% CI 0.50–0.73). The mean interobserver difference was 2.1°, and the limits of agreement were –15.7° to 20.0°.
A significant direct correlation between the occiput-spine angle value and the fetal head station as assessed by digital examination was found. More specifically, the lower the fetal station was at the time of ultrasound assessment the wider the occiput-spine angle value appeared ( Figure 4 ). A comparison between women who underwent an obstetric intervention (caesarean or vacuum delivery) as the result of labor arrest and those who underwent spontaneous vaginal delivery is shown on Table 1 . In the operative group due to labor arrest, the following variables were significantly different in comparison with the women who underwent spontaneous vaginal delivery: lower parity (5.3% vs 43.0% P < .01), smaller occiput-spine angle (121° ± 10.5° vs 127° ± 9.4°, P = .03), lower 5-minute Apgar score (9.9 ± 0.2 vs. 9.7 ± 0.6 P = .04), increased duration of the first stage (8.4 ± 4.3 vs 4.6 ± 2.8 hours, P < .001), and increased birth weight (3635.0 ± 391.3 vs 3335.1 ± 479.4 g, P = .01). Interestingly, a higher fetal station at clinical assessment was not significantly associated to an increased risk of operative delivery due to labor arrest (OR 2.21, 95% CI 0.83–5.85, P = .11).
| Maternal, obstetric and newborn characteristics | All, n = 98 | Spontaneous delivery (n = 79) | Operative delivery (n = 19) | P value |
|---|---|---|---|---|
| Maternal age, mean±SD | 32.6 ± 5.8y | 32.3 ± 5.9y | 34.2 ± 5.4y | .22 |
| Race (%) | >.99 | |||
| White | 88 (89.8%) | 70 (88.6%) | 18 (94.7%) | |
| Asian | 8 (8.2%) | 7 (8.9%) | 1 (5.3%) | |
| African | 2 (2.0%) | 2 (2.5%) | 0 (0.0%) | |
| Body mass index, mean | 26.9 ± 3.6 kg/m 2 kg/m 2 | 26.7 ± 3.6 | 27.9 ± 3.7 | .16 |
| Multiparity | 35 (35.7%) | 34 (43.0%) | 1 (5.3%) | <.01 |
| Gestational age, wk | 39.6 ± 1.2 | 39.5 ± 1.2 | 39.7 ± 1.3 | .44 |
| Premature rupture of membranes | 31 (31.6%) | 22 (27.8%) | 9 (47.4%) | .11 |
| Epidural analgesia | 49 (50.0%) | 37 (46.8%) | 12 (63.2%) | .31 |
| Induction of labor | 56 (57.1%) | 42 (53.2%) | 14 (73.7%) | .13 |
| By | ||||
| Vaginal prostaglandines | 11 (1.6%) | 10 (23.8%) | 1 (7.1%) | |
| Endovenous oxytocin | 53 (94.6%) | 39 (92.9%) | 14 (100.0%) | |
| Length of stage 1, min | 318.8 ± 209.2 | 274.1 ± 168.6 | 505.0 ± 259.4 | <.001 |
| Length of stage 2, min | 44.2 ± 42.0 | 40.4 ± 32.9 | 60.4 ± 66.9 | .69 |
| Time between ultrasound and delivery (min) | 234.5 ± 170.9 | 199.8 ± 139.8 | 378.5 ± 213.2 | <.001 |
| Occiput-spine angle, ° | 126 ± 9.8 | 127 ± 9.4 | 121 ± 10.5 | .03 |
| Station of the fetal head | –2.0 ± 0.1 | –1.9 ± 0.1 | –2.2 ± 0.1 | .11 |
| Birthweight, g | 3393.3 ± 476.9 | 3335.1 ± 479.4 | 3635.0 ± 391.3 | .01 |
| Apgar score, 1 min | 9.0 ± 0.8 | 9.1 ± 0.8 | 8.7 ± 1.0 | .17 |
| Apgar score, 5 min | 9.9 ± 0.3 | 9.9 ± 0.2 | 9.7 ± 0.6 | .04 |
| Apgar score <7 at 1 or 5 min | 4 (4.1%) | 2 (2.6%) | 2 (10.5%) | .17 |
| pH a | 7.2 ± 0.8 | 7.2 ± 0.9 | 7.3 ± 0.1 | .70 |
| Base excess b | −5.2 ± 3.4 | −5.3 ± 3.3 | −4.7 ± 3.9 | .23 |
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