Head-perineum Distance Research Articles

Utility of intrapartum transperineal ultrasound to predict cases of failure in vacuum extraction attempt and need of cesarean section to complete delivery

Objectives: We aim to evaluate the predictive capacity of intrapartum transperineal ultrasound (ITU) to predict cases of failure in fetal extraction in operative deliveries with vacuum. Prospective, observational study performed on 61 nulliparous women, ≥37 weeks, singleton pregnancies at full dilatation who underwent transperineal ultrasound before placement of vacuum to complete fetal extraction. Working on the transperineal longitudinal plane, we evaluated the following: Angle of Progression (AoP), Progression Distance (PD) and head direction. In the transverse plane, midline angle (MLA) and head–perineum distance were assessed. Vacuum extractions were classified as easy (EG) (three or less vacuum pulls), difficult (DG) (more than three vacuum pulls) or impossible (IG) (delivery completed by cesarean section). Occipito-posterior presentations were not evaluated.Results: Fifty-two patients were studied (26-EG, 19-DG and 7-IG). No differences in obstetric, intrapartum or neonatal characteristics were observed between study groups, with the following exceptions: weight at birth (3147 g-EG, 3523 g-DG and 3588 g-IG) and number of vacuum pulls (1.4-EG, 4.4-DG and 4.1-IG; p < 0.0005). The AoP pushing was 133.1° ± 13.6-EG, 112.8° ± 12.8-DG and 99.1° ± 8.9-IG (p < 0.0005); “head-up” direction was identified in 84.6% of EG, 36.8% of DG and 28.6% of IG (p < 0.001); PD were 37.0 ± 10.4 mm, 33.3 ± 23.3 mm and 20.8 ± 9.5 mm (p < 0.0005); MLA were 35.0° ± 19.6, 55.3° ± 24.4 and 76.0° ± 23.2 (p = 0.003); and head–perineum distances were 41.8 ± 6.6 mm, 49.2 ± 9.8 mm and 48.0 ± 3.4 mm (p = 0.072), respectively.Conclusion: We have observed that the presence of an AoP with pushing <105°, a PD <25 mm, a “head-down” direction and a >45° MLA are very unfavorable ITU parameters which can be used to identify cases of high risk of fetal extraction failure in vacuum-assisted deliveries.

A model to predict vaginal delivery in nulliparous women based on maternal characteristics and intrapartum ultrasound

Accurate prediction of whether a nulliparous woman will have a vaginal delivery would be a major advance in obstetrics. The objective of the study was to develop such a model based on maternal characteristics and the results of intrapartum ultrasound. One hundred twenty-two nulliparous women in the first stage of labor were included in a prospective observational 2-centre study. Labor was classified as prolonged according to the respective countries' national guidelines. Fetal head position was assessed with transabdominal ultrasound and cervical dilatation by digital examination, and transperineal ultrasound was used to determine head-perineum distance and the presence of caput succedaneum. The subjects were divided into a testing set (n = 61) and a validation set (n = 61) and a risk score derived using multivariable logistic regression with vaginal birth as the outcome, which was dichotomized into no/cesarean delivery and yes/vaginal birth. Covariates included head-perineum distance, caput succedaneum, and occiput posterior position, which were dichotomized respectively into the following: ≤40 mm, >40 mm, <10 mm, ≥10 mm, and no, yes. Maternal age, gestational age, and maternal body mass index were included as continuous covariates. Dichotomized score is significantly associated with vaginal delivery (P = .03). Women with a score above the median had greater than 10 times the odds of having a vaginal delivery as compared with those with a score below the median. The receiver-operating characteristic curve showed an area under the curve of 0.853 (95% confidence interval, 0.678-1.000). A risk score based on maternal characteristics and intrapartum findings can predict vaginal delivery in nulliparous women in the first stage of labor.

Transperineal ultrasonography for labor management: accuracy and reliability

To compare ultrasound measurements and clinical assessments of cervical dilatation, fetal head station and fetal head position. Prospective, observational study. Tertiary care center (Ankara University Hospital), labor ward. Forty-three women in labor. Women were prospectively evaluated through simultaneous examinations (79 in total). Dilatation, head station, and position were recorded via digital examination whenever possible and were compared with ultrasonographic measurements of dilatation, angle of progression, head-perineum distance, and head position by two independent operators. Operators were blinded to each other's findings. Agreement between sonographic measurements and digital examinations. Ultrasonographic assessment of dilatation correlated significantly with digital examination. The interclass correlation coefficient of the two methods was 0.82 (95% confidence interval 0.73-0.88). Ultrasonography showed a mean lower cervical dilatation of 10 mm (95% limits of agreement: -36 to 16 mm) compared with digital examination. The angle of progression and head-perineum distances correlated moderately with the palpated head station (Pearson's correlation coefficients: 0.55 and -0.42, respectively; p < 0.001). Position could only be assessed in 40 of 79 examinations by digital examinations and the results showed low agreement with ultrasound findings. We observed good agreement between clinical and ultrasound assessment of dilatation, moderate agreement in assessing fetal station and low agreement assessing head position.

Intrapartum transperineal ultrasound as a predictor of instrumentation difficulty with vacuum-assisted delivery in primiparous women

Objectives: To assess the capability of different intrapartum transperineal ultrasound parameters to predict the difficulty of vacuum extraction. This is a prospective observational study performed between 04/2012 and 03/2013 on 72 primiparous-women, ≥37-weeks with singleton pregnancies at full dilatation that underwent transperineal ultrasound before vacuum placement for foetal extraction. Working in a transperineal longitudinal plane we evaluated: progression-angle, progression-distance and head direction; in a transverse plane: midline-angle and head-perineum distance. The vacuum extractions were classified as easy-group (EG) (≤3 vacuum pulls), difficult/impossible-group (DG)(≥4 pulls). Occiput-posterior presentations were not assessed.Results: Fifty-two (52) patients were studied (26 patients per study group). No differences were observed in obstetric, neonatal or intrapartum characteristics between the study groups, with the following exceptions: new-born (NB) weight (3147 g versus 3540 g) and the number of vacuum pulls (1.4 EG versus 4.3 DG; p < 0.0005). The progression angle was 133.1° (123°–143°) in EG and 109.2° (97.2°–121.2°) in DG (p < 0.0005); up direction of foetal head was 88% versus 34.5% (p < 0.0005); progression distance was 37 mm (26.6–47.4) versus 29.9 mm (8.8–51; p = 0.003); midline angle was 35° (15.4°–54.6°) versus 59.7° (34.5°–84.9°; p = 0.0005); head-perineum distance was 41.9 mm (35.2–48.6) versus 48.9 mm (40.5–57.3; p = 0.017). The area under the Receiver Operating Characteristic (ROC) curve for the progression angle was 0.9 (95%CI, 0.82–0.99), and the midline angle was 0.8 (95%CI, 0.67–0.92).Conclusion: If previous to the placement of the vacuum cup the progression angle is ≤120°, the foetal head direction is horizontal or down, and the midline angle is ≥35°, there is an 85% chance that the delivery will require more than 4 vacuum pulls.

Relationship between intrapartum transperineal ultrasound measurement of angle of progression and head–perineum distance with correlation to conventional clinical parameters of labor progress and time to delivery

Objective: To assess whether angle of progression (AOP) and head–perineum distance (HPD) measured by intrapartum transperineal ultrasound (ITU) correlate with clinical fetal head station (station); and whether AOP versus HPD varies during uterine contraction and relaxation. In a subset of primipaorus women, whether these ITU parameters correlate with time to normal spontaneous delivery (TD). Methods: We evaluated prospectively 100 primiparous and multiparous women at term in active labor. Transabdominal and transperineal ultrasound (sagittal and transverse plane) were used to measure fetal head position and ITU parameters, respectively. Digitally palpated station and cervical dilatation were also noted. The results were compared using regression and correlation coefficients. Results: Station was moderately correlated with AOP (r = 0.579) and HPD (r = −0.497). AOP was highly correlated with HPD during uterine contraction (r = −0.703) and relaxation (r = −0.647). In the subgroup of primiparous women, natural log of TD has the highest correlation with HPD and AOP during uterine contraction (r = 0.742), making prediction of TD similar to that of using cervical dilatation. Conclusion: ITU parameters were moderately correlated with station. There was constant high correlation between AOP and HPD. Prediction of TD in primiparous women using ITU parameters was similar to that of using cervical dilatation.

Fetal head–perineum distance measured by transperineal ultrasound imaging as a predictive factor for successful induction of labor

Objective The aim of the study was to evaluate fetal head–perineum distance measured by transperineal ultrasound imaging as a predictive factor for successful induction of labor. Patients and methods In all, 100 women scheduled for induction of labor with the head engaged were examined by both transvaginal and transperineal ultrasound together with the Bishop score. Follow-up was performed and outcome was compared. Receiver operating characteristic (ROC) curve was performed to estimate the best cutoff value. Results Areas under the ROC curve for prediction of vaginal delivery were 78% [95% confidence interval (CI), 61–95%] for fetal head–perineum distance (P=0.04), 73% (95% CI, 61–94%) for cervical length (P=0.05), 42% (95% CI, 26–60%) for cervical width (P=0.044), and 59% (95% CI, 34–85%) for Bishop score (P=0.046). The ROC curves indicated that fetal head–perineum distance of 8.6 cm or less and cervical length of 3.3 cm or less were the best cutoff levels for predicting the mode of delivery. Conclusion Fetal head–perineum distance measured by transperineal ultrasound imaging is a good predictive factor for successful induction of labor in pregnant women with engaged head.

Sonographic prediction of vaginal delivery in prolonged labor: a two‐center study

To investigate whether head-perineum distance (HPD) measured by transperineal ultrasound is predictive of vaginal delivery and time remaining in labor in nulliparous women with prolonged first stage of labor and to compare the predictive value with that of angle of progression (AoP). This was a prospective observational study at Stavanger University Hospital, Norway and Addenbrooke's Hospital, Cambridge, UK from January 2012 to April 2013, of nulliparous women with singleton pregnancies with cephalic presentation at term with prolonged first stage of labor. We used transperineal ultrasound to measure HPD (shortest distance between the outer bony limit of the fetal skull and the perineum) and AoP (angle between a line through the long axis of the symphysis and the tangent to the fetal head) and transabdominal ultrasound to classify fetal head position. The main outcomes were vaginal delivery and time remaining in labor. Of 150 women enrolled, 39 underwent delivery by Cesarean section. The area under the receiver-operating characteristics curve for the prediction of vaginal delivery was 81% (95% CI, 73-89%) using HPD as the test variable and 72% (95% CI, 63-82%) using AoP. HPD was ≤ 40 mm in 84 (56%) women, of whom 77 (92%; 95% CI, 84-96%) delivered vaginally. HPD was > 40 mm in the other 66 (44%) women, of whom 34 (52%; 95% CI, 40-63%) delivered vaginally. AoP was ≥ 110° in 84 of the 145 (58%) in whom this was available and, of these, 74 (88%; 95% CI, 79-93%) delivered vaginally. AoP was < 110° in the other 61 (42%) women, of whom 35 (57%; 95% CI, 45-69%) delivered vaginally. Multivariable logistic regression analysis showed that HPD ≤ 40 mm (odds ratio (OR), 4.92; 95% CI, 1.54-15.80), AoP ≥ 110° (OR, 3.11; 95% CI, 1.01-9.56), non-occiput posterior position (OR, 3.36; 95% CI, 1.24-9.12) and spontaneous onset of labor (OR, 4.44; 95% CI, 1.42-13.89) were independent predictors for vaginal delivery. Both ultrasound methods were predictive for the time remaining in labor. Transperineal ultrasound measurement of HPD and AoP provide important information about the likelihood of vaginal delivery and the time remaining in labor in nulliparous women with prolonged labor.

Ultrasound parameters and clinical examination to assess fetal head station in labor

Purpose: Different ultrasound methods such as intrapartum translabial ultrasound (ITU) head station, angle of progression (AOP), head-perineum distance (HPD) and head-symphysis distance (HSD) have been proposed to examine the progress of labor. We analyzed the relationship between these ultrasound parameters and compared them with digital palpation.

Ultrasound in labor: is it time for a more simplified approach?

The use of transperineal intrapartum ultrasound assessment has been introduced in recent years to complement traditional digital evaluation, with the aim of providing an objective evaluation of fetal head descent1−5. Despite an increasing number of studies on the subject, the application of this method in everyday clinical practice remains difficult, probably because of the technical difficulty in obtaining most of the sonographic parameters that have been suggested thus far6, 7. However, Eggebø et al. have proposed a measurement that is simply obtained, i.e. the distance between the fetal skull and the perineum on transperineal ultrasound (the head–perineum distance (HPD))3 (Figure 1a). More recently we have proposed another simple index of fetal head station inspired by the obstetric literature, i.e. the distance between the lower edge of the maternal symphysis pubis and the fetal skull, along the infrapubic line (the fetal head–symphysis distance (HSD)) (Figure 1b)8. We studied HSD with three-dimensional ultrasound which gave us the opportunity to measure both HPD and HSD retrospectively using the volume datasets we had stored (Figure 2). As shown in Figure 3, we found good correlation between the two measurements (Pearson's r coefficient, 0.826; P < 0.001), suggesting that it may be possible to use the two parameters interchangeably in assessment of fetal head station. Whether HPD or HSD is more useful in difficult clinical situations (e.g. prior to instrumental delivery) remains to be elucidated. We stress that we evaluated both parameters in sonographic volumes that had been acquired in the mid-sagittal plane, whereas HPD is classically evaluated in the axial plane. However, due to the clear definition given by Eggebø et al. of the HPD as ‘the shortest distance from the outer bony limit of the fetal skull to the skin surface of the perineum’3, we do not think that rotation of the ultrasound probe would result in any difference in the evaluation of this parameter. To summarize, both HPD and HSD are objective, simple and reliable indices of fetal station evaluation in labor and correlate highly with each other. We strongly encourage prospective studies on the use of these measurements. However, we feel that there is already sufficient evidence to justify their use as complementary parameters in the labor ward, especially in clinically challenging situations when the highly subjective digital evaluation remains the only tool in the armamentarium of obstetric personnel. A. Youssef*, F. Bellussi, E. Maroni, G. Pilu, N. Rizzo and T. Ghi Department of Obstetrics and Gynecology, Sant'Orsola-Malpighi University Hospital, University of Bologna, Via Massarenti, 40138 Bologna, Italy *Correspondence. (e-mail: [email protected])

Comparison between ultrasound parameters and clinical examination to assess fetal head station in labor

Several ultrasound parameters, including intrapartum transperineal ultrasound (ITU) head station, angle of progression (AOP), head-perineum distance (HPD) and head-symphysis distance (HSD), have been suggested to assess fetal head station during labor. The aim of this study was to analyze the relationship between these ultrasound parameters and to compare them with digital palpation. We analyzed 106 stored volume dataset pairs that had been acquired at Stavanger University Hospital, Norway, from nulliparous women at term with prolonged first stage of labor. The volumes were acquired using a three-dimensional transducer applied between the labia majora in a mid-sagittal plane and perineally in a transverse plane. Digitally palpated head station and cervical dilatation were also noted. The results were compared using regression and correlation coefficients. There were good correlations between ITU head station and HPD (r = 0.71), between ITU head station and HSD (r = 0.74) and between HSD and HPD (r = 0.75). Palpated head station showed only moderate correlation with ITU head station (r = 0.52). Cervical dilatation showed a weak correlation with ITU head station (r = 0.30). The ultrasound parameters showed a high degree of correlation with each other, but only moderate correlation to vaginally palpated fetal head station.

P18.03: The fetal head‐symphysis distance correlates well with fetal head perineum distance on three‐dimensional intrapartum ultrasound

P18.03: The fetal head‐symphysis distance correlates well with fetal head perineum distance on three‐dimensional intrapartum ultrasound

Agreement between two‐ and three‐dimensional transperineal ultrasound methods in assessing fetal head descent in the first stage of labor

To study intraobserver repeatability and intermethod agreement between two- (2D) and three-dimensional (3D) transperineal ultrasound methods in assessing fetal head descent during the first stage of labor. Fetal head descent was measured with transperineal ultrasound as the fetal head-perineum distance and the angle of progression in 106 primiparous women with prolonged first stage of labor. A single obstetrician performed all the scans, and another obstetrician analyzed the acquired 2D images and 3D volumes, blinded to clinical assessments and labor outcome. Intraobserver repeatability and intermethod agreement between 2D and 3D methods were analyzed. The repeatability coefficient was ± 4.1 mm in 2D acquisitions and ± 1.7 mm in 3D acquisitions of fetal head-perineum distance. The intraclass correlation coefficients (ICC) were 0.94 for 2D and 0.99 for 3D measurements. The angle of progression repeatability coefficients were ± 6.7° using 2D and ± 5.7° using 3D ultrasound and ICCs were 0.91 and 0.94, respectively. The intermethod ICC for fetal head-perineum distance in 2D vs 3D acquisitions was 0.95 and for angle of progression it was 0.93; the intermethod 95% limits of agreement were - 5.8 mm to + 7.2 mm and - 8.9° to + 13.7°, respectively. Cohen's kappa for 2D vs 3D acquisitions was 0.85 using 40 mm as a cut-off level for fetal head-perineum distance and 0.79 using 110° as cut-off level for angle of progression. For one ultrasound operator the intraobserver repeatability and agreement between 2D and 3D ultrasound methods in prolonged first stage of labor were good. Given that 2D methods are simpler to learn and can be analyzed quickly online, 2D equipment might therefore be preferred in the labor room.

Prediction of delivery mode with transperineal ultrasound in women with prolonged first stage of labor

To investigate if fetal head-perineum distance and angle of progression measured with two-dimensional (2D) and three-dimensional (3D) transperineal ultrasound could predict outcome of labor in primiparous women with prolonged first stage of labor. This was a prospective observational study of 110 primiparous women with singleton cephalic presentation at term diagnosed with prolonged first stage of labor. Digital assessment of fetal station was related to the ischial spine. Fetal head descent was measured with transperineal ultrasound as the shortest distance from the fetal head to the perineum, and the angle between the pubic symphysis and the fetal head. Receiver-operating characteristics (ROC) curves were constructed and 2D and 3D data acquisitions were compared. The stored 3D volumes were assessed by an examiner blinded to all other data. Vaginal delivery vs. Cesarean section was the primary outcome. Cesarean section was performed in 25% of the women. Areas under the ROC curves for prediction of vaginal delivery were 81% (95% confidence interval (CI), 71-91%) (P < 0.01) and 76% (95% CI, 66-87%) (P < 0.01) for fetal head-perineum distance and angle of progression, respectively, as measured by 2D ultrasound and 66% (95% CI, 54-79%) for digital assessment of fetal station (P = 0.01). In 50% of women fetal head-perineum distance was ≤ 40 mm and 93% (95% CI, 83-97%) of them delivered vaginally vs. 18% (95% CI, 5-48%) with distance > 50 mm. In 48% of women the angle of progression was ≥ 110° and 87% (95% CI, 75-93%) of them delivered vaginally vs. 38% (95% CI, 21-57%) with angle < 100°. Results from 2D and 3D acquisitions were similar. Fetal head-perineum distance and angle of progression measured with 2D or 3D ultrasound can predict labor outcome, with similar predictive values for the two techniques.

Can Ultrasound Measurements Replace Digitally Assessed Elements of the Bishop Score?

It has been suggested that an objective assessment of cervical ripeness obtained by ultrasound might be preferable to digital assessment using the Bishop score in predicting successful induction. Although the predictive value of both individual ultrasound measurements and elements of the Bishop score appears to be limited, some combination of ultrasound and Bishop score measurements might be more useful. This prospective study compared the utility of cervical measurements obtained by ultrasound imaging before induction of labor with corresponding elements of the digitally determined Bishop score for predicting the probability of successful induction. The study group included 275 women, with singleton pregnancies at ≥37 weeks of gestation and a fetus in cephalic presentation, who were admitted for induction of labor at a university hospital in Norway. Immediately before labor induction, transperineal ultrasound was used to measure the distance from the fetal head to the perineum, the cervical length, posterior angle, and dilation. A Bishop score was then determined digitally by an operator without knowledge of the ultrasound measurements. Receiver operating characteristics (ROC) curves were used to evaluate the probability of a successful labor induction. Univariate analysis showed that predictive factors for successful induction included previous vaginal birth, maternal height, digital assessment of cervical dilatation, and certain sonographic measurements: a fetal head-perineum distance ≤40 mm, cervical length ≤25 mm, and posterior cervical angle >90 degrees (for all factors, P 90 degrees (aOR, 2.6; 95% CI, 1.1-6.1) had only borderline significance. ROC curve analysis showed that a scoring model combining ultrasound measurements of fetal head-perineum distance, cervical length, and posterior cervical angle with digital assessment of cervical dilatation discriminated between successful and failed induction at a curve value of 71 % (95% CI, 61 %-80%, P < 0.01). These findings suggest that a combination of digitally-assessed cervical dilation and ultrasound measurements of fetal head descent, cervical length, and position can predict success of labor induction.

Can ultrasound measurements replace digitally assessed elements of the Bishop score?

To compare elements of the Bishop score and corresponding sonographic measurements before induction of labor, and assess how predictive factors can be used in a clinical setting. Prospective comparative clinical study. Secondary referral center, university hospital. A total of 275 women scheduled for induction of labor. Fetal head descent to the perineum was assessed with transperineal ultrasound. Cervical length, posterior angle, and dilatation were evaluated with transvaginal ultrasound followed by Bishop score without knowledge of the ultrasound measurements. Univariable regression analyses of successful induction were significant for digital assessment of cervical dilatation, ultrasound measured fetal head-perineum distance < or =40 mm, ultrasound measured cervical length < or =25 mm, and ultrasound measured posterior cervical angle >90 degrees. After adjusting for maternal factors in a multivariable model, estimates were significant for previous vaginal birth (OR 5.3; 95% CI 2.1-13.9, p<0.01), but borderline for maternal height (OR 1.1; 95% CI 1.0-1.2, p=0.01) and ultrasound measured posterior cervical angle >90 degrees (OR 2.6; 95% CI 1.1-6.1, p=0.03). A scoring model combining ultrasound measured fetal head-perineum distance, cervical length, cervical posterior angle and digitally assessed cervical dilatation, discriminated successful and failed induction at 71% (95% CI 61-80%, p<0.01) area under the receiver-operating characteristics curve. Digital assessment of fetal head descent, cervical length and position can possibly be replaced with ultrasound measurements. Dilatation is best evaluated with digital assessment. Combination of these four factors can predict success of labor induction.

The secret cervix

Recently receiving much interest in the field of obstetric ultrasound are the assessment of its accuracy to predict spontaneous onset of labor at term, the success rate of induction of labor and the risk of Cesarean section after onset of spontaneous or induced labor. Two main factors drive current research. (i) Although the Bishop score1 remains the standard method for predicting the duration and safety of induced labor, digital cervical examination is subjective2 and the Bishop score may be of limited value in women with a low score3, 4. Ultrasound assessment of the cervix is more objective and reproducible5. (ii) The ultrasonographic measurement of cervical length is predictive of delivery before 37 weeks of gestation, the risk of spontaneous preterm delivery increasing as the ultrasonographic length of the cervix decreases6-8. Numerous studies, therefore, have compared ultrasonographic measurement of cervical length with digital cervical examination in the prediction of the time interval to delivery and the incidence of vaginal delivery in patients undergoing induction of labor9-20. Their results, however, are contradictory, probably because of the numerous weaknesses and/or differences in their methodologies (e.g. heterogeneous populations, including variable proportions of patients with an unripe cervix, absence of standardization of the method of induction (amniotomy and/or oxytocin and/or different prostaglandins), lack of power, secondary exclusion of patients) and the multiple and different endpoints used to evaluate success or failure of induction (e.g. total Cesarean section rate or vaginal delivery rate, duration of latent phase or total duration of labor, vaginal delivery within 24 h of induction, rate of active labor within 12 h, Cesarean section rate before dilatation reaches 8 cm). The different parameters used to assess the cervix both clinically (Bishop score or one of its components) and ultrasonographically (cervical length, dilatation, presence of wedging or funneling, cervical angle) also contribute to the differing results. Recently, Hatfield et al.21 reviewed the literature and performed a meta-analysis, evaluating sonographic cervical assessment to predict successful induction of labor; they concluded that it is not an effective predictor of successful labor induction and that it cannot predict any specific outcome (such as mode of delivery). However, studies investigating the usefulness of sonographic assessment of cervical length in the prediction of spontaneous onset of labor in low-risk women enrolled at 37 or more weeks of gestation consistently report more encouraging results. Seven studies22-28, including a total of 2308 patients, found cervical length measurement to be a significant predictor of the likelihood of the onset of spontaneous labor and/or of delivery within 7 days and/or delivery by 41 weeks. Overall, therefore, the published data show that ultrasonographic measurement of cervical length is a useful tool to predict imminent spontaneous delivery in preterm and term births, but that it is ineffective in predicting the success or failure of induction of labor. These apparently discordant results are in fact convergent: while cervical length is not a marker of cervical ripening, it may well be a marker of myometrial ripeness. The processes of cervical ripening and of uterine activity are independent29. Cervical and myometrial ripeness are often correlated, but this is not necessarily always the case; thus, it is possible that myometrial ripeness may sometimes precede cervical ripening and, conversely, that uterine activity is not coordinated with physiological preparedness for labor. We have shown previously22 that ultrasound measurement of cervical length between 39 + 4 weeks and 40 + 3 weeks of gestation is predictive of the onset of spontaneous labor within 7 days, while fetal fibronectin (fFN) assay is useful in predicting the risk of Cesarean section. We hypothesized that ultrasound examination of the cervix and fFN assay addressed two different physiological processes involved in parturition. In predicting the time to the onset of spontaneous labor, cervical length may be a marker of myometrial ripeness, whereas fFN is a confirmed marker of cervical ripening. Furthermore, the absence of preliminary myometrial ripeness among patients requiring prostaglandins for induction of labor may explain the absence of correlation between cervical length and induced labor duration18. Clearly, other sonographic approaches are necessary to predict the success rate of induction of labor and the risk of Cesarean section after induction. Eggebo et al.30 in this issue of the Journal contribute to this area of research, by evaluating fetal head–perineum distance as a predictive factor for vaginal delivery after induction of labor. Although its predictive value was similar to that of ultrasonographically measured cervical length and Bishop score, the main strength of this study is to open our minds towards the investigation of new approaches to better predict the outcome of induction of labor when the cervix is unripe. As unsatisfying as it may be, we must concede that the cervix has secrets as yet undiscovered by ultrasound.

Ultrasound assessment of fetal head–perineum distance before induction of labor

To evaluate fetal head-perineum distance measured by ultrasound imaging as a predictive factor for induction of labor, and to compare this distance with maternal factors, the Bishop score and ultrasound measurements of cervical length, cervical angle and occiput position. The study included 275 women admitted for induction of labor. The fetal head-perineum distance was measured by transperineal ultrasound imaging as the shortest distance from the outer bony limit of the fetal skull to the skin surface of the perineum. Cervical length and angle was measured by transvaginal ultrasound examination, and fetal head position was assessed by transabdominal ultrasound imaging. The Bishop score was assessed without knowledge of ultrasound measurements. Receiver-operating characteristics (ROC) curves were used for evaluation of the probability of a successful vaginal delivery. The time from induction to delivery was tested using Cox regression analysis with ultrasound measurements, parity and body mass index (BMI) as possible predictive factors. Areas under the ROC curve for prediction of vaginal delivery were 62% (95% CI, 52-71%) for fetal head-perineum distance (P = 0.03), 61% (95% CI, 51-71%) for cervical length (P = 0.03), 63% (95% CI, 52-74%) for cervical angle (P = 0.02), 61% (95% CI, 52-70%) for Bishop score (P = 0.03) and 60% (95% CI, 51-69%) for BMI (P = 0.05). The Cesarean delivery rate was 22% among nulliparous and 5% among parous women (P < 0.01). Parity, fetal head-perineum distance, cervical length and cervical angle were contributing factors predicting vaginal delivery within 24 h in a Cox regression model. Occiput posterior position had no significant predictive value. Fetal head-perineum distance measured by transperineal ultrasound examination can predict vaginal delivery after induction of labor, with a predictive value similar to that of ultrasonographically measured cervical length and the Bishop score. However, we judge none of these methods used alone to be good enough in a clinical setting.

The Bishop score components correlated to sonographic measurements prior to induction of labor

Objective: The Bishop score remains the gold standard for assessing favourability for induction of labor, but it is a subjective evaluation with limitations. The aims of this prospective study were to relate the components of the Bishop score to corresponding sonographic measurements and to assess predictive values for a successful labor induction. Methods: In 275 women the fetal head-perineum distance was measured through a transverse transperineal scan, and the cervical length, posterior cervical angle and cervical dilatation through a transvaginal scan. The Bishop score was assessed without knowledge of ultrasound measurements immediately after the scans. Correlation analyses were done, and receiver-operating characteristics (ROC) curves were used for evaluation of the probability of a successful vaginal delivery. Results: By sonographic assessment the cervix was closed in 219 (80%) of the women compared to 58 (21%) by digital assessment. Spearman\'s correlation coefficient for digital and ultrasound assessment of cervical length was 0.54 (p<0.01), fetal head decent 0.23 (p<0.01) and cervical position/angle 0.03. The predictive value for a vaginal delivery after induction of labor for Bishop score and ultrasound measurements is presented in figure 1. The best predictive factors for a vaginal delivery were digital assessment of cervical dilatation with 61%; 95% CI 51–71% (p=0.03) under the ROC curve area, and a combination of ultrasound measured fetal head-perineum distance, cervical length and cervical angle with 67%; 95% CI 56–77% (p<0.01) under the curve area. Conclusion: The correlation between ultrasound and digital assessments is weak. None of the factors used alone are good predictors of labor outcome. Combinations of factors improve the prediction.