Ultrasound assessment of fetal head–perineum distance before ...

Ultrasound Obstet Gynecol 2008; 32: 199–204Published online 5 June 2008 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/uog.5360Ultrasound assessment of fetal head–perineum distancebefore induction of laborT. M. EGGEBØ*, C. HEIEN*, I. ØKLAND*, L. K. GJESSING*, P. ROMUNDSTAD†and K. Å. SALVESEN‡*Department of Obstetrics and Gynecology, Stavanger University Hospital, Stavanger and †Department of Public Health, NorwegianUniversity of Science and Technology, ‡National Center for Fetal Medicine, Trondheim University Hospital (St Olav’s Hospital) andDepartment of Laboratory Medicine, Children’s and Women’s Health, Norwegian University of Science and Technology, Trondheim,NorwayKEYWORDS: Bishop score; cervical angle; cervical length; fetal head–perineum distance; induction of labor; occiputposterior; ultrasoundABSTRACTObjectives To evaluate fetal head–perineum distancemeasured by ultrasound imaging as a predictive factorfor induction of labor, and to compare this distancewith maternal factors, the Bishop score and ultrasoundmeasurements of cervical length, cervical angle andocciput position.Methods The study included 275 women admitted forinduction of labor. The fetal head–perineum distancewas measured by transperineal ultrasound imaging asthe shortest distance from the outer bony limit of thefetal skull to the skin surface of the perineum. Cervicallength and angle was measured by transvaginal ultrasoundexamination, and fetal head position was assessedby transabdominal ultrasound imaging. The Bishopscore was assessed without knowledge of ultrasoundmeasurements. Receiver–operating characteristics (ROC)curves were used for evaluation of the probability of asuccessful vaginal delivery. The time from induction todelivery was tested using Cox regression analysis withultrasound measurements, parity and body mass index(BMI) as possible predictive factors.Results Areas under the ROC curve for prediction ofvaginal delivery were 62% (95% CI, 52–71%) forfetal 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 deliveryrate was 22% among nulliparous and 5% among parouswomen (P < 0.01). Parity, fetal head–perineum distance,cervical length and cervical angle were contributingfactors predicting vaginal delivery within 24 h in a Coxregression model. Occiput posterior position had nosignificant predictive value.Conclusions Fetal head–perineum distance measured bytransperineal ultrasound examination can predict vaginaldelivery after induction of labor, with a predictive valuesimilar to that of ultrasonographically measured cervicallength and the Bishop score. However, we judge none ofthese methods used alone to be good enough in a clinicalsetting. Copyright © 2008 ISUOG. Published by JohnWiley & Sons, Ltd.INTRODUCTIONInduction of labor (IOL) is one of the most frequentobstetric procedures and occurs in 13–20% of termpregnancies 1,2 . IOL is associated with an increased riskof operative delivery 3,4 , with a Cesarean delivery rate ofaround 25% in nulliparous women and around 5% inparous women 5 .The Bishop score remains the ‘gold standard’ forassessing suitability for IOL 6 , but it is a subjectiveevaluation with limitations 7,8 . Parity and body mass index(BMI) are other factors predictive of labor induction 9–11 ,and transvaginal ultrasound measurements of cervicallength and cervical angle may be added to the list.Transvaginal ultrasound measurements perform betterthan the Bishop score in some studies 8,9,11 –14 , but notin others 15 –17 . A recent meta-analysis concluded thatCorrespondence to: Dr T. M. Eggebø, Department of Obstetrics and Gynecology, Stavanger University Hospital, 4068 Stavanger, Norway(e-mail: tme@lyse.net)Accepted: 1 March 2008Copyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd.ORIGINAL PAPER

200 Eggebø et al.transvaginal ultrasound imaging was equal to the Bishopscore, but the authors called for more research 18 .The timing of IOL for common indications suchas diabetes, post-term pregnancy and pre-eclampsia iscontroversial. If a successful labor outcome could bepredicted, IOL could be considered in cases with ‘softindications’ or on maternal request. Thus, any reliablemethods of preinduction assessment would be valuabletools in counseling women before IOL.Ultrasound measurement of the fetal head–perineumdistance has not been evaluated as a predictive factor inwomen scheduled for IOL, but it was found to predictlabor outcome for women with prelabor rupture ofmembranes at term 19 . The aim of the present study wasto evaluate fetal head–perineum distance measured bytransperineal ultrasound imaging as a predictive factorfor IOL, and to compare this distance with maternalfactors, Bishop score and ultrasound measurements ofcervical length, cervical angle and occiput position.METHODSIn all, 5487 women delivered at Stavanger UniversityHospital from January 2006 to March 2007. Womenwere eligible for the study if they had a live singletonpregnancy with cephalic presentation and a gestationalage of more than 37 completed weeks according to amid-trimester scan. Women with a previous Cesareansection were not eligible. Of the 4918 women eligible forinclusion in the study, 710 had IOL and, of these, 275(39%) were included in the study. The main reason fornon-inclusion was unwillingness to participate. The localethics committee approved the study and all women gavewritten informed consent. The characteristics of the studypopulation and the indications for IOL are presented inTable 1.Table 1 Characteristics of the study population (n = 275)Parameter% or median (range)Nulliparous 47Maternal age (years) 30 (18–45)Body mass index (kg/m 2 ) 24 (17–45)Gestational age (weeks) 40 (37–43)Caucasian 96Cesarean delivery 13Operative vaginal delivery 18Epidural analgesia 54Blood loss (mL) 300 (100–3400)Indication for induction of laborPost-term pregnancy 26Pre-eclampsia/hypertension 8Diabetes 3Fetal growth restriction 5Macrosomia 10Maternal request 12Prelabor rupture of membranes 20Oligohydramnios 3Other 13Two obstetricians and three specially trained midwivesperformed the ultrasound measurements with an EUBHitachi 5500 (Kashiwa, Japan) device with a 3.5–7.5-MHz multifrequency transabdominal transducer and a5.0–9.0-MHz multifrequency transvaginal transducer.The Bishop score was assessed after the scan andimmediately before IOL by the obstetrician on call,who was blinded to the ultrasound measurements. Allexaminations were performed with the woman in a supineposition and with an empty bladder.The fetal occiput position was evaluated by transabdominalultrasound imaging and classified as describedby Rane et al. 11 . The fetal head–perineum distance wasmeasured as the shortest distance from the outer bonylimit of the fetal skull to the skin surface of the perineumby a transperineal ultrasound examination in a transverseview, as described in a previous study 19 . The transducerwas moved and angled until the shortest distance tothe fetal head was visualized. The soft tissue was compressedwith firm pressure without creating discomfortfor the woman. The cervix was evaluated by transvaginalultrasound examination without putting pressure on thecervix. The cervical length was measured as describedby Valentin et al. 20 , and the posterior cervical angle wasassessed as described by Rane et al. 11 . The mean of threemeasurements was used in the calculations.IOL was performed according to the managementprotocol of the hospital before the study. Labor wasinduced with amniotomy followed by oxytocin if thecervix was favorable (Bishop score ≥ 6). If the cervix wasunfavorable, labor was induced with 25 µg misoprostolapplied vaginally every 4 h (up to a maximum of 100 µgin 24 h and a total maximum dose of 200 µg) untilcontractions were regular. Previous studies have useddiverse definitions for successful IOL 8,11,21 . We definedsuccessful IOL as resulting in a vaginal delivery, eitherspontaneous or operative, with no limit on the time forthis to occur.The fetal head–perineum distance was evaluated atcut-off levels of 40 mm, 45 mm and 50 mm, cervicallength at cut-off levels of 20 mm, 25 mm and 30 mm,and cervical angle using cut-off levels of 90 ◦ and 120 ◦degrees. The cut-off points were chosen according toprevious studies 8,11,19,22,23 and the fact that World HealthOrganization Stage 0 (fetal head at the ischial spines) isapproximately 5 cm from the perineum 24 . The fetal headposition was evaluated as occiput posterior comparedwith other positions 11 .In a previous study of women with prelabor rupture ofmembranes at term 19 , we reported Cesarean delivery ratesof 4% among women with a short fetal head–perineumdistance and 15% among women with a long distance.Using an alpha of 0.05 and a power of 0.8, and assuminga ratio of 2 : 1 for long to short distance, we calculatedthat 250 women were needed for an adequately poweredstudy.Data were entered into the statistical softwarepackage SPSS version 14.0 (Chicago, IL, USA). Forstatistical analyses, Mann–Whitney U-tests, Chi-squareCopyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd. Ultrasound Obstet Gynecol 2008; 32: 199–204.

Ultrasound assessment before induction of labor 201tests, Fisher’s exact tests and Pearson correlation wereused. The predictive value of maternal factors, ultrasoundmeasurements and Bishop scores for a successfulvaginal delivery was evaluated using receiver–operatingcharacteristics (ROC) curves, and the area under thecurve was used as discriminator. The induction–deliveryinterval within 24 h was evaluated using Kaplan–Meiersurvival analyses and Cox regression analyses. Womenexperiencing a Cesarean section or a time to delivery> 24 h were censored. In the Cox regression analyses,fetal head–perineum distance, cervical length, cervicalangle, occiput position, parity and BMI were tested aspossible predictive factors. In additional analyses, wealso adjusted for maternal age, gestational age, birthweight and head circumference as possible confounders.The Bishop score was not tested in the survival analysesbecause the induction methods were stratified accordingto this score. P < 0.05 was considered significant.RESULTSThe overall Cesarean delivery rate was 13%. Onehundred and thirty (47%) women were nulliparous. TheCesarean delivery rate among nulliparous women was22% compared with 5% among women with one ormore previous vaginal deliveries (P < 0.01).Table 2 shows test characteristics of maternal characteristics,the Bishop score and ultrasound measurementsas predictive factors for a successful vaginal delivery.Areas under the ROC curve for prediction of vaginaldelivery (Figure 1) were 62% (95% CI, 52–71%) forfetal 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), 60% (95% CI,51–69%) for BMI (P = 0.05), 51% (95% CI, 41–61%)for maternal age (P = 0.85) and 56% (95% CI, 45–66%)for gestational age (P = 0.27). The ROC curves indicatedthat fetal head–perineum distance ≤ 40 mm, cervicalSensitivity (%)100806040200 0 20 40 60 80 100False-positive rate (%)Figure 1 Receiver–operating characteristics curves for fetalhead–perineum distance ( ), cervical length ( ) and Bishopscore ( ) in predicting a vaginal delivery.length ≤ 25 mm and cervical angle > 90 ◦ were the bestcut-off levels for predicting vaginal delivery.The mean distance from the fetal head to the perineumwas 47.5 mm and the median was 46 (range, 26–108)mm. The Cesarean delivery rate was 4% among 73 womenwith a short distance (≤ 40 mm) and 16% among 202women with a long distance (P = 0.01). Characteristicsof labor outcomes stratified according to a cut-off level at40 mm are presented in Table 3.The mean cervical length was 27.6 mm and the medianwas 28 (range, 6–54) mm. The Cesarean delivery rate was8% among 112 women with a short cervix (≤ 25 mm)and 17% among women with a long cervix (P = 0.05).The cervical length and the fetal head–perineum distancewere correlated (Pearson r = 0.39, P < 0.01).Table 2 Test characteristics of ultrasound measurements, maternal characteristics and Bishop score in predicting vaginal deliverySensitivity(% (95% CI))FPR(% (95% CI))PPV(% (95% CI))NPV(% (95% CI)) LRFetal head–perineum distance (mm)≤ 40 29.3 (23.6–35.5) 8.3 (1.7–22.5) 95.9 (88.5–99.1) 16.3 (11.5–22.2) 3.5≤ 45 49.4 (42.9–55.9) 41.7 (25.5–59.3) 88.7 (82.1–93.6) 14.8 (9.4–21.7) 1.2≤ 50 67.4 (61.0–73.3) 47.2 (30.4–64.5) 90.4 (85.2–94.3) 19.6 (12.2–28.9) 1.4Cervical length (mm)≤ 20 26.4 (20.9–32.4) 13.9 (4.7–29.5) 92.6 (83.7–97.6) 15.0 (10.4–20.6) 1.9≤ 25 43.1 (36.7–49.6) 25.0 (12.1–42.2) 92.0 (85.3–96.3) 16.6 (11.2–23.2) 1.7≤ 30 62.3 (55.9–68.5) 44.4 (27.1–61.9) 90.3 (84.7–94.4) 18.2 (11.5–26.7) 1.4Cervical angle ( ◦ )> 90 87.9 (83.0–91.7) 61.1 (43.5–76.9) 90.5 (86.0–94.0) 32.6 (19.1–48.5) 1.4≥ 120 37.7 (31.5–44.1) 27.8 (14.2–45.2) 90.0 (82.4–95.1) 14.9 (9.9–21.0) 1.4Para 1+ 57.7 (51.2–64.1) 19.4 (8.2–36.0) 95.2 (90.3–98.0) 22.3 (15.5–30.4) 3.0BMI < 30 kg/m 2 84.9 (79.8–89.2) 77.8 (60.8–89.9) 87.9 (83.0–91.8) 18.2 (8.2–32.7) 1.1Bishop score ≥ 6 43.5 (37.1–50.1) 25.0 (12.1–42.8) 92.0 (85.4–96.3) 16.7 (11.3–23.3) 1.7FPR, false-positive rate; LR, likelihood ratio; NPV, negative predictive value; PPV, positive predictive value.Copyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd. Ultrasound Obstet Gynecol 2008; 32: 199–204.

202 Eggebø et al.Table 3 Characteristics of labor stratified into groups with short(≤ 40 mm) or long (>40 mm) fetal head–perineum distance100Fetal head–perineum distance≤ 40 mm(n = 73)> 40 mm(n = 202) P*Epidural analgesia (%) 47 56 0.17Cesarean delivery (%) 4 16 0.01Operative vaginal19 17 0.65delivery (%)Birth weight (g) 3577 (603) 3741 (585) 0.08Apgar score 1 min 8.68 (1.04) 8.44 (1.18) 0.04Apgar score 5 min 9.23 (0.64) 9.15 (0.80) 0.66Umbilical artery pH 7.26 (0.08) 7.24 (0.08) 0.07Umbilical artery−3.16 (3.57) −3.66 (3.36) 0.23base excessDuration of labor (min)Induction to delivery 540 (371) 809 (541) < 0.01Labor (stages 1 + 2) 259 (120) 321 (195) 0.06Labor stage 2 21 (20) 21 (21) 0.80Values are % or mean (SD). *Mann–Whitney U-test, Chi-squaretest or Fisher’s exact test.The mean cervical angle was 109 ◦ and the medianwas 110 ◦ (range, 60–150 ◦ ). The Cesarean delivery ratewas 33% among 43 women with a cervical angle ≤ 90 ◦compared with 9% if the cervical angle was > 90 ◦(P < 0.01). Cervical angle correlated with cervical length(Pearson r =−0.23, P < 0.01) and fetal head–perineumdistance (Pearson r =−0.19, P < 0.01).In all, 61 (22%) women had the fetal head in anocciput posterior position before the start of induction.The Cesarean delivery rate was 7% compared with15% among women with fetal heads in other positions(P = 0.13).The time from induction to delivery is demonstratedfor women with a fetal head–perineum distance ≤ 40 mmand those with a distance > 40 mm in a Kaplan–Meiersurvival plot (Figure 2). The difference between groupswas statistically significant (P < 0.001, log rank test).Twenty-four hours after induction 3% (95% CI, 0–10%)of women with a short distance and 11% (95% CI,7–17%) of women with a long distance were still in labor.In a multivariate Cox regression analysis, parity,fetal head–perineum distance ≤ 40 mm, cervical length≤ 25 mm and cervical angle > 90 ◦ predicted vaginal deliverywithin 24 h (Table 4). BMI and occiput posterior positionwere not contributing factors. Multivariable adjustmentsfor potential confounders did not change the results.There was one catastrophic event in the study group.One woman had a sudden cardiac arrest 10 h after IOL. ACesarean delivery was performed immediately. A healthybaby was delivered, but the mother died. A medicolegalautopsy was performed, and the conclusion was amnioticfluid embolism.DISCUSSIONFetal head–perineum distance measured by transperinealultrasound examination was found to be a predictor ofPercentage of women undelivered8060402000 5 10 15 20 25Time to delivery (h)Figure 2 Kaplan–Meier estimates of proportions of women notdelivered within 24 h, according to sonographically measured fetalhead–perineum distance (≤ 40 mm, ; > 40 mm, ). Womenexperiencing Cesarean section or a time to delivery > 24 hwerecensored. P < 0.001 (log rank test).Table 4 Relative risk (hazard ratio) for a vaginal delivery within24 h after induction according to fetal head–perineum distance,cervical length, cervical angle, occiput position, parity and bodymass index (BMI) determined by Cox regression analysisHazard ratio(95% CI) PFetal head–perineum distance (mm) 0.001> 40 1.00 (reference)≤ 40 1.67 (1.23–2.26)Cervical length (mm) < 0.001> 25 1.00 (reference)≤ 25 2.35 (1.75–3.14)Cervical angle ( ◦ ) 0.005≤ 901.00 (reference)> 90 1.92 (1.22–3.04)Fetal head position 0.13Occiput anterior or transverse 1.00 (reference)Occiput posterior 1.27 (0.93–1.75)Parity < 0.001Nullipara1.00 (reference)Para 1+ 3.25 (2.43–4.35)BMI (kg/m 2 ) 0.50≥ 301.00 (reference)< 30 1.14 (0.78–1.65)Women experiencing Cesarean section or a time to delivery > 24 hwere censored.vaginal delivery after IOL, and the predictive values weresimilar for this method and ultrasound-measured cervicallength and the Bishop score. However, we judge none ofthese methods used alone to be good enough in a clinicalsetting.Parity was the best predictor of vaginal delivery.BMI, Bishop score and ultrasound measurements offetal head–perineum distance, cervical length and cervicalangle predicted a vaginal delivery with around 60% underCopyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd. Ultrasound Obstet Gynecol 2008; 32: 199–204.

Ultrasound assessment before induction of labor 203the ROC curve area. Parity, fetal head–perineum distance,cervical length and cervical angle were statisticallysignificant contributing factors to vaginal delivery within24hinaCoxregressionmodel.The fact that parity was the single best predictor wasin accordance with previous studies 12,13 . We could notconfirm the importance of maternal age, which has beenemphasized in other studies 3,9 . This may be due to a ratherlow Cesarean delivery rate in our department. Some studiesrecommend ultrasound measurements, such as occiputposterior position, cervical length, cervical angle or cervicalwedging, as additional predictive variables 8–14,22 , butother authors disagree 15 –17 . Rozenberg et al. emphasizethat heterogeneous populations, lack of standardizationof the method of induction, different ultrasoundmeasurements and different endpoints can give conflictingresults 16 . Some authors have suggested modelswith combinations of maternal factors and ultrasoundmeasurements 10,11,21 .Tan et al. found ultrasound measurements of cervicallength and the Bishop score to be of similar value in predictinga Cesarean delivery 23 .Raneet al. found cervicalangle to be a predictive factor for successful IOL 11 ,andthese results are in accordance with our findings.Fetal head position had no predictive value for vaginaldelivery after IOL. This is in accordance with a study fromPeregrine et al. 25 , but not with other studies 11,26 .Mostfetuses were in the occiput posterior position close to thelimit values (4 o’clock or 8 o’clock 11 ), and in a previousstudy we found that 30% of fetuses moved the head duringan ultrasound examination before the start of labor 27 .In a recent editorial, Sherer has pointed out the needfor new methods for measuring fetal head descent 28 .Dietz and Lanzarone evaluated ultrasound-measuredengagement of the fetal head as a predictive factor foroutcome in spontaneous labors by relating the fetalhead to the symphysis pubis 29 . However, we believe thatmeasurement of fetal head–perineum distance is easier.Fetal head station is determined after digital assessmentof the relationship between the fetal head and the maternalischial spine 24 . In our experience, the ischial spine isdifficult to visualize with ultrasound imaging. Instead weused the outlet of the birth canal as a reference line.This is an intuitively useful reference line, being the ‘goalline’ of parturition. Because the birth canal is curved,there is an association, but not a direct relationship,between fetal head–perineum distance and fetal headstation. A schematic drawing of fetal head descent in acurved birth canal and the association with the remainingfetal head–perineum distance is illustrated in Figure 3.Measurements may vary with the degree of compressionof the soft tissue. In our experience, the examiner will meetfirm resistance when the perineal soft tissue is compressed,without causing discomfort to the woman. We have foundthe interobserver and intraobserver variability of themethod to be acceptable 19 . Thus, we believe that thismethod can be used to assess fetal head descent, eventhough the relationship with the well established conceptof fetal head station is less clear. An unclear relationshipFetal headRemainingdistancePerineumFigure 3 Schematic drawing of fetal head descent in a curved birthcanal showing association with remaining fetal head–perineumdistance.should not undermine the potential clinical value of theproposed method, which is easy and quick to perform.The method should be validated in new studies for headdescent during labor, in particular during protracted laborand before operative vaginal deliveries.The fetal head–perineum distance predicts laboroutcome after IOL. The clinical value of this ultrasoundmeasureddistance is similar to that of ultrasoundassessment of cervical length and the Bishop score. Futurestudies should evaluate if and how these predictive factorscan be combined.ACKNOWLEDGMENTSMargrethe Tangerud and Karin Stangeland did some ofthe ultrasonographic examinations.REFERENCES1. Gjessing LK. 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