Ninety-nine (34%) of the 288 women had an OP position of the fetal head on ultrasonography before induction of labor. Outcome data on fetal head position at delivery was missing in 17 women, and one woman had a fetus that turned to breech presentation before delivery. Therefore, further analysis regarding head position at delivery was performed on the remaining 270 women. Of these 270 women, 97 (36%) had an OP position on ultrasonography before induction of labor. At delivery, eight (8%) of these 97 were OP, 12 (12%) were OT, and 77 (79%) were OA.
Twenty-five babies were in an OP position at delivery. Before induction of labor, eight (32%) of these 25 women were OP, eight (32%) were OT, and nine (36%) were OA. Therefore, 68% of OP positions at delivery occurred due to a mal-rotation from a non-OP position during labor.
Obstetric outcome data were available for all 288 women (Table 3). This table also compares outcome data from those with an OP and those with a non-OP position on ultrasonography before induction of labor. There was no significant difference between the two groups for delivery within 24 hours, rate of vaginal delivery, vaginal delivery within 24 hours, instrumental delivery, abnormal cardiotocograph, use of oxytocin, use of epidural, birth weight, and length of time from the start of induction of labor to delivery. The total length of labor was significantly longer with an OP position before induction of labor compared with a non-OP position (median 606 versus 499 minutes). The length of labor and time from the start of induction of labor to delivery were also compared in the OP and non-OP groups using an analysis of variance (ANOVA) test. There was no evidence for heterogeneity of variance with either comparison. The length of labor was significantly longer in the OP group (F=5.83, P=.016), but there was no significant difference in the total time from the start of induction of labor to delivery (F=1.14, P=.287). Of the 25 infants in OP position at delivery, 24 (96%) were delivered by cesarean delivery and one (4%) by ventouse. Of these 24 women who had a cesarean delivery, two women had a failed ventouse before cesarean delivery, one had a cesarean delivery at full dilatation due to a prolonged second stage and a high head, and 21 had a cesarean delivery before full dilatation (nine for fetal distress, eight for failure to progress, and four for a failed induction of labor–failure to reach 4 cm). Logistic regression demonstrated that only parity and body mass index were independent predictors of cesarean delivery (OR 10.61, 95% CI 4.97–22.64, P=.000 and OR 1.11, 95% CI 1.11–1.26, P=.000, respectively). Occipito-posterior position before induction of labor did not independently predict the risk of cesarean delivery (OR 1.75, 95% CI 0.97–3.15, P=.06).
The data were then analyzed separately according to parity. Before labor, 69 (40%) of the 174 nulliparous women had an OP position on ultrasonography compared with 30 (26%) of the 114 multiparous women. Complete data on fetal head position at delivery were available in 159 nulliparous women and 111 multiparous women. Twenty-one (13%) of the nulliparous women had an OP position at delivery compared with four (4%) of multiparous women. Multiparous women were significantly less likely to have an OP fetal head position both on ultrasonography before induction of labor (χ2=5.43; P=.02) and at delivery (χ2=7.18, P<.01). The obstetric outcomes for the 288 women according to parity and ultrasound fetal head position are shown in Table 4. When obstetric outcomes for nulliparous and multiparous women were analyzed separately, the only significant difference between the two groups was that multiparous women with an OP position before induction of labor were more likely to deliver vaginally than multiparous women with a non-OP position. Overall, the risk of cesarean delivery was significantly higher in nulliparous compared with multiparous women (45% versus 11%, χ2=56.09; P<.01). This difference between nulliparous and multiparous women remained significant even when they were separated into OP and non-OP position groups before induction of labor (39% versus 0%, χ2=16.14, P<.01, and 49% versus 15%, χ2=22.82, P<.01, respectively).
Placental position on ultrasonography was compared with both fetal head and spine position on ultrasonography before induction of labor using a χ2 test. No significant association was found between fetal head position on ultrasonography and anterior (χ2=0.48, P=.49) or posterior placental position (χ2=1.14, P=.29) or fetal spine position on ultrasonography and anterior (χ2=0.93, P=.34) or posterior placental position (χ2=0.76, P=.39).
Of the 267 women in whom a fetal position was defined on abdominal palpation, the spine position was assessed correctly (in relation to the ultrasonographic spine position) in only 92 women (34%). Palpation was within 45° of the correct position in a total of 165 (62%) women, within 90° in 217 (81%), and more than 90° different in 50 (19%) women. The palpation was more likely to be correct when the fetus was lying with its spine in an occipito-anterior position (72 of 128, 56%) compared with non-OA positions (20 of 146, 14%). Of the 288 women where complete ultrasound data were available, the head and spine positions correlated exactly in 137 (48%) women, were within 45° in 274 (95%), and within 90° in 288 (100%).
Our ultrasound data show that fewer than 10% of OP positions before induction of labor remain OP at delivery and that two thirds of OP positions at delivery arise from a mal-rotation from an initial non-OP position. This holds true in both nulliparous and multiparous women. Our findings challenge conventional wisdom regarding the movements of the fetal head in labor.
The incidence of OP position before induction of labor in our study was 34%. One other study has agreed with our data of a higher incidence of OP position before induction of labor,22 compared with the reported incidence before spontaneous labor. One hypothesis to explain this finding is that OP position may in some way delay the spontaneous onset of labor, making induction of labor more likely.
Our findings indicate that 92% of OP positions before induction of labor rotated to an OA position and that 68% of OP positions at delivery developed through a mal-rotation during labor from an initially OA or OT position. This is contrary to standard teaching and other studies in spontaneous labor that state that failure of rotation occurs in up to 20% of cases, resulting in a persistent OP position and that most OP positions at delivery arise from a persistent OP position.3,4,11,26 Some of this variation in findings may be due to differences in the populations studied and the timing of assessment of position, either before or during labor or that the head movements in induced labor are inherently different from those after spontaneous labor. Gardberg and colleagues2 found similar results to ours, demonstrating that OP position at delivery develops more frequently (62%) from mal-rotation from a non-OP position than from failure of rotation from an initial OP position. Their study was, however, in a group of 408 women before both spontaneous and induced labor, rather than a solely induced labor group, supporting the fact that our findings could be extrapolated to a population of spontaneously laboring women.
Occipito-posterior position at delivery has been associated with many factors and outcome measures in the pregnancy and delivery, including short maternal stature,8 nulliparity,8 anterior placenta,27 induction of labor,9 variable fetal heart rate decelerations,5,28 use of epidural analgesia,6,8,9 use of oxytocin augmentation,6,8,9,11 increased length of labor,8,9 increased length of the second stage,10 increased instrumental delivery rate,6,8–10 increased cesarean delivery rate,6,8,9,11,12 increased episiotomy rate,10 perineal trauma,8,9,29,30 larger birth weights,6 and lower 1-minute Apgar scores.5,6,8,9,31 In our study, OP position before induction of labor was associated with a significantly longer labor than a non-OP position. However this finding is not likely to be clinically significant given the wide ranges of the length of labor and that there was no difference in the time from the start of the induction to the delivery. It has been proposed that the increased length of labor associated with OP position may be due to decreased uterine contractility,9 but the lack of association with oxytocin use in our study does not support this.
We did not find that OP position before induction of labor increased the risk of cesarean delivery or was associated with other adverse obstetric outcomes. Most groups have studied the associations of OP position at delivery rather than position before labor. It is only knowledge of position before or during labor that will be potentially clinically useful for prediction of successful outcome and, therefore, appropriate management.
We found that 96% of those with OP position at delivery had a cesarean delivery, but few conclusions can be drawn from these data because we did not examine fetal head position during labor or in the second stage. This, therefore, represents a group of women who are at very high risk of surgical delivery in whom either the head was in an OP position at a cesarean delivery in the first stage or at spontaneous vaginal delivery or in an OP position in the second stage immediately before a trial of instrumental delivery. The majority (84%) of these women were not deliverable vaginally because the cervix was not fully dilated.
We found that there was no difference in the incidence of OP position and non-OP position in white and nonwhite women before induction of labor, which is in keeping with findings in early labor.12 We found that OP position before induction of labor and at delivery is more common in nulliparous than multiparous women. This has been previously noted,8 but the reasons are poorly understood. Occipito-posterior position has been associated with reduced uterine contractility caused by other factors, such as epidural6 and the resulting interventions.9 It could be proposed, therefore, that inefficient uterine activity in labor itself in nulliparity predisposes to the persistent OP position. Our findings, however, do not support this theory; oxytocin requirement was not more common in the OP position group.
In all groups (all women, OP position and non-OP position), cesarean delivery was more common in nulliparous than multiparous women. This reflects the known association between nulliparity and cesarean delivery, especially after induction of labor,24 which appears to be irrespective of fetal position. The finding that multiparous women with an OP position are more likely to deliver vaginally than those with a non-OP position is likely to be due to the small numbers of multiparous women with an OP position and the low risk of cesarean delivery in multiparous women as a whole. This is supported by our finding that no other outcomes were significantly different.
This study has demonstrated the ease of use of transabdominal ultrasonography in determining fetal head position before induction of labor. Previous studies have shown that sonographic head position can reveal the high error rate of vaginal examination in determining fetal head position in the first16,17 and second stages of labor18–20 and that digital vaginal examination is more difficult17, 26 and less accurate16–18,21,26,32 in an OP than a non-OP position. Transperineal ultrasonography has also been used successfully to determine fetal head position when the head is low in the pelvis.20 However, there are few data on its use before induction of labor or comparing sonographic head position before induction of labor with head position at delivery. Previous studies before induction of labor have mainly included one element of a preinduction assessment: one study found that ultrasonographic fetal head position can be used to predict the risk of cesarean delivery and the induction-to-delivery interval.23 However, this has not been confirmed in other studies.24
Our study has shown that abdominal palpation is highly inaccurate in determining fetal head position, particularly in a non-OP position; the spine and head position correlated in only 48% of women. This lack of correlation in orientation of the fetal head and spine has been noted before,13,14 but it is often not clinically appreciated during abdominal palpation. If we assume that it is head position rather than spine position that will determine the position changes in labor, we can conclude that abdominal palpation of fetal position is not a clinically useful procedure. Vaginal determination of head position is only possible once the cervix has dilated, may be subjective, and was not assessed by this study. Determining fetal head position by ultrasonography can confirm position before labor or during labor when the clinician is unsure on vaginal examination. It has particular potential before instrumental delivery and in training.
These findings challenge traditional teaching about the diagnosis of and implications of an OP position at term. The widespread belief that OP position before induced (and spontaneous) labor is associated with poor obstetric outcome does not appear to be well founded. At present, women who are suspected of having a fetus in an OP position are often frightened by this finding and resort to various measures to try to change the fetal position.33 Our data suggest that they can be reassured that, although an OP position at this stage may be associated with a longer labor, it is not associated with an increased risk of cesarean delivery. This is likely to be due to the fact that fetal head position before induction of labor does not predict position late in labor or at delivery.
1. Caldwell WE, Moloy HC, D’Esopo DA. A roentgenologic study of the mechanism of engagement of the fetal head. Am J Obstet Gynecol 1934;28:824–41.
2. Gardberg M, Laakkonen E, Sälevaara M. Intrapartum sonography and persistent occiput posterior position: a study of 408 deliveries. Obstet Gynecol 1998;91:746–9.
3. Cunningham FG, Leveno KJ, Bloom SL, Heath JC, Gilstrap LC, Wenstrom KD. Williams obstetrics. 22nd ed. New York (NY): McGraw Hill; 2005.
4. Myerscough PR. Munro Kerr’s operative obstetrics. 10th ed. London: Baillière Tindall; 1982.
5. Ingemarsson E, Ingemarsson I, Solum T, Westgren M. Influence of occiput posterior position on the fetal heart rate pattern. Obstet Gynecol 1980;55:301–4.
6. Sizer AR, Nirmal DM. Occipitoposterior position: associated factors and obstetric outcome in nulliparas. Obstet Gynecol 2000;96:749–52.
7. Phillips RD, Freeman M. The management of persistent occiput posterior position: a review of 552 consecutive cases. Obstet Gynecol 1974;43:171–7.
8. Ponkey SE, Cohen AP, Heffner LJ, Lieberman E. Persistent fetal occiput posterior position: obstetric outcomes. Obstet Gynecol 2003;101:915–20.
9. Fitzpatrick M, McQuillan K, O’Herlihy C. Influence of persistent occiput posterior position on delivery outcome. Obstet Gynecol 2001;98:1027–31.
10. Neri A, Kaplan B, Rabinerson D, Sulkes J, Ovadia J. The management of persistent occipito-posterior position. Clin Exp Obstet Gynecol 1995;22:126–31.
11. Akmal S, Tsoi E, Howard R, Osei E, Nicolaides KH. Investigation of occiput posterior delivery by intrapartum sonography. Ultrasound Obstet Gynecol 2004;24:425–8.
12. Akmal S, Kametas N, Tsoi E, Howard R, Nicolaides KH. Ultrasonographic occiput position in early labour in the prediction of caesarean section. BJOG 2004;111:532–6.
13. Ververs IA, de Vries JI, van Geijn HP, Hopkins B. Prenatal head position from 12–38 weeks. I. Developmental aspects. Early Hum Dev 1994;39:83–91.
14. Ververs IA, de Vries JI, van Geijn HP, Hopkins B. Prenatal head position from 12–38 weeks. II. The effects of fetal orientation and placental localization. Early Hum Dev 1994;39:93–100.
15. Akmal S, Tsoi E, Nicolaides KH. Intrapartum sonography to determine fetal occipital position: interobserver agreement. Ultrasound Obstet Gynecol 2004;24:421–4.
16. Sherer DM, Miodovnik M, Bradley KS, Langer O. Intrapartum fetal head position. I. Comparison between transvaginal digital examination and transabdominal ultrasound assessment during the active stage of labor. Ultrasound Obstet Gynecol 2002;19:258–63.
17. Akmal S, Tsoi E, Kametas N, Howard R, Nicolaides KH. Intrapartum sonography to determine fetal head position. J Matern Fetal Neonatal Med 2002;12:172–7.
18. Sherer DM, Miodovnik M, Bradley KS, Langer O. Intrapartum fetal head position. II. Comparison between transvaginal digital examination and transabdominal ultrasound assessment during the second stage of labor. Ultrasound Obstet Gynecol 2002;19:264–8.
19. Kreiser D, Schiff E, Lipitz S, Kayam Z, Avraham A, Achiron R. Determination of fetal occiput position by ultrasound during the second stage of labor. J Matern Fetal Med 2001;10:283–6.
20. Chou MR, Kreiser D, Taslimi MM, Druzin ML, El Sayed YY. Vaginal versus ultrasound examination of fetal occiput position during the second stage of labor. Am J Obstet Gynecol 2004;191:521–4.
21. Akmal S, Kametas N, Tsoi E, Hargreaves C, Nicolaides KH. Comparison of transvaginal digital examination with intrapartum sonography to determine fetal head position before instrumental delivery. Ultrasound Obstet Gynecol 2003;21:437–40.
22. Tribalat S, Lewin D, Tochon C. Orientation of the fetal head at the time of engagement in induced labor [in French]. J Gynecol Obstet Biol Reprod 1986;15:937–9.
23. Rane SM, Guirgis RR, Higgins B, Nicolaides KH. The value of ultrasound in the prediction of successful induction of labor. Ultrasound Obstet Gynecol 2004;24:538–49.
24. Peregrine E, O’Brien P, Jauniaux E. Clinical and ultrasound parameters to predict the risk of cesarean delivery after induction of labor. Obstet Gynecol 2006;107:227–33.
26. Souka AP, Haritos T, Basayiannis K, Noikokyri N, Antsaklis A. Intrapartum ultrasound for the examination of the fetal head position in normal and obstructed labor. J Matern Fetal Neonatal Med 2003;13:59–63.
27. Gardberg M, Tuppurainen M. Anterior placental location predisposes for occiput posterior presentation near term. Acta Obstet Gynecol Scand 1994;73:151–2.
28. Sokol RJ, Roux JF, McCarthy S. Computer diagnosis of labor progression. Am J Obstet Gynecol 1975;122:253–60.
29. Pearl ML, Roberts JM, Laros RK, Hurd WW. Vaginal delivery from the persistent occiput posterior position: influence on maternal and neonatal morbidity. J Reprod Med 1993;38:955–61.
30. Sultan AH, Kamm MA, Hudson CN, Bartram CI. Third degree obstetric anal sphincter tears: risk factors and outcome of primary repair. BMJ 1994;308:887–91.
31. Gardberg M, Tuppurainen M. Persistent occiput posterior presentation: a clinical problem. Acta Obstet Gynecol Scand 1994;73:45–7.
© 2007 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.
32. Rayburn WF, Siemers KH, Legino LJ, Nabity MR, Anderson JC, Patil KD. Dystocia in late labor: determining fetal position by clinical and ultrasonic techniques. Am J Perinatol 1989;6:316–9.