Materials and Methods
Data were obtained from the Cardiff Births Survey, population-based information on all deliveries from South Glamorgan Health Authority in the United Kingdom since 1965. The period January 1990 to December 1998 yielded 17,432 nulliparas who delivered singleton cephalic-presenting infants at term (defined at 37–42 weeks)—the so-called standard obstetric patient. Two hundred sixty-four women who had elective cesareans were excluded. Data were incomplete for 387 deliveries, which also were excluded. Therefore, 16,781 deliveries were available for analysis.
Occipitoposterior position was defined as delivery face-to-pubes, or labor that required rotation from an occipitoposterior position for delivery to occur. Factors examined for association with occipitoposterior delivery were completed weeks of gestation (determined by ultrasonography, usually in the first trimester), epidural analgesia in labor, induction of labor (by any means), augmentation of labor with oxytocin, maternal age at delivery (grouped as under 16 years, 16–19, 20–23, 24–27, 28–31, 32–35, 36–39, at least 40) and fetal birth weight (grouped as under 2500 g, 2500–2999 g, 3000–3499 g, 3500–3999 g, 4000–4499 g, at least 4500 g. Modes of delivery were grouped as spontaneous vaginal, instrument (including vacuum extraction and all forms of forceps delivery), and emergency cesarean. Apgar scores were taken at 1 and 5 minutes postdelivery.
Statistical analysis was done with SPSS for Windows version 8.0 (SPSS Inc., Chicago, IL). Analysis of factors associated with occipitoposterior delivery was done with logistic regression that generates an odds ratio (OR) for each factor and takes into consideration the confounding effect of one variable upon another. Analysis of delivery and fetal outcome was done with χ2test.
The association of obstetric factors with occipitoposterior delivery for 16,781 deliveries is shown in Table 1. There was a rising incidence of occipitoposterior position with increasing birth weight that was highly statistically significant (Table 2). Incidence of occipitoposterior position increased with maternal age but did not reach statistical significance. Epidural analgesia in labor and oxytocin augmentation are strongly associated with occipitoposterior delivery. Gestation and labor induction were not significant.
Distribution of mode of delivery was significantly different between occipitoanterior and occipitoposterior deliveries. Spontaneous vaginal delivery rates for women who delivered occipitoanterior and occipitoposterior were 61.8% and 14.6%, respectively. Instrumental vaginal delivery rates were 24.4% and 43.7% and emergency cesarean delivery rates were 13.7% and 41.7% for occipitoanterior and occipitoposterior positions, respectively. χ2 analysis of those different delivery profiles was χ2 = 772.76, P < .001. The percentage of infants with low Apgar scores (under 8) at 5 minutes was 2.4% for occipitoanterior and 3.2% for occipitoposterior deliveries, which did not reach statistical significance (χ2 = 2.18, P = .14); however, a power analysis for low Apgar scores gave a power of only 31%, inadequate to draw any firm conclusions.
We believe this is the first study to use a large dataset to examine obstetric factors and outcomes associated with occipitoposterior position in standard nulliparas. New findings were: association of epidural analgesia in labor and oxytocin augmentation with occipitoposterior position and increased incidence of instrument and cesarean deliveries compared with the occipitoanterior position within that group. Although infants delivered in the occipitoposterior position were more likely to have had obstetric interventions, there was no significant difference in the percentage of infants with low Apgar scores at 5 minutes and those delivered in the occipitoanterior position. However, those latter data should be viewed cautiously because the statistical power was low, probably owing to low incidence of low Apgar scores in both groups.
A review of six standard textbooks on obstetrics9–14 found that factors in labor believed to be associated with development or persistence of occipitoposterior position are size and flexion of the fetal head and shape of the pelvis, the anthropoid and android pelvic shapes being the most significant. Although birth weight is possibly related to fetal head size, none of those texts report epidural analgesia or labor augmentation as factors possibly associated with occipitoposterior position.
The incidence of occipitoposterior position depends on the time of pregnancy or labor at which it is examined11 but it is generally accepted that at the onset of labor 10–15% of fetal heads lie in that position. Llewellyn-Jones10 stated that approximately 65% will rotate to occipitoanterior and a further 20% to occipitotransverse; however, the incidence of occipitoposterior position at delivery was 4–5% and the figures at best gave an incidence of occipitoposterior position at just over 2%. Therefore, most occipitoposterior positions at delivery must arise from an initial occipitoanterior or occipitotransverse position and are not persistence of occipitoposterior from the beginning of labor as was commonly believed. That was borne out by a Finnish study8 that showed it with ultrasound.
Our data show a rise in the incidence of occipitoposterior position with increasing birth weight. One study3 showed that infants delivered occipitoposterior were heavier, but the continuous increase with increasing birth weight has not been reported, to our knowledge. It is possible that it is more difficult for larger fetuses to rotate as labor progresses, and that those might be the infants who have persistence of occipitoposterior position with the highest level of operative intervention.8
There are two possible explanations for the strong association of epidural analgesia and occipitoposterior position. It is acknowledged that occipitoposterior labors are more painful and might result in increased usage of this very effective form of analgesia. It also is possible that epidural analgesia leads to increased occipitoposterior positions by making uterine activity less efficient, or that it allows the pelvic floor to relax, preventing rotation to occipitoanterior. Finnish studies3 found no association between epidural and occipitoposterior position, but within that relatively small patient group the epidural rate was very low.
Our data do not support the theory of Briggs6 that the younger pelvis is not mature and therefore predisposes to malpresentation, because in nulliparas the incidence of occipitoposterior position was lowest under age 16 years. A possible explanation for rising incidence of occipitoposterior position with maternal age was that the uterus, like other organs, becomes increasingly inefficient with age.15 Inefficient uterine contractions might manifest themselves as malposition because of failure to rotate. The figures did not reach statistical significance and it might be possible that a larger population is required to show significant effect of maternal age on occipitoposterior position.
It is often stated anecdotally that the lengths of the first and second stages of labor are increased with occipitoposterior position, and there is some evidence to support that.3 The reasons for prolonged first stage of labor when occipitoposterior position is present are unclear, but it is possible that inadequate uterine action leads to failure of cervical dilation and rotation of the fetal head, hence increased need for augmentation. Augmentation then might enable progressive increase in cervical dilation, but if the fetal head has been molded in the occipitoposterior position it might be difficult for it to rotate, even with adequate contractions.
The fact that labor induction and gestation are not associated with occipitoposterior position is not surprising. The association between occipitoposterior position and induction of labor has been shown2 and refuted.8 In theory, advancing gestation could be associated by the fact that birth weight increases. When subjected to logistic regression that takes into account possible confounding effects of one variable upon another, gestation is not significant.
Nulliparas with occipitoposterior position have a very low spontaneous vaginal delivery rate with concomitant high rates of instrument and cesarean delivery. A lower incidence of spontaneous vaginal delivery and increased incidence of instrument delivery associated with occipitoposterior delivery has been shown.2 That latter study found no increase in cesarean delivery rates with occipitoposterior position, unlike the data presented here.
Older textbooks9–11 state that management of occipitoposterior position in the second stage of labor should depend on the station of the fetal head. They suggest that if the fetal head is high in the pelvis (above the ischial spines), then consideration should be made to rotation and vaginal delivery, a practice that would not be countenanced today. If the head is low, then rotation might be difficult and instrument delivery face to pubes might be required. Modern obstetricians might take the view that it is better to opt for cesarean in either of those circumstances.
Although our study did not examine the effects of occipitoposterior delivery on fetal and maternal morbidity, it has been reported that occipitoposterior delivery is associated with a higher episiotomy rate,2, a higher frequency of third16 and fourth degree tears and a higher incidence of fetal injury.5
It was suggested that the age at which women have their first child is rising.17 In view of the data presented here, we may well see a rise in the incidence of occipitoposterior position in the future because improved maternal nutrition (leading to increased birth weight), increasing age of nulliparas and increased availability of epidural anesthesia (possibly leading to a higher degree of uptake) are all associated factors. If women are aware of the increase in maternal and fetal morbidity associated with occipitoposterior position, they might elect for cesarean for persistence of occipitoposterior position in the second stage of labor rather than attempting vaginal delivery. Of course that would lead to a further rise in cesarean delivery rate.
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