The results of the univariate analysis of the association between intrapartum variables and OBPP are given in Table 2. The estimated risk of OBPP was increased by multiparity, vertex at the level of the ischial spine (versus below the ischial spine) at time of application, indication due to delay of the second stage of labor (versus fetal distress), use of fundal pressure, shoulder dystocia, 6 or more vacuum tractions (versus < 6 tractions) to complete delivery, vacuum extraction time of 10 minutes or more (versus < 10 minutes) from start of traction to birth, and fetal birth weight of 3,999 g or more. Intrapartum factors not associated with OBPP in the newborn included augmentation of labor, epidural analgesia, use of vacuum silicone cup, cup detachment, and operator experience (Table 2).
A correlation was found between vacuum extraction time from start of extraction to birth (minutes) and OBPP (Table 2). Analysis with univariate logistic regression (Table 2) showed that the probability of OBPP increased by OR 2.6 for vacuum extraction time of 10 minutes or more. Analysis by a combination of spline functions and logistic regression revealed that the probability of the risk of OBPP in vaginal vacuum deliveries increased continuously in relation to extraction time. At 5 minutes of extraction time, the estimated risk of OBPP was 0.8%, whereas at 25 minutes of extraction time, it was close to 4% (Fig. 1).
To adjust for potential confounders, all factors in the univariate model were processed by multivariate logistic regression analysis. Parity, indication for assisted delivery, delay in the second stage of labor, fetal head at the level of the ischial spine at vacuum application time, 5 or more vacuum tractions, vacuum application time, use of vacuum silicone cup, and epidural anesthesia were not significantly related to OBPP, whereas shoulder dystocia, fetal birth weight of 3,999 g or more, and fundal pressure all independently increased the risk, even after statistical adjustment. When birth weight was included as a continuous variable, the risk of OBPP was more than 7 times higher in infants of birth weight greater than 3,999 g, compared with newborns of lower weight (Table 3).
Of all cases of OBPP, 20.5% (n = 21) of the infants to nulliparous women had a birth weight of 4,500 g or more, and 24.6% of these deliveries were complicated by shoulder dystocia. A similar analysis of multiparous women resulted in 7.1% infants with a birth weight of 4,500 g or more, and 7% of these deliveries were complicated by shoulder dystocia.
This retrospective study aimed to identify the risk factors of OBPP relative to the duration of vacuum application. Such detailed data are usually not easily obtained from standard birth records but were able to be retrieved from the national evaluation protocol combined with the Medical Birth Registry in Sweden.
One factor that initiated the present study was the increasing incidence of OBPP during the last decade in Sweden.16 Vacuum extraction is an important part of obstetric practice, and this mode of delivery has recently been linked to an increased risk of OBPP in the infant.1,9 There is an ongoing debate, as to whether OBPP is caused by a prolonged second stage of labor or is related to the vacuum extraction itself. In this study we do not focus primarily on fetal birth weight because the infant's weight is unknown at the time of vacuum application. Furthermore, the increased risk of OBPP related to high birth weight is well known from previous studies.1,9,16 Three factors remained as independent risk factors: shoulder dystocia, fetal birth weight of 3,999 g or greater, and the use of fundal pressure during the second stage of labor. Multiple logistic regression analysis revealed that shoulder dystocia was the predominant obstetric risk factor for OBPP in vacuum-assisted deliveries. In the present study, the incidence of OBPP in vacuum-assisted deliveries was 1.1%, which is 5.5-fold higher than in the total population of vaginal deliveries16 (Table 2). This result agrees with most previous studies, in which the incidence ranged from 0.4%9 to 3.7%.15 However, an incidence up to 17.6% was reported by others.14 The reason for these discrepant findings is unclear, but the fact that most studies were not population-based could be a contributory explanation.
Nulliparous women are subjected to vacuum deliveries more often than parous women (Table 1), which is in agreement with previous studies.10,15 On the contrary, the relative risk of OBPP is higher among parous than nulliparous women undergoing vacuum deliveries (Table 2). We have no definite explanation for this finding, but it could relate to the fact that multiparous women are giving birth to larger infants.2 In several previous studies, the increased risk of OBPP among parous women has been associated to high infant birth weight.6 It is also well known that infants who weigh 3,999 g or more, irrespective of parity, are at greater risk of arrest of descent of the fetal head, vacuum-assisted delivery, shoulder dystocia, and OBPP.1,5,9,11,17 The reason for the lower percentage of infants weighing 4,500 g or more among parous women, compared with nulliparous women, may be the fact that a higher proportion of large babies are delivered spontaneously by parous women, whereas nulliparous women more often need vacuum extraction assistance.
There are data to suggest that infants born to nulliparous women are at greater risk of OBPP because of shoulder dystocia in vacuum-assisted deliveries. Of all infants with this mode of delivery, 4.3% (586/13,716) had a birth weight of 4,500 g or more. Shoulder dystocia is clearly a problem associated with large babies, especially in nulliparous women. Vacuum duration of 10 minutes or more was associated with an increased risk of OBPP in the univariate analysis (Table 2), and the risk of OBPP in the infant increased continuously as a function of the duration of extraction (Fig. 1). We found no data available in the literature for comparison, but it has been suggested that extraction should be interrupted if 3 consecutive pulls do not result in noticeable progress.11 On the other hand, prolonged extraction per se is perhaps not the cause of OBPP. More likely, cephalopelvic proportion or insufficient contractions cause prolonged labor associated with a higher of risk of instrumental delivery, shoulder dystocia, and OBPP. Such an explanation is supported by Beall et al,7 demonstrating that the duration of the second stage of labor was associated with shoulder dystocia but that there was no association with operative vaginal delivery.
The association between external fundal pressure and OBPP was an interesting result (Tables 2 and 3). One explanation for such a relationship could be that fundal pressure could impact the anterior shoulder behind the pubic symphysis and indirectly cause shoulder dystocia. The effect of fundal pressure during the second stage of labor has been studied previously only to a limited extent.5,11,17 These studies suggest an association between fundal pressure and a difficult vacuum procedure17 and that the need for fundal pressure could act as a warning sign for shoulder dystocia.5 In the study by Gherman et al,11 the authors found that 12% of deliveries in which fundal pressure was applied result in permanent OBPP, which is much higher than the 1.7% found in the present study. It is not clear exactly how fundal pressure is associated with a risk of OBPP. However, there is an association between fundal pressure and both prolonged second stage and high birth weight, indicating that the need for fundal pressure may serve as an indicator of a disproportionately large fetus in relation to the birth canal.
Shoulder dystocia was found to be the most significant risk factor for OBPP in our study (Tables 2 and 3). Most children with OBPP are large, with the majority (65.4%) weighing more than 3,999 g at birth (Table 2). It is known that infants of 3,999 g or more are at greater risk of complications during delivery.1,9,11 In deliveries complicated by shoulder dystocia, the risk of OBPP was 30%, and the association with OBPP was very strong, both in the univariate and in the multivariate analysis (Tables 2 and 3). A high risk has been observed also in other studies, and a frequency as high as 53% was reported by Gilbert et al.9 The different results obtained probably relate to the fact that there is no universal definition of shoulder dystocia and that few studies are population-based. The mechanism whereby shoulder dystocia causes OBPP is unknown. It probably relates to application of force, compression or traction on the brachial plexus, and the need to use ancillary maneuvers to deliver the shoulders.7,13 Even if a close relationship exists, most deliveries (81%) resulting in OBPP were not complicated by shoulder dystocia (Table 2). However, we suspect that less severe difficulties in delivery of the shoulders, with the application of some traction and force to the cervical roots, could contribute in the etiology of OBPP, and still these cases may not be classified as shoulder dystocia.
In conclusion, this study indicates that, in the present population of vacuum-assisted deliveries, shoulder dystocia, fetal birth weight of 3,999 g or more, the use of fundal pressure, and prolonged vacuum extraction time were all significant risk factors for OBPP. An important decrease in OBPP could be achieved if a patient population at risk for shoulder dystocia could be identified. A cesarean delivery would be indicated in these cases to prevent the poor outcome in the infant. Unfortunately, there is no effective strategy for predicting shoulder dystocia, and still the majority of deliveries resulting in infants with OBPP were not complicated by shoulder dystocia. So the question remains: how could OBPP be prevented? We suggest that prospectively performed studies are warranted to better understand those critical factors in obstetric procedures that are associated with OBPP. Such knowledge may form the basis for modification of obstetric management so as to reduce the risk of OBPP in the infant.
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© 2005 The American College of Obstetricians and Gynecologists
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