Breech or nonvertex presentation complicates approximately 3–4% of term deliveries.1,2 The majority of these will be delivered by cesarean as was seen in 2002 when 86.9% of women with breech presentation underwent cesarean delivery.3 A meta-analysis of planned cesarean delivery compared with planned vaginal delivery for breech presentation found decreased rates of short-term maternal morbidity and mortality in the vaginal delivery group,4 but two large studies showed low risk of severe maternal complications in women undergoing cesarean delivery.5,6 The risks of neonatal morbidity and mortality by mode of delivery are also under debate.6,7 External cephalic version, or manually turning the fetus in utero, is one method of reducing the rate of breech delivery at term, but success rates vary from 35% to 86%.7
The uncertainty surrounding the ideal management of breech presentation calls for a better understanding of the etiology of nonvertex position of the fetus. It is hypothesized that the normal kicking movements of the fetus allow it to assume a vertex position by the time of delivery. Previously reported risk factors for breech presentation include nulliparity and grand multiparity, advanced maternal age, contracted pelvis, and uterine anomalies or lesions. Pregnancy complications that have been associated with nonvertex presentation include preterm delivery, congenital malformations, abnormal amniotic fluid levels, and placenta previa or cornual implantation of the placenta. The association of birth weight with breech presentation has been somewhat mixed. A previous population-based, case-control study of 11,771 patients using Washington State birth certificates additionally found that risk of breech presentation was increased with hydrocephalus, pre-existing diabetes, congenital malformations, smoking, and late or no prenatal care.8
The natural history of fetal position has not been systematically studied in a large population. This current paper presents the findings from a large prospective study of women undergoing ultrasonographic surveillance. We examined the natural history of fetal position throughout pregnancy and the likelihood for nonvertex delivery by maternal and fetal characteristics.
MATERIALS AND METHODS
The population for this study consisted of participants from the Routine Antenatal Diagnostic Imaging with Ultrasound (RADIUS) trial, a practice-based multicenter study of pregnant women who were at low risk for adverse outcomes of pregnancy. The trial was carried out between November 1987 and May 1991 and was designed to test the hypothesis that routine screening with standardized ultrasonography at two time periods would reduce perinatal morbidity and mortality. A detailed description of the trial is available elsewhere.9
In brief, English-speaking pregnant women 18 years of age or older, whose last menstrual period was known to within 1 week and who were recruited before 18 weeks of gestation, were eligible for this study. Exclusion criteria included women who had previous stillbirth, prior small-for-gestational age infant, irregular menstrual cycles, greater than 3-week discrepancy between uterine size and dates, pre-existing diabetes mellitus, chronic hypertension, and chronic renal disease. Eligible women were randomly assigned to an ultrasound-screening group, in which two routine ultrasound examinations were performed at 15–22 weeks and at 31–35 weeks of gestation, or to a control group with no scheduled ultrasounds. In both groups, clinically indicated ultrasounds could be performed at any time.
With limited exceptions, all ultrasound examinations in this study were performed in one of the 28 ultrasound laboratories participating in the trial. The evaluation was standardized and included assessment of placental location, amniotic fluid volume according to protocol by Phelan et al,10 uterine and adnexal pathology, fetal number, fetal biometry, and anatomic survey of the fetus.
Baseline information for the participants was collected at recruitment and included demographic characteristics and reproductive history. Antepartum and intrapartum information was abstracted from medical records. Quality control procedures for ultrasonography and data abstraction were implemented. Institutional review boards at each study site approved this study.9
The RADIUS trial individually randomized 15,151 women by a computer-generated scheme; 7,617 women were in the screening group, and 7,534 were in the control group. There were no differences in the baseline characteristics of the two groups. The trial found no differences in perinatal outcomes between the two groups.
In this current study, we used data from women who were assigned to receive ultrasound screening. We excluded 69 women with multifetal pregnancies, 229 women whose infants were found to have congenital defects (postnatal diagnosis), and 274 women who delivered before 36 weeks. The data from 7,045 women who received two routine ultrasound examinations at 15–22 weeks and 31–35 weeks of gestation and who delivered at 36 weeks of gestation or later were analyzed. Information on 15,027 ultrasound examinations, including some that were performed for clinical indications, was available. Delivery information was also available for all of the participants. Subjects who underwent successful external cephalic version at or beyond 36 weeks of gestation were considered to have a nonvertex position at delivery.
We compared the percentage of ultrasounds demonstrating the fetus in vertex, breech, transverse, or variable positions at 15–22 weeks with the percentage of fetuses in the vertex or nonvertex position at 31–35 weeks. We examined the characteristics of women with infants who were nonvertex at delivery compared with those of women (controls) who delivered vertex infants. For smoking in pregnancy, any smoking was considered a positive response. Amniotic fluid volume level was considered normal if amniotic fluid index was greater than 6 cm. To examine the association of estimated fetal weight and birth weight with fetal position at term, subjects were divided into percentiles of estimated fetal weight at late ultrasound examination and percentiles of birth weight: less than 10th percentile, 10th to 90th percentiles, and 90th percentile or more.
For bivariate comparisons, we used χ2 or Fisher exact test for categorical data and Student t test for continuous variables. Multivariable log binomial regression analysis was then carried out evaluating maternal and fetal characteristics (smoking in pregnancy, number of previous pregnancies, number of previous abortions, parity, birth weight, gestational age, amniotic fluid level and placental position at late ultrasound examination, and estimated fetal weight at 31–35 weeks of gestation) as risk factors for nonvertex position at delivery. For those characteristics that were found to be associated with nonvertex presentation at delivery, odds ratios were calculated, adjusting for the effects of the other characteristics. To assess how well the logistic regression model fits the data, Pearson χ2 goodness-of-fit was used (P value close to 1 indicates a good fit).11 Adjusted risk difference was also calculated. Statistical analysis was performed with SAS 9.0 (SAS Institute Inc., Cary, NC).
Table 1 shows the change in fetal position between early and late ultrasound examinations. In early pregnancy, defined as 15–22 weeks, 57% of fetuses had nonvertex position, including breech, transverse, and variable presentations. By 31–35 weeks, 10% had nonvertex position, but there was no apparent association between nonvertex presentation early in pregnancy and nonvertex position at 31–35 weeks. Ninety women underwent external cephalic version, with a success rate of 60% (54 of 90).
Table 2 demonstrates the distribution of selected maternal and fetal characteristics for infants that were vertex and nonvertex at delivery. There were no significant demographic differences between the two groups. Mothers who delivered nonvertex infants were more likely to have smoked during pregnancy than those who delivered vertex infants (17% compared with 12%, P=.005). Mothers delivering nonvertex infants were more likely to be nulliparous (59%) compared with those whose infants who presented in cephalic presentation (45%) (P<.001). Amniotic fluid level was also found to be significantly associated with fetal position at term, as was placental position at late ultrasound.
Infants who were nonvertex at delivery had a lower mean weight compared with those who were vertex, but they were also more likely to have delivered at an earlier gestational age. Both estimated fetal weight by ultrasound at 31–35 weeks and birth weight percentile adjusting for gestational age at birth indicate that there is no significant difference in fetal size between vertex- and nonvertex-presenting infants.
Of all the variables examined, only four characteristics (parity, smoking status during pregnancy, decreased volume of amniotic fluid, and placental position at late ultrasound examination) were found to be associated with nonvertex presentation at delivery in the fully adjusted multivariable model (Table 3). As parity increased from zero to one previous birth, there was a 42% (95% confidence interval [CI] 26–55%) decrease in the risk of nonvertex delivery, and in women with more than one previous birth, the risk of nonvertex delivery decreased by 49% (95% CI 28–63%), compared with primiparous women. When compared with those infants born to women who did not smoke during pregnancy, infants born to those who smoked during pregnancy had a 47% increase in risk of nonvertex position at delivery (95% CI 10–96%). The absolute risk difference was 2%. A decreased volume of amniotic fluid at 31–35 weeks increased the risk of nonvertex position at delivery by a factor of 3.74 (95% CI 1.85–7.53). Fundal position of the placenta at late ultrasound examination was associated with a significant increase in the risk of nonvertex position at delivery by a factor of 1.85 (95% CI 1.23–2.78).
Nonvertex position at 31–35 weeks was a risk factor for nonvertex position at delivery, and this risk increased with each increasing week of gestation as shown in Figure 1. However, by 35 weeks, a nonvertex fetus still had a 45% chance to convert spontaneously and deliver in a vertex position.
By examining a large cohort of women who received ultrasound evaluation throughout pregnancy, we were able to determine what characteristics might predict term nonvertex presentation. Although some of the risk factors are unchangeable, identifying potentially modifiable risk factors might allow for preventive strategies. Furthermore, these may contribute to understanding the etiology of nonvertex presentation.
Controversy exists concerning the relationship between fetal weight and risk of breech delivery.8,12,13 When adjusted for gestational age, birth weight in the current study was not associated with nonvertex position. Furthermore, there was no difference in the percentage of the infants who delivered vertex compared with nonvertex when divided into percentiles of estimated fetal weight at late ultrasound examination or percentiles of birth weight at a given gestational week. This was in direct contrast to a large cross-sectional study examining fetal size as a risk factor for breech birth, in which term breech presentation was associated with smaller fetal size for gestational age when divided into birth weight gestational age percentiles.12 It appears from the current study that, in low-risk pregnancies at gestational ages near term, weight is not associated with risk of breech delivery.
This study found that smoking may be a modifiable risk factor for nonvertex delivery, even after controlling for potential confounders such as fetal weight. It is, therefore, unlikely that intrauterine growth restriction related to smoking is the etiologic factor for the increased risk of breech delivery at term. Small studies have demonstrated an association between decreased intrauterine movement of the fetus and cigarette smoking.14–17 Smoking was also associated with breech birth in a large, population-based, case-control study.8 Our hypothesis is that cigarette smoking may decrease fetal muscle tone and movements in utero. One can imagine that, with decreased kicking near term, the fetus may not have the motor capacity to move into a vertex position. Smoking is not the only risk factor for decreased movements in the fetus. For example, diminished fetal movements have been noted in diabetic pregnancies,18 and diabetes has been found to be a risk factor for breech delivery.8
Decreased intra-amniotic fluid also appears to increase the risk for nonvertex delivery. We hypothesize that lower fluid level is associated with a reduction in the space available in the intrauterine environment for the fetus to accomplish the necessary motions to move into a vertex position. This theory is somewhat supported by numerous reports of increased external cephalic version success with normal or elevated fluid levels as compared with reduced fluid levels.19,20
A recent multicenter randomized controlled trial investigating external cephalic version beginning at 34 weeks of gestation compared with 37 weeks of gestation found that early external cephalic version could reduce risk of nonvertex presentation at delivery.21 Breech presentation at term is associated with nonvertex presentation on ultrasound earlier in pregnancy, and this risk increases with gestational age from 31 to 35 weeks. However, we found that even with nonvertex presentation at 35 weeks, the rate of spontaneous version is still high, arguing against early intervention for nonvertex position.
This study is unique in that it offers a longitudinal view of the natural history of fetal position throughout pregnancy. The findings can aid clinicians in counseling patients regarding management of the nonvertex fetus near term. The study also demonstrates yet another potentially adverse outcome that may be related to smoking in pregnancy. The association between smoking and nonvertex presentation at term may be useful in smoking cessation counseling during the preconception and prenatal periods and may motivate the pregnant woman who wants to reduce her chances of cesarean delivery.
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