Abstract
Objective
We sought to estimate when rates of maternal pregnancy complications increase beyond 37 weeks of gestation.
Methods
We designed a retrospective cohort study of all low-risk women delivered beyond 37 weeks gestational age from 1995 to 1999 within a mature managed care organization. Rates of mode of delivery and maternal complications of labor and delivery were examined by gestational age with both bivariate and multivariable analyses.
Results
We found that among the 119,254 women who delivered at 37 completed weeks and beyond that the rates of operative vaginal delivery (OR 1.15, 95% CI 1.09, 1.22), 3rd or 4th degree perineal laceration (OR 1.15, 95% CI 1.06, 1.24), and chorioamnionitis (OR 1.32, 95% CI 1.21, 1.44) all increased at 40 weeks as compared to 39 weeks of gestation (p<0.001), and rates of postpartum hemorrhage (OR 1.21, 95% CI (1.10, 1.32), endomyometritis (OR 1.46, 95% CI 1.14, 1.87) and primary cesarean delivery (1.28, 95% CI 1.20, 1.36) increased at 41 weeks of gestation (p< 0.001). The cesarean indications of nonreassuring fetal heart rate (OR 1.81, 95% CI 1.49, 2.19) and cephalo-pelvic disproportion (OR 1.64, 95% CI 1.40, 1.94) increased at 40 weeks of gestation (p<0.001)
Conclusion
We found that the risk of maternal peripartum complications increase as pregnancy progresses beyond 40 weeks of gestation. Management of pregnancies that progress past their EDC should include counseling regarding the risks of increasing gestational age.
Condensation
Rates of operative delivery, postpartum hemorrhage, chorioamnionitis, perineal lacerations, and endomyometritis all increase beyond 40 weeks of gestation.
Keywords: complications of pregnancy, postterm, post-dates, cesarean delivery
Introduction
Postterm pregnancy has been defined for more than two decades as a pregnancy that persists beyond 294 days or 42 weeks of gestation.1 The postterm pregnancy, which likely occurs among less than 5% of gravidas2 has been associated with an increased perinatal mortality rate.3,4,5 Perinatal morbidity has also been noted to be higher in postterm pregnancies including meconium and meconium aspiration syndrome,6 oligohydramnios,7 macrosomia,8,9 fetal birth injury,10 rate of nonreassuring fetal heart rate or fetal distress in labor,11 and rates of cesarean delivery.5,12 While it is well established that these risks are increased in postterm pregnancy, what is less well elucidated is whether these risks increase prior to 42 weeks’ gestation.
While the rate of stillbirth has been examined by week of gestation13,14 most studies have examined the rates of perinatal complications in a dichotomous fashion comparing patients below and beyond a particular gestational age. While this may be necessary to examine uncommon complications with enough statistical power, it confuses the fundamental clinical question – what are the rates of complications at the current week of gestation as compared to the next week of gestation? By examining rates of complications by week of gestation, we previously demonstrated that at a single community hospital the rates of meconium, operative vaginal delivery, severe maternal perineal lacerations, and primary cesarean delivery all increased prior to 42 weeks of gestation.15 We also found the rate of preeclampsia increased when the appropriate denominator of all women pregnant at a given week of gestation was considered.16 Further, when we examined short term neonatal complications at a tertiary academic center, we noted that neonatal pH < 7.0, base excess < −12, and 5-minute Apgar score < 7 all increase prior to 42 weeks of gestation.17
The question remains, how does perinatal morbidity change throughout term gestations? Further, do the increases in adverse outcomes noted among an older, referral population persist in the broader obstetric population? Given these questions, we examined the rates of maternal obstetric outcomes and complications by week of gestation among a fully insured population of parturients.
Methods
We designed a retrospective cohort study which included pregnant women delivered within the Kaiser Permanente Medical Care Program’s (KPMCP) Northern California Region from January 1, 1995 to December 31, 1999 after obtaining IRB approval. Patients were eligible for inclusion in our study if they delivered a live birth beyond 37 weeks of gestation during the study period at any one of 12 KPMCP delivery hospitals in Northern California or if they delivered at the Alta Bates Medical Center in Berkeley, CA under the care of KPMCP physicians. The KPMCP is a mature managed care organization with fully integrated information systems that employ a common medical record number across all facilities. The obstetric care provided throughout these hospitals includes attending obstetricians, OB/Gyn residents, and clinical nurse midwives. Maternal demographic data were obtained by scanning the KPMCP hospitalization database and linking maternal records using methods we have described elsewhere.18,19,20 Diagnoses were obtained by utilizing ICD-9 codes from discharge abstracts entered by trained medical coders. In order to obtain a relatively low-risk population, exclusion criteria included chronic hypertension, preeclampsia, pre-existing or gestational diabetes, placenta previa, multiple gestation, non-vertex presentation, and fetal anomalies. All data were then entered into a STATA v.7 dataset (Stata Corp, College Station, TX).
Using week of gestation as the primary predictor variable, we examined its association with the following outcomes: mode of delivery and its indications, prolonged labor beyond 24 hours, 3rd or 4th degree perineal laceration, postpartum hemorrhage (PPH), chorioamnionitis, and postpartum endomyometritis. These particular outcomes were chosen because of both existing evidence of their association with postterm pregnancies as well as biological plausibility between post term pregnancies and each of the outcomes. Further exclusion criteria were utilized for some of these outcomes: patients with a prior cesarean were excluded when examining mode of delivery and cesarean delivery prior to the onset of labor was excluded when examining length of labor. Statistical comparisons of proportions were made with the chi-square test and were deemed statistically significant if p<0.05.
In addition to the outcome variables of interest, the following potential confounders were controlled for in multivariable logistic regression models: maternal age, ethnicity, education, parity, length of labor, induction of labor, birthweight, and type of anesthesia. Maternal age and birthweight were noted to affect the outcomes of interest in a continuous fashion and thus were entered into the models in continuous form. In order to assess goodness of fit for the multivariable models we utilized the Hosmer-Lemeshow chi-square test.21 For this test, if the p-value is large, the model is well calibrated; if the p-value is small the model may need to be further modified.
Results
After exclusion of pregnancies complicated by multiple gestation, diabetes mellitus, gestational diabetes, chronic hypertension, preeclampsia, placenta previa, breech presentation, and congenital anomalies there were 119,254 singleton, low-risk pregnancies that were analyzed. Of these, 16.1% (19,229) were aged 35 or older, and all of the four major ethnic groups had a sample size of at least 10,000 (Table I). The overall cesarean rate was 13.8%, with a primary cesarean rate of 10.0%. This rate was greater among nulliparas (16.3%) as compared to multiparas (4.5%) with a p<0.001.
Table I.
Demographics and Descriptive Obstetric Outcomes
| Variable | Number (% of Total N=119,254) |
|---|---|
| Maternal Age | |
| < 18 | 3,928 (3.3%) |
| 18 – 34 | 96,097 (80.6%) |
| >= 35 | 19,229 (16.1%) |
| Education | |
| Less than 12 years | 15,068 (12.6%) |
| High School Graduate | 70,361 (59.0%) |
| College Graduate | 33,825 (28.4%) |
| Ethnicity | |
| African-American | 11,468 (9.6%) |
| Asian | 28,891 (24.2%) |
| Hispanic | 18,234 (15.3%) |
| Caucasian | 57,985 (48.6%) |
| Native American | 510 (0.4%) |
| Unknown | 2,166 (1.8%) |
| Nulliparous | 50,005 (42.3%) |
| Birthweight >= 4,000 gms | 17,424 (14.6%) |
| Cesarean Delivery | 16,485 (13.8%) |
| Operative Vaginal Delivery | 11,113 (9.3%) |
When the rate of primary cesarean was examined by increasing week of gestation, there was a large increase between 40 and 41 (9.0% vs. 14.0%, p<0.001) weeks of gestation which increased to 21.7% among pregnancies beyond 42 weeks of gestation (Table II). When these rates were considered by parity, nulliparous women mirrored the large increases seen by week of gestation after 40 weeks, while the increases were less among multiparas (Table II). These increases were also seen in the rates of operative vaginal delivery, which nearly doubled from 9.4% at 39 weeks to 17.4% at 42 weeks or greater. When the indication for cesarean of nonreassuring fetal heart rate, previously known as “fetal distress”, was examined by week of gestation (Table III), these increases actually began a week earlier with those at 40 weeks of gestation having a rate higher than those at 39 weeks of gestation (19.6 % vs. 13.7 %, p<0.001). Again, the rate of this indication increased further beyond 41 (23.5%) and 42 (27.5%) weeks of gestation. This was also true for the cesarean indication of cephalo-pelvic disproportion (CPD) with rates at 40 weeks (26.2 %, p<0.001) being higher than at 39 weeks (14.9 %). The rate of the CPD indication further increases beyond 41 weeks of gestation (Table III). When operative vaginal delivery for nonreassuring fetal heart rate were examined, rates increased at 41 weeks of gestation (Table III).
Table II.
Mode of Delivery by Week of Gestation
| Gestational Age | |||||
|---|---|---|---|---|---|
| Weeks | (n) | Primary Cesarean | Primary Cesarean Nulliparas | Primary Cesarean Multiparas | Operative Vaginal Delivery |
| 37 | (5,550) | 9.9% | 14.4% | 5.8% | 8.9% |
| 38 | (14,130) | 8.2% | 12.8% | 4.5%* | 8.8% |
| 39 | (26,884) | 8.8% | 14.4% | 4.4% | 9.4% |
| 40 | (40,364) | 9.0% | 14.9% | 4.0% | 10.9%*** |
| 41 | (16,946) | 14.0%*** | 21.9%*** | 5.1%** | 13.3%*** |
| ≥42 | (2,964) | 21.7%*** | 30.8%*** | 7.9%*** | 17.4%*** |
Statistical significance as compared to the rate of complication in the prior week of gestation
<0.05,
<0.01,
<0.001 using chi-square test
Cesarean analyses exclude all patients with a prior cesarean, operative vaginal delivery excludes all patients with a cesarean delivery
Table III.
Indication for Delivery by Week of Gestation
| Gestational Age Weeks (C/S n) | 1° Cesarean Nonreassuring fetal heart rate | 1° Cesarean CPD | Op Vag Del Nonreassuring fetal heart rate |
|---|---|---|---|
| 37 (548) | 13.3% | 9.7% | 12.5% |
| 38 (1,168) | 14.5% | 14.4% | 10.9%** |
| 39 (2,364) | 13.7% | 14.9% | 10.0% |
| 40 (3,651) | 19.6%*** | 26.2%*** | 11.5% |
| 41 (2,368) | 23.5%*** | 31.4%*** | 13.6%** |
| ≥42 (645) | 27.5%*** | 38.0%*** | 22.1%*** |
Statistical significance as compared to the rate of complication in the prior week of gestation
<0.05,
<0.01,
<0.001 using chi-square test
CPD = cephalo-pelvic disproportion
When the rates of maternal complications of labor and delivery were examined by week of gestation, we found that the rates of 3rd or 4th degree perineal lacerations, postpartum hemorrhage, chorioamnionitis, and prolonged labor were all increased among women delivering at 40 weeks compared to 39 weeks of gestation (Table IV). The rate of 3rd or 4th degree perineal lacerations increased from 38 to 39 weeks and with each incremental week thereafter. The rate of endomyometritis did not increase until women at 41 weeks were compared to those at 40 weeks of gestation. Of note, these complications were associated with an increased postpartum length of stay as was the gestational age at delivery. For example, the mean length of stay at 39 weeks was 1.41 days (95% CI – 1.39, 1.42) whereas at 41 weeks it was 1.54 days (95% CI – 1.50, 1.57) and at 42 weeks it increased to 1.70 days (95% CI – 1.62, 1.79) with all differences being statistically significant.
Table IV.
Maternal Complication Rates by Week of Gestation
| Gestational Age | ||||||
|---|---|---|---|---|---|---|
| Weeks | (n) | 3rd or 4th Degree Laceration | Postpartum Hemorrhage | Chorioamnionitis | Endomyometritis | Prolonged Labor (>24 hours) |
| 37 | (6,168) | 3.4% | 1.1% | 2.3% | 1.3% | 7.3% |
| 38 | (15,773) | 3.6% | 2.5%*** | 2.5% | 1.2% | 7.9% |
| 39 | (30,685) | 4.0%* | 2.5% | 2.7% | 1.3% | 9.2%*** |
| 40 | (44,722) | 4.6%** | 3.1%*** | 3.7%*** | 1.4% | 11.2%*** |
| 41 | (18,638) | 6.7%*** | 4.1%*** | 5.1%*** | 2.0%*** | 14.9%*** |
| ≥42 | (3,268) | 9.1%*** | 5.0%* | 6.0%* | 2.9%** | 19.7%*** |
Perineal laceration rates are among vaginal deliveries.
Statistical significance as compared to the rate of complication in the prior week of gestation
<0.05,
<0.01,
<0.001 using chi-square test
In order to examine the contribution of the increased cesarean rate to the postpartum complications, we performed subgroup analyses by mode of delivery for endomyometritis and postpartum hemorrhage. In women with a cesarean delivery, the rate of endomyometritis was 5.1% at 39 weeks, 6.7% at 40 weeks, 7.7% at 41 weeks, and 9.7% at 42 weeks and greater (p<0.001). In women with a vaginal delivery, the rate of endomyometritis was 0.64% at 39 weeks, 0.69% at 40 weeks, 0.72% at 41 weeks, and 0.83% at 42 weeks and greater (p=0.075). The rates of postpartum hemorrhage increased similarly by week of gestation with both trends being statistically significant (p<0.001). Finally, to examine the contribution of operative vaginal delivery to perineal lacerations, the rates were examined by subgroup. Women with a spontaneous vaginal delivery experienced statistically significant increased rates of 3rd or 4th degree perineal laceration from 39 weeks (1.9%) to 40 weeks (2.2%) to 41 weeks (3.6%) through 42 weeks and greater (5.1%, p<0.001). The increases were proportionately smaller though the magnitude was larger for women with an operative vaginal delivery increasing from 24.4% at 39 weeks to 28.2% at 42 weeks and greater (p=0.02)
When both the mode of delivery as well as the rates of perinatal complications were examined controlling for potential confounders, the differences exhibited in the univariate analyses generally persisted (Table V). The Hosmer-Lemeshow chi-square test of goodness of fit was >0.05 for all models except primary cesarean. However, it was >0.05 for the multivariable models examining the indications for primary cesarean. The indication of nonreassuring fetal heart rate for cesarean was 1.8 times more likely at 40 weeks as compared to 39 weeks which increased to 2-fold beyond 41 weeks even when controlling for maternal age, parity, ethnicity, education, induction of labor, use of epidural, and birthweight. The rates of CPD also increased beyond 39 weeks of gestation even when controlling for these same confounders. Rates of 3rd or 4th degree perineal lacerations, postpartum hemorrhage, chorioamnionitis, and endomyometritis were controlled for all of the potential confounders listed above as well as mode of delivery and exhibited increases consistent with the univariate comparisons except for postpartum hemorrhage which did not exhibit an increase until 41 weeks of gestation.
Table V.
Association of Gestational Age with Perinatal Outcomes in Multivariable Models*
| Outcome | 40 wks GA | 41 wks GA | ≥42 wks GA | |||
|---|---|---|---|---|---|---|
| OR | (95% CI) | OR | (95% CI) | OR | (95% CI) | |
| Primary Cesarean Delivery | 0.95 | (0.89, 1.01) | 1.28 | (1.20, 1.36) | 1.77 | (1.59, 1.98) |
| 1° C/S for nonreassuring fetal heart rate | 1.81 | (1.49, 2.19) | 2.00 | (1.63, 1.46) | 2.11 | (1.60, 2.80) |
| 1° C/S for CPD | 1.64 | (1.40, 1.94) | 2.06 | (1.74, 2.45) | 1.93 | (1.52, 2.46) |
| Operative Vaginal Delivery | 1.15 | (1.09, 1.22) | 1.29 | (1.20, 1.37) | 1.64 | (1.45, 1.85) |
| 3rd or 4th Degree Laceration | 1.15 | (1.06, 1.24) | 1.58 | (1.44, 1.73) | 1.88 | (1.61, 2.21) |
| Postpartum Hemorrhage | 1.06 | (0.99, 1.15) | 1.21 | (1.10, 1.32) | 1.27 | (1.08, 1.50) |
| Chorioamnionitis | 1.32 | (1.21, 1.44) | 1.60 | (1.45, 1.77) | 1.71 | (1.44, 2.02) |
| Endomyometritis | 1.08 | (0.88, 1.33) | 1.46 | (1.14, 1.87) | 1.76 | (1.09, 2.84) |
| Prolonged Labor (> 24 hrs) | 1.12 | (1.03, 1.22) | 1.85 | (1.70, 2.02) | 4.13 | (3.67, 4.64) |
Each outcome was examined in a separate multivariable analysis and compared to pregnancies delivered at 39 weeks gestation, controlling for maternal demographics, length of labor, induction, use of epidural, birthweight, and mode of delivery (except for C/S and operative vaginal delivery). Of note, the Hosmer-Lemeshow chi-square test of goodness of fit was >0.05 for all models except primary cesarean delivery (<0.01), it was >0.05 for the models examining indications for primary cesarean.
Comment
We found that the rates of cesarean, operative vaginal delivery, and maternal complications of labor and delivery all increased prior to 42 weeks of gestation among a cohort of fully insured pregnant women within a large managed care organization. These increases which occurred at either beyond 39 weeks (prolonged labor, operative vaginal delivery, 3rd or 4th degree perineal laceration) or 40 weeks (postpartum hemorrhage, endomyometritis, primary cesarean), persisted when controlling for potential confounders. Furthermore, when the indications of nonreassuring fetal heart rate and cephalopelvic disproportion were examined they also increased beyond 39 weeks of gestation. Of note, the odds ratios when comparing 40 to 39 weeks’ gestation range from 1.1 to 1.8, however the effect of change also depended on the baseline risk which ranges from 1–3% for postpartum hemorrhage, chorioamnionitis, and endomyometritis up to 15% for cesarean delivery. Thus, while it is statistically significant, the effect of increasing morbidity was lower for these less frequent complications.
At least some component of the rise in the maternal complications noted may have been contributed by the increase in operative vaginal and cesarean deliveries. However, even when subgroup analyses of the postpartum complications were performed by mode of delivery, all of the trends continued to exhibit statistically and clinically significant increases except for the rate of endomyometritis among women undergoing vaginal delivery. Thus, for this complication alone, the increase in and among women experiencing cesarean deliveries accounted for the bulk of the increase by gestational age. Thus, for the majority of these complications, even if the increases in operative deliveries could be avoided, it appears that the increases in maternal complications would persist.
The designation of postterm pregnancy as those pregnancies which persist beyond 42 weeks of gestation or 294 days has existed for more than 25 years.1 A recent ACOG Technical Bulletin22 persisted with this designation despite evidence that both maternal and neonatal complications appear to increase prior to 42 weeks of gestation. Further, this document did not discuss the theoretic impact of decreasing both cesarean deliveries as well as intrauterine fetal demise which might occur if earlier antenatal testing or induction of labor were recommended. For example, evidence from a 1988 study23 demonstrated a decreased rate of stillbirths and nonreassuring fetal heart rate during labor in a group of patients who began testing at 41 weeks as compared to the control group who began testing at 42 weeks gestational age. Most recently, the gestational age at which clinical concern should be raised was questioned24 in a study which asserted that concerns regarding the morbidity and mortality of pregnancies at and beyond term should be weighed against the risks of induction of labor. Certainly, the clinical evidence supporting induction of labor earlier than 42 weeks of gestation is favorable with a number of studies25,26,27,28 suggesting that the rates of cesarean delivery are clinically similar between induction and expectant management and with improved dating criteria.29,30 In fact, a recent meta-analysis of this literature found an overall effect of a lower cesarean delivery rate among those women who underwent induction of labor (20.1%) as compared to expectant management (22.0%).28
Our results offer further support to reconsider what recommendations we make to women who progress beyond their due date. When this does happen, a woman and her physician need to compare the risks of induction of labor versus the risks of expectant management to both herself and her fetus. At 41 weeks of gestation, the majority of the evidence26,27,28 supports a lower rate of cesarean with induction of labor, and by virtue of earlier delivery, the rate of intrauterine fetal demise would also be lower. We would also offer that other maternal complications such as postpartum hemorrhage, chorioamnionitis, and endomyometritis that may also be associated with neonatal complications are also lower one and two weeks prior to the 42 week threshold.
As with any retrospective cohort study, there were limitations to our analysis. As compared to the prospective randomized, controlled study that demonstrated a reduction in the rate of cesarean delivery with induction,25 our study may have potential confounders that were not identified that could vary between the different gestational ages. We attempted to control for identifiable confounders such as differences in maternal demographics, medical history, and obstetric management. Regardless of these attempts to address confounding, we cannot ignore possible bias by practitioners. For example, in the case of indication for cesarean a clinician may be more likely to use a particular indication more commonly at a particular gestational age. However, we think it unlikely that such bias may exist as early as 40 weeks of gestation, given the current definition of postterm pregnancy being 42 weeks of gestation and beyond. Another possible bias is that the actual dating of pregnancies may differ from that recorded. It has been noted that more pregnancies are misdated prolonged than foreshortened because of the use of last menstrual period dating which tends to bias in that direction.2 However, the correction of this bias would only augment the differences we found which increased with increasing gestational age. Finally, with any multivariable model, there is concern that the model may not represent all subgroups within the population. However, we found that the p-values for the Hosmer-Lemeshow goodness of fit tests were all > 0.05 except for the overall cesarean delivery models. Subgroup cesarean models for indication for cesarean delivery were not statistically significant. An additional limitation of these data was the lack of information on antenatal testing. As these data represent a number of different Kaiser institutions in Northern California, management will differ between institutions. Because of the lack of these data, we cannot explore whether women who had earlier antenatal testing were less likely to experience complications on labor and delivery as has been previously demonstrated.23
Given these theoretic limitations, we believe that we offer further evidence that the management of postterm pregnancy and what is designated as postterm pregnancy should be readdressed. Clinicians may use this evidence to counsel patients who wish to continue expectant management beyond 41 weeks of gestation regarding the increasing risks of complications of labor delivery with increasing gestation. Future research should focus on the use of improved pregnancy dating, antepartum testing, and induction of labor to determine whether perinatal morbidity and mortality can be reduced.
Footnotes
Dr. Caughey is a Women’s Reproductive Health Research Scholar, sponsored by the National Institute of Child Health and Human Development, Grant # HD01262. This project was supported by the Agency for Healthcare Research and Quality (grant number P01 HS010856, “Promoting Effective Communication and Decision Making for Diverse Populations”). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the AHRQ.
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