According to the National Vital Statistics for 2011, the birth rate for older women, specifically those aged 35–39 and 40–44 years, has been steadily rising over the past several years.1 In parallel with this trend, the cesarean delivery rate has been consistently increasing as well.1 Numerous investigators have demonstrated that older women have higher rates of cesarean deliveries.2–5 The relationship between cesarean delivery and age is unclear but likely is influenced by other maternal and fetal factors. Women giving birth today are not the same as the cohort of women used to create the Friedman labor curves, which are often used today to identify normal compared with abnormal labor progress.6–8 Based on more recently published literature, women today are older, heavier, vary in ethnicity, and are more likely to undergo assisted reproduction.1,9,10 They are also more likely to have a labor induction or augmentation and an epidural and less likely to have an operative vaginal delivery.11–16 These factors may affect labor progress and may be partially responsible for the increasing cesarean delivery rate.11–16 Furthermore, based on recent data, labor progress in a more contemporary population may be different than previously depicted in the Freidman curves.17–19
Hence, as the obstetric population continues to change, a better understanding of the relationship between maternal age and labor progress is necessary. This may help to optimize labor and ultimately help reduce cesarean deliveries in the U.S. The purpose of the current study is to characterize labor progress and length in women according to maternal age.
MATERIALS AND METHODS
We performed a secondary analysis from the Consortium on Safe Labor database. The Consortium on Safe Labor is a collaboration among 12 geographically dispersed clinical centers with 19 total participating community and academic hospitals. After data were collected at each center, it was transferred to a data collecting center where data inquiries, cleaning, and logic checking were performed. Validation studies confirmed a high level of accuracy with greater than 95% concordance between the data set and medical charts. Specifically, validation studies on four of the 20 outcomes examined (shoulder dystocia, cesarean delivery for nonreassuring fetal heart rate, neonatal intensive care unit admission for respiratory conditions, and neonatal asphyxia) were performed by hand abstraction of eligible charts.20 The goal of the Consortium on Safe Labor was to construct a database from electronic medical records that would describe contemporary labor progression in the U.S. There were 228,562 deliveries (87% of which occurred during 2005–2007) in the database. We included women with a known maternal age at delivery who had a singleton, live, term (37 weeks of gestation or greater) birth in cephalic presentation. Women with prior cesarean deliveries were excluded. To avoid intraperson correlation, we selected the first delivery for each patient from the database. Abnormal neonatal outcomes defined as 5-minute Apgar score less than 7, congenital anomalies, birth injury, or neonatal intensive care unit admissions, were excluded to maintain consistency with the original study from the data set.17 Women were grouped into four maternal age categories: younger than 20 years of age, 20–29 years of age, 30–39 years of age, and 40 years of age or older and then stratified by parity (ie, nulliparous or multiparous). Demographics and maternal characteristics among the age categories were compared with the younger than 20 year olds (referent) using Pearson’s χ2 and analysis of variance tests.
To estimate the duration of labor, an interval-censored regression analysis was used to determine median traverse times with 95th percentiles, assuming the labor data had a normal distribution.21,22 The median traverse times represent the time to progress centimeter by centimeter, starting at 3 cm to complete dilation. Women with at least two cervical examinations, regardless of whether they reached 10 cm and irrespective of the ultimate delivery route, were included in the median traverse times analysis. The analysis was adjusted for covariates including race, admission body mass index, preexisting diabetes and gestational age at the time of admission, labor induction, labor augmentation, epidural use, and birth weight. The youngest age category (younger than 20 years of age) within each parity cohort was selected as the referent for this statistical analysis. The median and 95th percentile times for the second stage of labor were also calculated for each maternal age category with and without epidural use and stratified by parity. Each age category was compared with the younger than 20-year-old category with generalized linear models, adjusting for race, admission body mass index, preexisting diabetes, gestational age at the time of admission, labor induction, labor augmentation, birth weight, and operative vaginal deliveries and excluded women who delivered by cesarean in the second stage. A P value <.05 was considered statistically significant.
A repeated-measures analysis with an eighth-degree polynomial model was used to construct mean labor curves by parity using cervical dilation in centimeters. The curves were constructed in a backward fashion, in which the starting point was set at the first time when dilation reached 10 cm (time=0) and the time was calculated backwards (eg, 60 minutes before complete dilation=−60 minutes). After the labor curve models had been completed, the x-axis (time) was reverted to a positive value (ie, instead of −12 to 0 hours, it was transformed to 0–12 hours). The labor curves only included gravidas who reached 10 cm and had at least two cervical examinations and did include those who may have had a cesarean delivery in the second stage. The labor curves started at 3 cm so as to allow for convergence in an eighth-degree polynomial model, but women who were admitted at earlier dilations also contributed data to the labor curves. All statistical analyses were performed using SAS 9.2 (PROC MIXED for the repeated-measures analysis and PROC LIFEREG for interval censored regression, PROC GLM for second stage of labor). This study was approved by the institutional review board at all participating sites.
Of the 120,442 women who met inclusion criteria, 55,115 (45.8%) were nulliparous and 65,327 (54.2%) were multiparous (Fig. 1). Among nulliparous and multiparous women, those who were aged younger than 20 years of age were more likely to be of black race, whereas those aged 20 years and older were more likely to be of white race (P<.001; Table 1). As age increased, so did the incidence of preexisting diabetes, labor inductions, epidural use, operative vaginal deliveries, and cesarean deliveries in both parity groups, whereas admission dilation and effacement decreased (P<.02). Operative vaginal deliveries included all vacuum and forceps-assisted deliveries.
For nulliparous women, time to progress centimeter to centimeter decreased as age increased up to 40 years old (Table 2). The time to progress from 4 to 10 cm decreased as age increased up to 40 years old with nulliparous women younger than 20 years old progressing the slowest (median 8.5 hours compared with 7.8 hours in those 20–29 years of age group and 7.4 hours in the 30–39 years of age group, P<.001). Those 40 years of age or older took 8.0 hours to progress from 4 to 10 cm; however, this did not differ compared with the referent group (P>.05). The second stage of labor with and without an epidural increased directly with age (P<.001), and epidural use was associated in an increase in the duration of the second stage by approximately 0.4 hours in all age groups.
According to the median traverse times, multiparous women progressed through labor faster as age increased (Table 3). The time to traverse from 4 to 10 cm was 8.8 hours and 6.5 hours for the youngest and oldest groups, respectively. The second stage of labor with and without an epidural was not different among age groups and epidural use was associated with an increase in the length of the second stage by 0.2 hours in all groups.
Figure 2 demonstrates the mean labor curves for nulliparous women (n=18,953; Fig. 2A) and multiparous women (n=24,989; Fig 2B) who reached 10-cm dilation by each maternal age category. For both nulliparous and multiparous women, those younger than 20 years old progressed through labor the slowest. In nulliparous women, those younger than 20 years of age and those 40 years of age or older progressed through labor at markedly different rates, whereas the two middle age categories progressed at similar rates (Fig. 2A). In contrast, the labor curves for multiparous women demonstrated that women younger than 20 years old and those women 20–29 years of age progressed through labor at similar rates and slower than the two oldest age categories, which is in agreement with the median traverse times (Fig 2B). In nulliparous women, there was no apparent inflection point, whereas the multiparous women had an inflection point around 6 cm, after which the rate of cervical change increases markedly.
We initially hypothesized that the first stage of labor would progress more slowly as maternal age increased. Instead, we demonstrated that the first stage of labor progressed more quickly with increasing maternal age as reflected by the median traverse times, especially in multiparous women, and by the labor curves in both nulliparous and multiparous women. These findings are independent of important covariates such as labor induction or augmentation. The labor curves suggest a difference in labor patterns between the youngest and oldest parturients for both nulliparous and multiparous women. All these findings are especially important for clinical practice because the cohort of parturients studied represents a contemporary group of women.
Our median traverse times and total labor lengths are in agreement with the original studies by Friedman.6,23 We have additionally found a relationship between maternal age and labor progression seen in our labor curves that has not been previously demonstrated. In 1965, Friedman and Sachtleben8 evaluated labor length in 3,329 nulliparous women and found no important difference in the course of labor that could be ascribed to maternal age when comparing the young (younger than 18 years old) and the older (older than 35 years old) parturient. However, the authors note that their study may not have been powered to find a significant difference. In their analysis of 26 control-matched gravidas, Sokol et al24 concluded advanced maternal age was not associated with labor prolongation. With respect to the second stage length, our findings agree with other studies in that the second stage increases directly with age with and without an epidural.6–8,25,26
Greenberg et al27 assessed how maternal age affects the first and second stage labor lengths as a whole. In their analysis of 31,976 births, they concluded that nulliparous women had increasing median lengths of labor as age increased, but this pattern was only present up to 34 years, after which the length of labor decreased in the 35–39 years and 40-year-old and older groups. Furthermore, younger women had a slightly longer first stage than older women (6.1 hours in those younger than 20 years and 5.7 hours in those older than 40 years, P=.02). This bimodal distribution in which first stage labor length is similar among the youngest and oldest groups according to the traverse times is similar to our work and warrants more investigation because it differs from the trend in our labor curves. Because the difference in first-stage labor length was so small among the groups, they concluded that clinical applicability may be limited to the 95th percentile values only.27 In terms of the second stage, their findings were in agreement with ours.27
Our study is not without limitations. The labor curves for older women (mainly those 40 years and older) may not represent true labor if a woman delivered by cesarean before achieving complete dilation, possibly as a result of physicians' preferences because these women would not be included in the labor curves. However, Adashek et al4 found that increasing maternal age was independently associated with increased cesarean delivery rates but did not find any controllable physician factors for this association. There were statistically significant differences in most maternal and labor characteristics among the age groups as a result of the large sample size of our study (Table 1), yet this may be of limited clinical significance. The subjectivity of cervical examinations must be considered as well as the lack of uniform labor protocols among all the institutions that participated in the Consortium on Safe Labor. To assess true spontaneous labor, we attempted to perform a secondary analysis excluding all parturients who received augmentation or induction agents. Because labor augmentation and induction were relatively common in our study cohort, averaging approximately 60%, this analysis was not feasible as a result of a much smaller sample size. Harper et al28 demonstrated that induced labor progresses strikingly slower than spontaneous labor. Therefore, our estimates of labor length are likely conservative. Including all women who were augmented or induced may be considered a strength of our study design in that it allows for accurate representation of a more contemporary obstetric cohort compared with prior reports that focused only on spontaneous labor. The effect of maternal age on spontaneous compared with augmented or induced labor warrants further study, for which we lacked an adequate sample size to assess.
The strengths of our work include being a multicenter cohort study generalizable to women across the U.S. Our aim was to assist health care practitioners in understanding the influence of maternal age on labor progress. Further research to assess the physiologic aging process on myometrial tissues is also needed. In general, our findings confirm the need for a better understanding of the mechanism underlying uterine contractility as well as the effect of age on uterine musculature. Given that maternal age influences the first and second stages of labor in opposite directions according to our findings, we speculate that there is likely a very fundamental difference in the aging process of smooth compared with striated muscle.
Based on our analysis, we conclude that contemporary labor practices should take into account the changing age profiles of obstetric populations in the U.S. Our findings suggest that younger women progress through labor at a slower rate and older women progress faster. We therefore recommend that maternal age be taken into consideration before intervening with cesarean delivery. This is especially true for young nulliparous women in whom delivery route may have a major effect on future pregnancy outcomes.
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