The Effect of Delaying Childbirth on Primary Cesarean Section Rates

Abstract

Background

The relationship between population trends in delaying childbirth and rising rates of primary cesarean delivery is unclear. The aims of the present study were (1) to characterize the association between maternal age and the outcome of labor, (2) to determine the proportion of the increase in primary cesarean rates that could be attributed to changes in maternal age distribution, and (3) to determine whether the contractility of uterine smooth muscle (myometrium) varied with maternal age.

Methods and Findings

We utilized nationally collected data from Scotland, from 1980 to 2005, and modeled the risk of emergency cesarean section among women delivering a liveborn infant in a cephalic presentation at term. We also studied isolated myometrial strips obtained from 62 women attending for planned cesarean delivery in Cambridge, England, from 2005 to 2007. Among 583,843 eligible nulliparous women, there was a linear increase in the log odds of cesarean delivery with advancing maternal age from 16 y upwards, and this increase was unaffected by adjustment for a range of maternal characteristics (adjusted odds ratio for a 5-y increase 1.49, 95% confidence interval [CI] 1.48–1.51). Increasing maternal age was also associated with a longer duration of labor (0.49 h longer for a 5-y increase in age, 95% CI 0.46–0.51) and an increased risk of operative vaginal birth (adjusted odds ratio for a 5-y increase 1.49, 95% CI 1.48–1.50). Over the period from 1980 to 2005, the cesarean delivery rate among nulliparous women more than doubled and the proportion of women aged 30–34 y increased 3-fold, the proportion aged 35–39 y increased 7-fold, and the proportion aged ≥40 y increased 10-fold. Modeling indicated that if the age distribution had stayed the same over the period of study, 38% of the additional cesarean deliveries would have been avoided. Similar associations were observed in multiparous women. When studied in vitro, increasing maternal age was associated with reduced spontaneous activity and increased likelihood of multiphasic spontaneous myometrial contractions.

Conclusions

Delaying childbirth has significantly contributed to rising rates of intrapartum primary cesarean delivery. The association between increasing maternal age and the risk of intrapartum cesarean delivery is likely to have a biological basis.

Abstract

Gordon Smith and colleagues find a linear increase in the log odds of cesarean delivery with advancing maternal age from 16 years upwards, and that increasing maternal age is associated with reduced spontaneous uterine activity but increased multiphasic spontaneous myometrial contractions.

Editors' Summary

Background.

The rising rates of cesarean sections have been a concern for over two decades. An acceptable rate of cesarean sections is between 10% and 15% for countries in the developed world, according to the World Health Organization (WHO). However, the estimated rate in the United Kingdom was 20% in 2004. In Canada, it was estimated at 22.5% in 2001–2002. And in the United States, the rate was 30.2% in 2005 (rising 46% since 1996).

This increase may have implications for the mother, baby, healthcare providers, and policy makers. Though it is difficult to directly compare risks between vaginal and cesarean deliveries, higher mortality and morbidity rates are associated with the latter. Risks encountered during the operation may include anesthetic complications and difficulty in stopping bleeding. Later risks include infections, wound healing problems, and increased risk of problems in subsequent pregnancies including malpresentation, placenta previa, and uterine rupture.

Why Was This Study Done?

The trend of increased rates of cesarean sections with maternal age appears to be consistent in different countries and has previously been reported by several epidemiological studies. However, it remains unclear why the risk of having cesarean section is associated with advancing maternal age. Could the association reflect a biological effect of advanced age, or is it a consequence of physician and maternal preference?

The researchers aimed to (1) characterize the association between maternal age and the outcome of labor, (2) determine the proportion of the increase in primary cesarean rates that could be attributed to changes in maternal age distribution, and (3) determine whether the contractility of uterine smooth muscle (myometrium) varied with maternal age.

What Did the Researchers Do and Find?

To address aims (1) and (2), the researchers analyzed data collected over the period 1980 to 2005 by the Scottish Morbidity Record (SMR2), which has been demonstrated to be 99% complete since the late 1970s and free from substantial errors in more than 98% of the records in most of the specific fields used for their analysis.

Their analysis showed a linear association between the risk of having a cesarean section and advancing maternal age in first pregnancies. The cesarean rate also more than doubled over the study period. They estimated that 38% of the additional procedures would have been avoided if maternal age distribution had remained the same as in 1980. Therefore they conclude that a substantial part of the increase may be associated with the trend of delaying of first childbirth.

They then hypothesized that this trend is a result of a biological effect of aging on the contractility of the uterus. This hypothesis was further evaluated with aim (3) of the study, where they biopsied the uteri of 62 women (of mixed parity) undergoing routine elective cesarean delivery to test their contractility. They found that advancing age was associated with impaired uterine function as evidenced by a reduced degree of spontaneous contraction and the type of spontaneous contraction.

What Do These Findings Mean?

This study adds to the evidence that advancing maternal age is associated with higher rates of cesarean sections. It also suggests a possible mechanism for this association, i.e., impaired uterine function. Though it was not studied here, the researchers hypothesize that impaired uterine contractility may be a consequence of prolonged stimulation of the uterus by estrogen and progesterone, resulting from a prolonged interval between menarche and first birth. Further research is needed to understand the determinants and management of dysfunctional labor in older women to help design strategies for reducing population cesarean delivery rates without adversely affecting maternal and infant outcomes.

Additional Information.

Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0050144.

• NHS Choices is a patient information Web site developed to help patients take control of their health care

• Wikipedia has a section on cesarean section (Note that Wikipedia is an internet encyclopedia that anyone can edit)

• MedlinePlus also has information on cesarean section