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 > or =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.

Figure 1. Maternal Age and the Risk of Cesarean Delivery

Proportion of women being delivered by emergency intrapartum cesarean section in relation to age of mother (n = 583,847). Bars are binomial 95% CIs.

Figure 2. Stratified Analysis of Maternal Age and Risk of Cesarean Section

Adjusted odds ratio for a 5-y increase in maternal age (bars are 95% CIs) stratified by maternal age, year of delivery, week of gestation, and deprivation category. Interactions were statistically significant for year of delivery (p < 0.001), week of gestation (p = 0.002), and deprivation category (p = 0.002) using the likelihood ratio test. Odds ratios are adjusted for maternal age, height, deprivation category, onset of labor, week of gestation, sex, birth weight percentile of infant, and year. The vertical dashed line indicates unity.

Figure 3. Time Trends in Maternal Age Distribution and Cesarean Section Rates in Scotland between 1980 and 2005

(A) Proportion of women per year delivering within the age ranges 30–34, 35–39, and ≥40 y. (B) Proportion of women per year delivered by emergency cesarean section.

Figure 4. Maternal Age and the Duration of Labor and Risk of Operative Vaginal Birth

(A) Mean duration of spontaneous labor in relation to maternal age (n = 409,703). Bars are 95% CIs of the mean. (B) Proportion of nulliparous women who required operative vaginal delivery in relation to maternal age among the 518,787 women delivered by a means other than emergency cesarean section. Bars are binomial 95% CIs.

Figure 5. Myometrial Contractility in Relation to Maternal Age

(A) Trace of isometric tension from myometrial strip obtained from a 40-y-old woman being delivered by planned cesarean section. The blue line represents the 15 min before addition of potassium, and the red line 7 min in the presence of 50 mM of potassium. The area under the curve is the space between the tension trace and the baseline, indicated by the blue and red lines. The log₁₀ of the ratio of these two areas is the contraction unit. The spontaneous contraction before the addition of potassium and the contraction following potassium being washed out are both multiphasic. (B) Mean spontaneous contractile activity (quantified as contraction units) of isolated strips of myometrium obtained from women (n = 62) at the time of planned cesarean section in relation to the age of the donor. Regression line: y = 0.1078 + (−0.0174 × age); 95% CI for slope, −0.0326 to −0.0022. (C) Proportion of spontaneous contractions that were multiphasic in relation to maternal age (n = 62, 181 samples). See text for regression analysis of (B) and (C).