July 1, 2008 Impaired contraction of the uterus (womb) may play a part in the association of advancing maternal age with increased cesarean rates according to new research. In a two part study, Professor Gordon Smith and colleagues (Cambridge University, UK) analyzed a large Scottish data base to characterize the association between maternal age and outcome of labor, specifically looking at what proportion of the increase in primary cesarean rates could be attributed to changes in maternal age distribution.
They then went on to study samples of myometrium (muscle from the wall of the uterus) taken during routine cesarean sections to determine whether muscle contractility varied with maternal age.
Cesarean sections are associated with higher mortality and morbidity rates compared with vaginal deliveries. Rates of caesarean section have dramatically increased over the last 20-30 years throughout the developed world. This increase is a concern as it may have implications for the mother, baby, healthcare providers, and policy makers. According to the World Health Organization (WHO), the UK's cesarean rate was 20% in 2004, Canada's was 22.5% in 2001-2002 and the United States' was 30.2% in 2005 -- all above the WHO's proposed acceptable rate of between 10% and 15% for countries in the developed world.
Previous epidemiological studies have reported a trend of increased rates of cesarean sections with maternal age which is consistent in different countries. However, the reason for these increased rates remains unclear. Moreover, it was unclear to what extent, if any, the trend of delaying childbirth had contributed to the increased cesarean rates.
This study was carried out in two parts. First, the researchers analyzed data collected by the Scottish Morbidity Record (SMR2) from 1980 to 2005 -- over 0.5m available records. A linear association between the risk of having a cesarean section and advancing maternal age in first pregnancies was found. They observed striking changes over the period of study.
The proportion of women aged 30-34 increased three fold, the proportion aged 35-39 increased seven-fold and the proportion aged 40 or older increased 10-fold. Over the same period, the cesarean rate more than doubled. The researchers estimated that around 38% of the additional procedures would have been avoided if maternal age distribution had remained the same as in 1980. They therefore concluded that a substantial proportion of the increase in cesarean section rates may be due to the trend of delaying first childbirth.
Second, they hypothesized that the increased risk of caesarean section among older women is a result of a biological effect of aging on the ability of the muscle of the uterus to contract. They evaluated this hypothesis by examining biopsies from the uteruses of a separate group of 62 women (of mixed parity) undergoing routine elective cesarean delivery in Cambridge. 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.
The analysis of Scottish data adds to the evidence that advancing maternal age is associated with higher rates of cesarean sections. Moreover, it indicates that the trend of delaying childbirth has substantially contributed to recent increases in cesarean rates. The researchers' further work at the NIHR Cambridge Biomedical Research Centre at the Rosie Hospital in Cambridge, UK suggests a possible mechanism for this association, i.e., impaired contraction of the uterus. Although further work will be needed in other populations, as the authors comment, "understanding the determinants and management of dysfunctional labor in older women is central to designing strategies for reducing population cesarean delivery rates without adversely affecting maternal and infant outcomes."
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- Smith et al. The Effect of Delaying Childbirth on Primary Cesarean Section Rates. PLoS Medicine, 2008; 5 (7): e144 DOI: 10.1371/journal.pmed.0050144
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