The proportion of mothers at no indicated risk decreased from 46% of all births in 1991 to 42% in 1998 but has since levelled off (). However, the primary caesarean rate for this exceptionally low risk group rose 67% between 1991 (3.3%) and 2001 (5.5%), with a gradual increase from 1991 to 1996 and a rapid one thereafter.
Percentage of all live births to women with no indicated risk factors* and associated rate of primary caesarean delivery,† United States, 1991-2001, and percentage change 1991-2001
What is already known on this topic
The overall and primary caesarean rate is growing rapidly in the United States and worldwide, and the likelihood of a caesarean is strongly related to age of the mother and parity
What this study adds
A new category for analysis has been created—the “no indicated risk” caesarean
The proportion of no indicated risk primary caesareans is growing rapidly in the United States, adding to the overall rise in the primary caesarean rate
Older, primiparous mothers were much more likely to have a no indicated risk primary caesarean; almost one fifth (19.5%) of primiparous mothers aged over 34 had such a delivery in 2001. More than 5% of multiparous mothers over 34 who had had previous vaginal births also had a no indicated risk primary caesarean in 2001. Among mothers under 30 with no indicated risk, the primary caesarean rate grew by more than half (58%) between 1991 and 2001 to 4.9%.
The raw numbers of births also illustrates this trend. In 2001, 80 028 no indicated risk primary caesareans took place in the United States, an increase of 25 162 since 1996. This represented 25.8% of the total increase (97 659) in primary caesareans between 1996 and 2001.
We used multivariate logistic regression analysis (SAS version 8) to examine changes in primary caesarean rates after controlling for parity; maternal ethnicity, age, and education; birth weight; and data year (1991, 1996, or 2001) (see table on bmj.com
). We ran models for all mothers, including parity as a variable, and for first time mothers only. Age was a major factor, particularly among first time mothers. For primiparous mothers aged over 40, the odds of having a caesarean were 5.4 times that for mothers aged 20-24. In the multivariate analysis, the overall increase between 1991 and 1996 disappeared, but the odds of having a no indicated risk primary caesarean in 2001 were almost 50% higher than the odds for comparable mothers in 1996.