Is There a Link Between C-section Births and Evolutionary Changes?
by Lisa DiFalco
According to the CDC, over 32 percent of births in America are C-sections. This far exceeds the World Health Organization’s range of 10 to 15 percent given for C-sections that are medically necessary. Do evolutionary advantages play a role in C-section births, and are C-section births contributing to evolutionary changes? Essentially, this chicken or the egg question is part of a new theory that is being put forth in a new study shared via CBS News. With the increase in C-section use, there may be a widening gap between baby size and female pelvis width. The consequences of C-sections on delivered infants leave much to be understood, as do the consequences of C-sections over successive generations of pregnant women. Explore the new theory that seeks to establish an evolutionary link related to C-section births.
The New Theory
Essentially, researchers have brought forth a new theory; they believe that “Evolution favored small female pelvises and large newborns.” In addition, they believe the rise in C-section deliveries may be helping to create a bigger gap between the size of mothers, pelvises, and the size of newborns. They estimate that there is now a 10 to 20 percent increase in this gap that is directly related to regular use of C-sections.
Despite evolutionary changes, the female pelvis has stayed small. The human female birth canal size is quite different than in other mammals, in terms of the ratio between canal size and infant size. Wenda Trevathan, professor emeritus of anthropology at New Mexico State University, said:
“The dimensions of the infant head and shoulders are very close to and even exceed the dimensions of the mother’s birth canal in humans.”
She continues with:
“Labor contractions are probably painful for most mammals. But, I think it’s safe to say that the long labor required to birth a human baby is more painful and difficult than the apparently shorter labors of other mammals, including apes.”
Phillip Mitteroecker, assistant professor with the Department of Theoretical Biology at the University of Vienna, Austria, and colleagues present a mathematical model in the new study available in the journal, Proceedings of the National Academy of Sciences. They believe that data presented supports the idea of evolution favoring larger offspring as it would help the species survive. Mitteroecker said:
“Medical data show that larger newborns have higher survival rates and are less affected by several diseases.”
C-section rates have increased significantly, with many performed without medical necessity. Originally, Cesarean births were performed to save a child when a mother was dead or dying. Now, according to the U.S. National Institute of Child Health and Development, approximately one of every three births in the U.S. is by Cesarean. The new study does not necessarily support more C-section deliveries, as vaginal births may be beneficial for an infant’s immune system, as supported by previous studies. Trevathan said:
“The risks of this form of delivery in the absence of medical necessity are great for both mom and baby, especially when the child’s lifelong immune function is considered.”
Study authors estimate that the rise in C-section deliveries over a period of 60 years have increased the gap between newborn size and pelvic size, therefore affecting evolutionary changes in terms of delivery. Cases of C-sections delivered due to obstructed labor have risen from 30 out of 1,000 births to 33-36 out of every 1,000 births, if the mathematical model presented is accurate. This marks a 10 to 20 percent increase since the ’50s. C-section deliveries may have been an intervention that has intervened with human selection. Mitteroecker said:
“Without modern medical intervention such problems often were lethal and this is, from an evolutionary perspective, selection.”
However, it is difficult to determine if obstructed labor is on the rise or if there is any relation to C-section delivery. 60 years is nothing when it comes to evolutionary time spans. However, potential changes are interesting to note and for healthcare professionals to be aware of when working with pregnant women. Biological anthropologist Karen Rosenberg shared:
“It would never occur to me that Cesarean sections would not have an effect. The idea that human behavior affects our evolution is a central idea in understanding evolution. All kinds of things—when we cook our food, when we share food, when we build shelters—everything we do as cultural animals has the potential to affect our biology.”
What Evolutionary Changes Are on the Horizon?
It is not known what impact C-sections have on evolution. In addition, while obstructed deliveries are one reason for Cesareans, other factors may include medically necessary deliveries due to diabetes or obesity, obstetrics ward policies or malpractice concerns. The findings should be taken as an initial foray into the effects, if any, that C-section deliveries may have on the pelvis size and infant size of future generations. A considerable amount of additional research is necessary to corroborate any link between C-sections and evolutionary changes.
Physicians, case managers and healthcare professionals should be aware of new findings when it comes to healthy deliveries and optimal infant and mother outcomes. C-sections that are deemed medically necessary and promote the health of the parties involved are a useful and necessary option.