J R Soc Med 2003;96:559-561
doi:10.1258/jrsm.96.11.559
© 2003 Royal Society of Medicine
Rising caesarean section rates: can evolution and ecology explain some of the difficulties of modern childbirth?
W A Liston FRCOG
Department of Obstetrics, Simpson Centre for Reproductive Health, Royal
Infirmary of Edinburgh—Little France, 51Little France Crescent,
Edinburgh EH16 4SA, Scotland, UK
 |
INTRODUCTION
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|---|
There is concern in many countries about rising rates of caesarean
section.
In England and Wales, for example, the figure is about
22%,
1 whereas the
World Health Organization favours 10-15%. Most comments
on the rising trend,
whether from the medical profession or
from the public, express disapproval,
an example being the report
of a Parliamentary Select Committee. In response,
obstetricians
in the UK have set up studies such as the National Sentinel
Audit
(England and
Wales)
1 and the
National Audit
(Scotland).
2 In
Canada
the matter is being addressed by the Ontario Women's Health
Project.
3 These
bodies note that in primary emergency caesarean section
(i.e. caesarean
section in primigravid women in labour) there
are two main
indications—failure to progress in labour
and fetal distress. To counter
these they advocate generous
use of oxytocin in the failure-to-progress group
(a strategy
that can precipitate fetal distress) and careful evaluation
of the
fetus via scalp pH in the fetal-distress group. But such
exhortations have had
scant effect on the caesarean section
rate. Further, when a caesarean section
has been done for the
first baby, there is a tendency to deliver subsequent
babies
in the same way; thus, an increase in the primary caesarean
section
rate has a multiplier effect on the whole rate. Why
is it that modern human
childbirth is so frequently associated
with difficulty? Only occasionally has
anyone attempted to explain
this.
4 While the
proximate causes for primary caesarean section are
failure to progress and
fetal distress, the ultimate causes
may lie in changes in human ecology.
|
EVOLUTION
|
|---|
Looking around in nature we see that natural selection is a
brilliant
designer. Richard
Dawkins
5 cites the
bat and its echolocation
device as an example. Every living organism displays
evidence
of the complexity and inventiveness of natural selection. Yet,
when
it comes to ‘natural childbirth’ modern
Homo sapiens
performs poorly—despite the mother's investment
of huge resources during
the nine months of gestation. The explanation
is that cultural evolution has
outstripped biological evolution.
The species H. sapiens evolved as a large-brained omnivorous
primate hunter-gatherer in Africa. Its ancestors were hominid and
australopithecans living in Africa who had developed over several million
years from earlier primates. Modern H. sapiens emerged in Africa less
than 100 000 years
ago,6 and through a
combination of cleverness, curiosity and greed spread through all the
habitable earth. The large brain contributed to this success but made the
process of birth more critical. There is a much tighter fit than in hominid
and pongid ancestors between the human infant head and the mother's
pelvis.
For most of its history H. sapiens lived in its own ecological
niche, that of the hunter-gatherer. The total world population was small,
possibly 10 000 000. The species lived in scattered bands containing 100-150
individuals, nomadic and consuming the edible animals, fruit and vegetables
that they encountered. A large amount of exercise was taken to obtain food. A
woman's reproductive career began shortly after menarche at the age of
17-18.
All that changed with the invention of agriculture about 10 000 years
ago.7 With huge
increases in population and later industrialization the life of modern woman
and man bears little relation to that of the hunter-gatherer. Because
biological evolution cannot keep pace, man is a hunter-gatherer living in a
21st century world. Admittedly, where selection pressures have been very
strong (e.g. malaria and the haemoglobinopathies) there have been genetic
changes, but the species retains much of the physiology of pre-agricultural
times. Whereas hunter-gatherers went through tens of thousands of generations
there have been only 500 generations of agriculturalists and just a few in the
industrial era. Physicians and nutritionists have therefore proposed that
certain modern diseases, particularly heart disease and type 2 diabetes, are
caused by a maladaption to our current lifestyle. Similar arguments can be
applied to reproductive health and obstetric performance.
|
CHANGES IN HUMAN ECOLOGY
|
|---|
There are four chief ways in which this misfit between biology
and
lifestyle could affect childbirth—diet, population
density, exercise and
reproductive behaviour. The diet in palaeolithic
times was by most accounts
richer in protein and poorer in carbohydrate,
with a different pattern of
fats.
8,9
It was also very varied.
In particular the carbohydrate component had little
refined
starch and sugar with much more fibre. The agriculturalists
then moved
to a diet with less protein and fat, and more complex
carbohydrate. The modern
western diet contains a super-abundance
of food, especially sugar and fat with
less protein than that
of early upper palaeolithic man. In poorer parts of the
world
where protein is scarce, food consists largely of complex carbohydrate,
but
western tendencies and fast food are spreading to all parts
of the
globe.
8,9
What of population density? The advent of agriculture enabled people to
live in villages and cities rather than small settlements, and gave rise to a
great increase in population density. In consequence, infectious diseases,
especially droplet infection and waterborne disease, became very common
particularly in infancy and childhood. Conditions became even worse in the
slums of the industrial revolution, where desperate living conditions were
compounded by deficiency
diseases.10 Such
conditions are still encountered in mega-cities of the poor world.
What is not widely known is that the invention of agriculture and the
development of settled living had pronounced affects on physical stature.
Study of skeletons points to adverse changes in the
teeth11 and a
general reduction of
height.8,12-14
Angel15 has charted
the patterns over thousands of years. Humans were tall in early upper
palaeolithic times and did not become as tall again until the late 20th
century in Western Europe and the USA. The rich were always taller than the
poor. Better nutrition and living conditions were associated with some
increases in stature, but it is only now in prosperous parts of the globe,
with adequate children's nutrition and control of infectious diseases by
sanitation and vaccination, that humans are again reaching their full
potential height. That this process can take several generations is
illustrated by data in immigrants to the United
States.16
With growth in stature goes growth in pelvic size. Short stature has long
been known as a risk factor for difficult labour; indeed, the first regular
use of caesarean delivery was in short women with contracted
pelves.17
Baird18 emphasized
this in Aberdeen in the 1950s, and numerous studies have confirmed it. For
example, in Western
Australia19
primigravid women with height less than 160 cm had a rate of caesarean section
four times that in women over 164 cm. Natural selection designed women to be
tall, with good pelvic capacity to allow delivery. Though many women are now
tall, large numbers even in prosperous areas, have not grown to their full
genetic potential.
Another change noticeable in primiparous women is a rising prevalence of
obesity. This reflects a worldwide
trend20 which
results from a combination of lack of exercise and a diet high in refined
carbohydrates and fat. Obesity predisposes to difficult labour. A woman with a
body mass index > 30 has nearly three times the risk of caesarean section
of a woman with BMI <
20.4,21
Hunter-gatherers, from such evidence as we have in remnant populations, were
largely slim and physically
active.22
Why obesity has a bad influence on childbirth is not clear. The most
plausible explanation is that obese women tend to have large babies: the
maternal pre-pregnancy weight is the best marker for fetal
weight.23 Fat women
have bigger babies either because the baby is proportionate to their size or
because both birthweight and maternal weight result from maternal diet or
components of that diet (e.g. proportions of carbohydrate and
protein).24 Data
from
Scotland25,26
and China27 point
to an increase in fetal macrosomia, and it seems that babies are getting
bigger as women are getting fatter. Bigger babies undoubtedly have higher
caesarean section
rates.19,28
In communities where the babies are small, the caesarean section rate is
low.29
The final important change in human ecology is maternal age at first
delivery. Hunter-gatherers had their first child when aged 17 or 18. Culture,
education, contraception and abortion have allowed women to delay first
childbirth. Wherever we look in the western world, women are putting off
having their first
baby.30-32
This has biological disadvantages, with more infertility, more miscarriages
and more caesarean sections. From
Baird's18 work
onwards, many studies point to an increasing incidence of caesarean section
with maternal age. Why age has this effect is not obvious. Perhaps labour is
made more difficult by changes in the connective tissues and the collagen in
the uterus and
cervix.33 Whereas
from a cultural, economic and educational viewpoint there are great advantages
in delaying first childbirth, from a biological standpoint increasing maternal
age is a disadvantage.
|
CONCLUSION
|
|---|
Changes in diet, population density, exercise and reproductive
behaviour
mean that primigravid women are commonly shorter,
older and fatter than is
ideal for first childbirth. These adverse
factors have been well recorded: a
paper by Cnattinguis
et
al.4 was
entitled ‘Obstacles to reducing cesarean rates in
a low cesarean
setting: the effect of maternal age, weight,
height’; Read
et
al. showed that primigravidae aged under
20 and over 164 cm in height had
an emergency caesarean rate
of 2.0% while those over 35 and less than 160 cm
had a rate
of
30.8%.
19 Yet, some
audits, including the National Sentinel
Audit, have ignored these factors and
their importance is not
widely appreciated by either the medical professions
or the
general public. The lowest caesarean rates in the western world
are in
the Nordic
countries
1 and in
the Netherlands, where women
are tall and there is little disparity between
health and nutrition
among different sectors of society.
When these anthropomorphic factors are added to influences such as maternal
preference we cannot realistically expect rates of caesarean section to
decline greatly if at all. But is this such a bad thing? The morbidity from
elective caesarean section is extremely
low,34 though
higher than that from straightforward vaginal delivery. There may even be
long-term benefits for women's health, such as less pelvic floor damage. The
risk to the baby is also less than in primigravid
labour.35 The main
advantage of achieving a vaginal delivery of a first baby is that subsequent
vaginal deliveries will be easier for the mother and less risky for the
baby.35 The
financial costs may or may not be
greater.34 What is
clear is that the caesarean sections with the greatest morbidity and mortality
for mother and baby are emergency sections after prolonged labour. It is these
emergency operations that need to be avoided by better methods of
prediction.
There is a widespread perception that the increasing caesarean section rate
is driven by obstetricians. This is only partly true, and critics of the
profession should acknowledge the strong relevance of age and obesity, and the
positive aspects of the operation. For the world in general the best public
health policy is to feed and look after children properly, particularly the
girls.
|
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INTRODUCTION
EVOLUTION
