Researchers
in Denmark carried out a study between 1996 and 2002,
with the results being published in September 2007.
Using a questionnaire, they interviewed more than 92,000
women about their exercise patterns during pregnancy.
The women's responses were documented through computer
assisted telephone interviews.
The authors
of the study concluded that there appears to be an
association between exercise in early pregnancy and the
risk of subsequent miscarriage. They do highlight that
the results should be treated with caution as the method
they used to obtain the information could have led to
bias which then affected the study results.
The
telephone interviews were carried out using computer
assisted technology. The women were asked "Now that you
are pregnant do you engage in any kind of exercise?". If
they answered yes to this question, they were then
asked:
"What kind
of exercise do you engage in?"
"How many times a week do you engage in [this
exercise]?"
"How many minutes a time do you engage in [this
exercise]?"
"Do you engage in other kinds of exercise?"
If they
answered yes to the final question, they then looped
through the series of questions again to allow them to
answer for all types of exercise they did. The
researchers categorised the answers into six categories:
high impact exercise (jogging, ball games, and racket
sports), low impact exercise (aerobic
for pregnant women, aerobic, dance, and walking/hiking),
workout or fitness training, bike riding or horseback
riding, swimming, and other. When asking questions about
other factors for analysis of the results they included:
-
mother's
age
-
previous
miscarriages
-
education level & employment status
-
coffee
consumption during pregnancy
-
smoking
during pregnancy
-
amount
of physical exertion at work
-
history
of an eating disorder
-
body
mass index before pregnancy
-
history
of fertility treatment
-
history
of chronic disease
-
number
of pregnancies and previous live births
Miscarriage
was defined as the death of an infant before 22 weeks
gestation (not including terminations). They compared
the miscarriage rates of women who exercised with those
who did not. The findings indicate that the risk of
miscarriage triples in women who exercise strenuously
when compared with those who do not exercise.
There are
several issues worth considering before assuming that
exercise increases the risk of miscarriage.
Previous
history of exercise not taken into account
The
researchers do not appear to have asked the women
whether they had exercised prior to their pregnancy. We
are unable to determine whether strenuous exercise in
itself is a risk factor for miscarriage or whether it is
only a risk factor if the woman has exerted herself more
in pregnancy than she normally would.
Bias due to
self disclosure about smoking and coffee drinking
The women
were all asked to report whether they smoked or drank
coffee during pregnancy. Although the women were not
being interviewed face to face, the social judgments
made towards women who smoke during pregnancy could have
led women who did smoke to not disclose this
information. The same is true, although possibly to a
lesser degree, with coffee consumption since it is now
well known amongst pregnant women that drinking excess
coffee can impact the health of a developing baby and
may increase the miscarriage rate.
Assumptions
about exercise and health leading to bias
It is not
clear what the women were told the study was examining.
If they were told the study was looking at the links
between exercise and miscarriage, there is a possibility
that they assumed it was looking at whether exercise
DECREASED the risk of miscarriage. If this is the case,
women may have overstated how much exercise they did
during pregnancy. We would assume that women are more
likely to say they did more exercise than they really
did, rather than say they were sedentary, since
generally exercise is considered a beneficial activity.
Miscarriage
itself may affect recall about exercise patterns
Nearly 77%
of the women who miscarried were interviewed after the
miscarriage had taken place. The miscarriage itself may
have affected how the women recalled the amount of
exercise they did during pregnancy. Bearing in mind that
grief affects our recall of events, and guilt is often a
factor in miscarriage, it is possible that the women
overstated the amount of exercise they did.
Healthy
pregnancies may lead to more women stopping exercise
The authors
point out another source of bias. It is well documented
that women with healthy pregnancies are more likely to
experience nausea in the first trimester, when compared
with women who will subsequently miscarry. If women
experience nausea, they may be more likely to stop
strenuous exercise in early pregnancy because they feel
ill. This would mean that rather than the exercise being
a cause of the miscarriage, it was possible because the
woman was going to miscarry anyway.
Number of
minutes of exercise is different from intensity of
exercise
The authors
asked women in the interviews how many minutes of
exercise they did. However, they did not ask the women
what levels of intensity they used when exercising. Two
women doing an aerobic workout may do it at very
different levels. Walking and hiking were included in
low impact exercise for example, but there is a
significant difference between walking to pick the kids
up from school and hiking in the forest. Without
knowledge of intensity of exercise it is difficult to
draw any conclusions simply from the number of minutes
that a woman exercised.
Miscarriage rates are inconsistent with "typical"
miscarriage rates
In the study
only 3.4% of the women miscarried. This is significantly
lower than the number of pregnancies typically expected
to miscarry which is usually quoted as between 15-20%.
It is not clear as to why the miscarriage rate was so
low in this study population but this could have been a
confounding factor that impacted the results.
The authors
acknowledge the many areas of potential bias in the
study and state themselves that it is not possible to
draw any conclusions or make any recommendations based
on their findings. Interestingly though, many of
the news reports that followed the publishing of this
paper used headlines that appeared to draw conclusions.
Inquirer:
Exercise during
pregnancy can lead to miscarriage
The Australian: Heavy
exercise can up miscarriage risk
News (news.com.au): Exercise can cause miscarriage
The Guardian: Exercise can increase risk of miscarriage
It is worth thinking about how these headlines affect
women and how they behave during their pregnancy as a
result of reading them. The newspaper articles do not
evaluate the research or look at some of the
questionable areas. It is unlikely that the reporters
have even looked at the research itself. In most cases,
they receive a brief article from a news agency such as
Reuters, which is then replicated across the world in
each news publication.
Many news articles concluded with: But the findings
do argue in favor of a review of exercise guidelines for
pregnant women, the authors suggest.
What the researchers actually said in their conclusion
was: In spite of the findings of this study, we do,
however, think that it is too early to draw any public
health inferences on this basis. Many positive effects
of exercise are well established, and the findings of
this study need to be replicated.
You can read
the full text of the research here:
http://www.blackwell-synergy.com/doi/pdf/10.1111/j.1471
-0528.2007.01496.x
