Exercise in pregnancy Part 1: Update on pathophysiology

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Exercise in pregnancy
Part 1: Update on pathophysiology
Lisa Stevenson, MD, CCFP, DIP SPORTS MED (CASM)
OBJECTIVE To review the current data regarding the safety of exercise in pregnancy.
QUALITY OF EVIDENCE MEDLINE search was limited to English literature between 1987 and 1995
with the key MeSH words exercise and pregnancy. Other sources included the Sports and Fitness
Database between 1991 and 1995 and a manual search for relevant articles. Previous studies on the
safety of exercise in pregnancy are limited in many ways and exhibit difficulties in controlling for
confounding factors and in establishing associations while maintaining maternal and fetal safety.
MAIN FINDINGS Concerns about the effect of exercise on pregnancy include exercise-induced
hyperthermia, birth weight changes, fetal heart rate responses, miscarriage, labour patterns, maternal
injury, and maternal weight gain. Exercise during pregnancy also has many potential benefits.
CONCLUSIONS Important gaps in our knowledge of exercise and pregnancy remain. Available data
suggest, however, that moderate exercise on a regular basis during a healthy pregnancy has minimal
risk for women and their fetuses.
OBJECTIF Passer en revue les donnees actuelles concernant l'innocuite de l'exercice pendant la
grossesse.
QUALITE DES PREUVES La recherche dans MEDLINE s'est limitee 'a la documentation de langue
anglaise publiee entre 1987 et 1995 en utilisant le langage MESH et les mots cles <<exercise» et
<< pregnancy»>. Les autres sources consultees furent la base de donnees << Sports and Fitness >> couvrant
la periode de 1991 'a 1995 ainsi qu'une recension manuelle des articles pertinents. Les etudes
anterieures sur l'innocuite de l'exercice pendant la grossesse sont deficientes 'a plusieurs egards et
n'ont pas reussi 'a controler les facteurs de confusion et 'a etablir des associations entre l'exercice et la
securite de la mere et du foetus.
PRINCIPAUX RESULTATS Parmi les preoccupations entourant les effets de l'exercice sur la grossesse,
on retrouve notamment l'hyperthermie induite par l'exercice, les changements de poids 'a la naissance,
les variations du rythme cardiaque foetal, les fausses couches, les effets sur le travail, les blessures de
la mere et le gain de poids maternel. Pendant la grossesse, l'exercice comporte de nombreux
avantages potentiels.
CONCLUSIONS Nos connaissances entourant les effets de l'exercice sur la grossesse sont deficientes
et incompletes. Lorsque la grossesse se deroule normalement, les donnees disponibles indiquent
toutefois que l'exercice regulier d'intensite moderee presente un minimum de risques pour la mere et
son foetus.
Can Fam Physician 1997;43:97-104.
VOLL43: JANUARY * JANVIER 1997 + Canadian Family Physician Le Medecin defamille canadien 97-It- FOR PRESCRIBING INFORMATION SEE PAGE 131
CME
Exercise in pregnancy
omen of all ages are enjoying the benefits
of regular physical activity. Approximately
15% of women of reproductive age exer-
cise regularly.' Of these women, most
would like to continue exercising throughout
pregnancy.'
Several basic questions are, however, still unre-
solved. Can pregnant women begin or continue a vig-
orous exercise program during pregnancy safely?
Does exercise adversely or positively affect the out-
come of a pregnancy? Is an infant of an exercising
mother healthier or compromised? Is there some
optimal level of exercise? These questions have been
debated for many years and are the subject of much
research.
To answer these questions, the American College
of Obstetrics and Gynecology in 1985 published
guidelines for exercise in pregnancy.2 These guide-
lines lacked scientific support and were based mainly
on common sense. They tended to be conservative
and stressed exercise safety above all else, often frus-
trating fit and healthy pregnant women.3 A demand
for reliable, scientifically based, and flexible guide-
lines prompted the American College of Obstetrics
and Gynecology to update them in 1994.4 The new
guidelines are much more flexible, but still tend to be
conservative.
Several excellent review articles have been pub-
lished on this subject.',' This review is specifically
directed at family physicians who must deal with
practical questions and controversies daily. In this
first article of a two-part series, available data regard-
ing the safety of exercise in pregnancy and the limita-
tions of these studies will be reviewed. Concerns
about mothers and fetuses and the benefits of exer-
cise in pregnancy will be discussed. In the second
article, practical guidelines, based on current knowl-
edge, will be provided for prescribing and monitoring
an exercise program for pregnant women and their
babies.
Method
The method used to obtain the research papers cited
in these articles included a MEDLINE search limited
to the English language between 1987 and 1995 and
Dr Stevenson is a family physician practising obstetrics
with a special interest in sports medicine. She is past
Chair ofthe Canadian Academy ofSports Medicine's
(CASM) Womens Issues in Sports Medicine committee.
She is a primary care sports medicine consultant at Sport
Care, Women's College Hospital, Toronto.
the Sports and Fitness Database between 1991 and
1995. The key MeSH words were exercise and preg-
nancy. These sources produced several excellent
recent reviews on the subject, which enabled a fur-
ther manual search of the medical literature. I
attempted to include the most recent well-designed
studies examining safety of exercise in pregnancy. I
excluded case reports, research that examined the
effect of exercise on menstrual cycles and the exer-
cising women's ability to get pregnant, or research
that did not provide new data relevant to the subject
reviewed.
Limitations of previous studies
The difficulty with studying the effect of exercise on
pregnancy is that expectant mothers cannot be
exposed to exhaustion for fear of jeopardizing them-
selves and their fetuses. Also, it is very difficult to
separate the physiologic effects of pregnancy from
the effects of exercise. Other confounding variables
include genetic factors, socioeconomic factors, nutri-
tional factors, and environmental factors.9
Many studies do not describe their specific pre-
scriptions for type of exercise or intensity, duration,
and frequency of exercise.5 Often subjects' prior level
of conditioning is not considered. Most of the
research to date has been on healthy, nonsmoking,
white, previously fit women with low-risk pregnan-
cies.'0 There is little research on the effects of exer-
cise on women at risk for adverse pregnancy
outcome.'0 All of this makes it difficult to compare
one study with another or to apply the results to all
pregnant women.9 Often control subjects are not
used.9 Currently no good randomized, controlled
studies in the literature assess exercise in pregnancy.
We depend mostly on nonrandomized cohort and ret-
rospective studies for our recommendations.
Many of the concerns surrounding exercise in
pregnancy were raised from adverse outcomes noted
in animal studies. Results from human studies are
quite different. Various reasons are given to explain
these discrepancies. Species have different physio-
logic responses to exercise in pregnancy.6 As well,
many of the animal studies were performed on unfit
pregnant species pushed to exhaustion, while human
studies involved more physically fit pregnant women
exercising at more moderate levels."
Finally, a type of bias could be introduced. When
subjects are chosen for a study, any pregnant women
who develop complications are advised to stop the
exercise and are eliminated from the study. This
could distort the original data through an exercise
98 Canadian Family Physician Le Medecin defamille canadien + VOL43: JANUARY *JANVIER 1997
CME
Exercise in pregnancy
"bias."9 It is also important to notethat thus far few
studies have been done on human subjects, and they
often have insufficient numbers of patients for statis-
tical power. Because complications in pregnancy are
extremely rare, many subjects are required to detect
an abnormal pregnancy outcome."
Theoretical concerns and possible benefits
Several theoretical concerns surround the effects of
exercise on pregnancy. Each of these concerns, as
well as the available data that support or refute them,
will be examined.
Exercise-induced hyperthermia. A nonpregnant
runner can raise her rectal temperature above 390C
and even to as high as 400C on a hot, humid day.'
Pregnant animal studies show that increases in core
body temperature above 390C in the first trimester
can increase the incidence of neural tube defects.7"2
A recent review looked at this issue.'2 Analysts
found that most of the human studies were retro-
spective and looked at maternal heat exposure of
varying types (sauna, hot tub, and fever) and for
varying durations with different results.'2 Very few
have looked at maternal heat exposure induced
by exercise.'2
No prospective studies to date have found any
association between maternal temperature increased
by exercise and congenital malformations.'3",4 The
human data suggest that women allowed to self-pace
their level of exercise during pregnancy do not ele-
vate core body temperature sufficiently to harm a
fetus.7 Exercise seems to improve the body's ability
to dissipate heat, and this continues in the pregnant
state. In addition, pregnancy-induced plasma volume
expansion increases skin temperature and skin blood
flow, enhancing heat dissipation."1'4 Lacking sufficient
data to support or refute the risk of hyperthermia in
pregnancy, women should continue to be cautious,
particularly in the first trimester.
Exercise, blood flow redistribution, and birth
weight. Exercise leads to diversion of blood flow
away from the internal organs to exercising muscles
and skin to allow for heat dissipation. Splanchnic
blood flow can decrease up to 50% of resting values at
moderate exercise intensities and drop a further 30%
with prolonged high-intensity exercise.' Since the
uterine blood flow is part of the splanchnic circula-
tion, the concern is that during exercise blood, with
its supply of glucose and oxygen, will be shunted
away from the placenta and the developing fetus to
the exercising muscles. Ultimately the concern is
that fetal malnutrition will lead to a decrease in birth
weight and intrauterine growth retardation.
It has been technically difficult to measure the
uterine blood flow during exercise.7 However, most
studies have shown that birth weight is either
unchanged or slightly increased by exercise.91015,16
Hatch et all0 (Table 19"101519) found that previously fit
prenatal patients with no history of adverse pregnan-
cy outcomes (ie, miscarriage, intrauterine growth
retardation, preterm birth) who expended more than
4200 kJ (1000 kcal) weekly in recreational activity
had babies who were 276 g heavier than inactive
patients. If they were not conditioned or had a history
of adverse pregnancy outcome, however, the effect of
exercise on birth weight was either equivocal or neg-
ative, but these results were based on small
numbers.10 Sternfeld et al16 (Table 19,101`-9) found
that even the most vigorous exercisers showed no
difference in infant birth weight compared with
sedentary controls.
A few recent studies showed a different trend.'171820
A study by Clapp17 in 1990 (Table 19"10"19) showed
that a select group of mothers who exercised (run-
ning or aerobics) above 50% of their preconception
levels throughout pregnancy delivered infants that
were on average 400 g lighter than control subjects'
infants. Clapp and Capeless20 further explored neona-
tal morphometrics in another study and found the
effect was observed only if women continued exer-
cise beyond 28 weeks of pregnancy. If women
stopped exercising in the last trimester, they deliv-
ered infants that were slightly heavier at term (on
average 200 g). For women who exercised to term,
the explanation for the drop in weight was a 220-g dif-
ference in fetal fat mass. Head circumference and
crown-to-heel lengths were similar and all fetal out-
comes were good. The authors concluded that exer-
cise did restrict storage of fetal fat near the end of
pregnancy, but it had no effect on the other parame-
ters of fetal growth.20
Bell et al'8 (Table 1910,1i9) separated pregnant
subjects by their frequency of exercise sessions at
25 weeks and found a significant increase in fetal
birth weight among women exercising three times
weekly compared with sedentary women. Once
exercise increased to 5 sessions or more weekly,
however, fetal birth weight declined steadily.18
Differences in study populations, study designs,
and data analysis have been used to explain the dis-
crepancies between the studies.16 Currently we
VOL 43: JANUARY * JANVIER 1997 +Canadian Family Physician . Le Medecin defamille canadien 99
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CME
]Exercise in pregnancy
cannot assume that heavy exercise causes a
decline in birth weight; studies indicate that a ran-
domized trial is required to sort out this issue. We
must also determine whether a decline in infant
birth weight and fat storage is a detriment to fetal
well-being.
Exercise and fetal distress. Studies reveal con-
flicting data for this measure of fetal well-being. The
normal fetal heart rate ranges from 120 to 160 beats
per minute. The most common response to exercise
is a rise of approximately 10 beats per minute in
fetal heart rate during exercise, which returns to
baseline 10 to 20 minutes after exercise is over.5"5
The actual rise and fall depends on the intensity and
duration of maternal exercise.5"5 The cause for this
change in fetal heart rate is unknown, but it is
believed to be a protective response in the fetus.
The fetus increases its blood circulation in response
to a minor drop in the oxygen available via the pla-
centa because of shunting blood from the uterus to
exercising muscles."7
More worrisome are the reports of a few
patients with transient fetal bradycardia during and
immediately after exercise.7 This bradycardia could
represent a symptom of underlying uteroplacental
insufficiency.7 Some of these studies had potential
monitoring artifacts, however, and others involved
women who were untrained and exercised to
exhaustion for short periods.1"5'7 To date no
increase in mortality or morbidity has been linkedto exercise-induced bradycardia among women
with normal pregnancies.7,15 A recent study by
Webb et al'5 (Table 19Plo2"9) documented only one
incident of fetal bradycardia during exercise and
two during recovery from exercise. All of these
cases returned rapidly to the normal baseline and
none were associated with adverse fetal outcome.15
The researchers also found that the fetal heart rate
response was similar in conditioned and sedentary
pregnant women despite the fact that the condi-
tioned women were exercising at a significantly
higher work rate.15
Clapp17 (Table 1910o15-19) did not find any increase
in fetal distress during exercise and, in fact, demon-
strated a decrease in the incidence of meconium
staining, abnormal fetal heart rate patterns, cord
entanglement, and low Apgar scores in the exercising
groups. Sternfeld et al16 also found no difference in
the rate of these complications between exercising
pregnant women and pregnant women exercising
less than once weekly.
Exercise and miscarriage. There is no informa-
tion on the safety of beginning regular exercise in
the period just before conception or very early in the
first trimester. Only a few studies have looked at the
effect of continuing regular exercise in the first
trimester. One author looked at two runners who
were unknowingly pregnant and continued to train
vigorously, running up to 100 km weekly until the
18th to 20th week of pregnancy, and both runners
delivered healthy babies.2' A prospective study of
158 previously fit women who continued to exercise
at a level above current guidelines during their preg-
nancies found no significant difference in the rate of
spontaneous abortion, congenital abnormalities, or
implantantion problems.22
Exercise and labour. During acute exercise, output
of catecholamines increases, particularly of noradren-
aline, which could stimulate uterine activity.5 Is the
risk of preterm delivery increased among exercising
pregnant women? To date no studies have shown an
increased risk of preterm labour or an increased inci-
dence of premature rupture of membranes among
exercising pregnant women not already at risk for
these conditions.9"6"7 Clapp's study'7 of vigorously
exercising women (Table 19,10,15-19) suggested that
exercising mothers deliver at term significantly earli-
er (5 days on average) if they continue to exercise
throughout pregnancy; 75% of exercising mothers
delivered before or on their due date versus 50% of
control subjects.
It has also been suggested that less medical inter-
vention, for example oxytocin, forceps, and cesarean
section, is required for women who exercised
throughout pregnancy.'6"7 Most studies suggest that
exercise has no effect on the length of labour.9"5
Clapp'7 found that the active stage of labour (4-cm
dilation to delivery) was 30% shorter on average in
the exercising group and that second stage arrest
decreased significantly. Anecdotally some physicians
find that the strong perineal musculature of an ath-
lete can prolong second-stage labour. There is insuffi-
cient research in this area to comment, as most
studies do not separate the length of the first and sec-
ond stages of labour.9
Exercise and maternal injury. Pregnant women
undergo profound changes in structure, metabo-
lism, cardiovascular function, and endocrine func-
tion. Posture changes with an increase in lumbar
lordosis and a forward tilt of the pelvis.23 The centre
of gravity shifts forward and a weight gain of 10 to
VOL43: JANUARYe JANVIER 1997. Canadian Family Physician . Le Mdecin defamille canadien 101
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Erenea*
Exercise in pregnancy
16 kg is common. All of these physiologic changes
can lead to an increase in low back pain and make
walking, turning, lifting, and stooping more
difficult.23 Blood volume also increases significantly
and basal metabolic rate, oxygen consumption, and
sensation of dyspnea increase.23 All these changes
can alter the balance, coordination, and exercise tol-
erance of pregnant women.
Impact exercise, involving tremendous torque
and shear forces on a large mobile pregnant
uterus, could cause injury to the mother and the
fetus. Therefore, it has been recommended that
pregnant women limit exercise requiring sudden
changes in body position and highly coordinated
body movements.2'4 Injury rates, however, are not
reported to increase when exercising in
pregnancy.' The incidence of musculoskeletal com-
plaints has actually decreased among women exer-
cising during pregnancy.5"6 The reason could be
that women are more cautious and controlled when
exercising in pregnancy. In late pregnancy some
women complain of lower abdominal discomfort,
but this can usually be relieved with lower abdomi-
nal support during exercise.
As pregnancy progresses, the hormonal levels
that aid in the relaxation and mobilization of the
pelvic, sacroiliac, and sacrococcygeal joints increase
along with the increase in weight.23 This hormonal
change can make pregnant women more vulnerable
to joint injury. To date there are no specific reports of
exercise-associated injuries to support this concern,
but until more data are available, it is probably pru-
dent to advise care and caution.24
Exercise and maternal fitness. It is very difficult
to assess maternal fitness in pregnancy. Maximal
oxygen consumption tests (VO2m.) to determine aer-
obic fitness in pregnant women are rarely performed
due to safety concerns for both mother and fetus.
Submaximal testing is often used and extrapolated to
estimate peak VO2ma values.5
The effect of exercise training during pregnancy
on aerobic capacity has been investigated in both
cross-sectional and prospective trials. At this time, it is
unclear whether pregnancy alone has a negative or a
positive effect on aerobic capacity. Changes in blood
volume and cardiac output are natural consequences
of pregnancy and are reflected in maintenance or ele-
vation of absolute VO2max (L/min) measures.
However, an overall decrease in exercise performance
is observed. This decline in exercise performance is
accompanied by a drop in relative VO2ma (mL/kg of
body weight every minute) due to the progressive
increase in body weight during gestation.525
Routine exercise programs of moderate intensity
appear consistently to achieve a training effect just
as they do in nonpregnant women. Pregnant exercis-
ers have a lower heart rate response to a given sub-
maximal workload and demonstrate improved
physical work capacity compared with sedentary
8,15pregnant women.'
Exercise and maternal weight gain. Most stud-
ies show that fat accumulation and total weight gain
in pregnant exercising women increases as expected
and is not decreased by physical conditioning.9'10'16 A
study by Clapp and Little (Table 19"1015-9) showed a
different trend.19 This study looked at 79 recreational
athletes who exercised at basic conditioning levels
for health and recreation. Their total weight gain was
significantly lower than pregnant women who did not
exercise: 13 kg versus 16 kg (both still within the nor-
mal range). The rate of weight gain was significantly
lower in the exercising group after the 15th week of
pregnancy. Clapp and Little also found that five site
skin-fold thicknesses were significantly decreased in
the exercising group after the 30th week of pregnan-
cy. All the women delivered healthy infants.19
Sternfeld et al'6 reported no difference in maternal
weight gain even between the heaviest exercisers
and the sedentary controls. They did not address
body fat measurements.'6
Possible benefits of exercise in pregnancy. Further
reported benefits, although with fewer supporting data,
suggest that exercise decreases the incidence of
depression and anxiety and increases self-esteem in
pregnant women.5"' Exercise also could give women a
sense of control over their bodies in a situation where
they have reducedcontrol. Many women subjectively
feel that their labour is easier and that they recuperate
more quickly postpartum.5 Exercise could also be an
alternative and safe therapeutic approach to prevention
and treatment of gestational diabetes.526 Active women
report fewer physical symptoms of pregnancy, ie, nau-
sea, heartburn, leg cramps, insomnia.'6 It is relatively
certain that moderate regular exercise during an other-
wise healthy pregnancy is likely to improve well-being
when compared with a sedentary lifestyle.'6
Conclusion
Current data regarding the safety of exercise in preg-
nancy were summarized in a recent meta-analysis
by Lokey et al9 (Table 191915019). They showed no
102 Canadian Family Physician . Le Medecin defamille canadien * VOL43: JANUARY * JANVIER 1997
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Exercise in pregnancy
significant difference in outcome variables between
exercising and control pregnant females. No differ-
ence in length of gestation, maternal weight gain,
length of labour, infant birth weight, or Apgar scores
appeared.9 This meta-analysis also looked at a group
of exercising women who exceeded the American
College of Obstetrics and Gynecology's guideline rec-
ommending an exercise frequency of 3 or more days
weekly, at an intensity of < 140 beats per minute, for a
duration of < 15 minutes with no bouncy or jerky
movements.2
Women who exceeded these guidelines exercised
an average of 3 to 6 days weekly, at an intensity of 140
to 152 beats per minute for 30 to 60 minutes; the
mode of exercise included jogging, biking, swim-
ming, and weight lifting. Again, no difference in the
outcome variables was measured. The authors cau-
tiously concluded that a moderate level of training did
not show any adverse effects on mothers or fetuses
during pregnancy, although they suggested more
research was needed to be conclusive.9
Important gaps remain in our knowledge of exer-
cise and pregnancy. Current data suggest that moder-
ate regular exercise during a low-risk pregnancy has
minimal risk for fetuses and beneficial effects for
exercising mothers-to-be. The previous guidelines
seem increasingly too restrictive for previously fit
and healthy exercising women. There is likely an
optimal and safe level of exercise, but this level is still
not clearly established. Future research must clarify
this safe limit by looking at influencing factors, such
as prior conditioning, gestation stage, activity type
and intensity, and maternal weight and caloric intake.
We must also look more closely at the influence of
exercise on the complications of pregnancy. At this
stage it seems reasonable that a large prospective,
randomized trial of low-risk prenatal patients be car-
ried out considering the positive effects of exercise in
pregnancy identified thus far. 4
Acknowledgment
I thank Dr Frankie Fraulin for his invaluable help in the
preparation ofthis manuscript.
Correspondence to: Dr Lisa Stevenson, Sport Care,
Women's College Hospital, 76 Grenville St, Toronto, ON
M5S 1B2; telephone 1-800-363-9353
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104 Canadian Family Physician Le Medecin defamille canadien * VOL 43: JANUARY * JANVIER 1997
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