According to a new study accepted for publication in The Endocrine Society's Journal of Clinical Endocrinology & Metabolism (JCEM), regular moderate-intensity aerobic exercise led to a modest reduction
    in offspring birth weight without restricting the development of
    maternal insulin resistance.


    There is a large body of
    evidence demonstrating the influence of the in utero environment on
    growth trajectory in postnatal life. Increased size at birth is
    associated with greater risk for the development of obesity in
    childhood. This study is the first to demonstrate a significant effect
    of non-weight bearing exercise (such as stationary cycling) on birth
    weight.


    "Our findings show that regular aerobic exercise
    alters the maternal environment in some way that has an impact on
    nutrient stimulation of fetal growth, resulting in a reduction in
    offspring birth weight," said Paul Hofman, MD, of the University of
    Auckland in New Zealand and co-author of the study. "Given that large
    birth size is associated with an increased risk of obesity, a modest
    reduction in birth weight may have long-term health benefits for
    offspring by lowering this risk in later life."


    This study is
    also the first to evaluate changes in insulin sensitivity in response
    to aerobic exercise training during pregnancy. Maternal insulin
    resistance is essential in increasing nutrient availability to the
    fetus and has been correlated with birth size. Exercise has been shown
    to reduce insulin resistance but a major reduction in insulin
    resistance may adversely affect a pregnancy by severely restricting
    fetal nutrition. However, findings from this study suggest that regular
    exercise during pregnancy does not cause the same reduction in insulin
    resistance that occurs in exercising non-pregnant individuals.


    "The
    physiological response to pregnancy appears to supersede the chronic
    improvements in insulin sensitivity previously described in response to
    exercise training in non-pregnant individuals," said Hofman. "This may
    be an important finding for athletes who want to continue regular
    training during their pregnancy as it suggests that training will not
    have a major adverse impact on insulin resistance."


    In this
    randomized trial, researchers assigned 84 first-time mothers to either
    exercise or control groups. Participants in the exercise group utilized
    stationary cycling and were individually prescribed to a maximum of
    five sessions of 40 minutes of aerobic exercise per week. The exercise
    group was instructed to maintain the exercise program until at least 36
    weeks gestation. Insulin sensitivity was assessed at 19 and 34-36 weeks
    gestation using an intravenous glucose tolerance test. Birth weight and
    BMI at birth were measured within 48 hours of birth.


    Exercise
    training had no effect on maternal body weight or BMI during late
    pregnancy. Furthermore, exercise had no effect on insulin resistance
    from baseline to late gestation, and did not affect any other
    parameters of glucose regulation. Offspring of exercisers were on
    average 143 â± 94 grams lighter than their control counterparts, however
    there was no difference in birth length. Exercise training also
    resulted in lower offspring BMI.


    Other
    researchers working on the study include: Sarah Hopkins, Wayne Cutfield
    and Lesley McCowan of The University of Auckland in New Zealand; and
    James Baldi of Northern Arizona University in Flagstaff, Ariz. The
    article, "Exercise Training in Pregnancy Reduces Offspring Size without
    Changes in Maternal Insulin Sensitivity," will appear in the May 2010
    issue of JCEM.


    Founded in 1916, The Endocrine
    Society is the world's oldest, largest and most active organization
    devoted to research on hormones and the clinical practice of
    endocrinology. Today, The Endocrine Society's membership consists of
    over 14,000 scientists, physicians, educators, nurses and students in
    more than 100 countries. Society members represent all basic, applied,
    and clinical interests in endocrinology. The Endocrine Society is based
    in Chevy Chase, Maryland. To learn more about the Society and the field
    of endocrinology, visitwww.endo-society.org.