The influence of exercise during pregnancy and its effect on birth weight and pregnancy outcome remains uncertain. The most recent technical bulletin from the ACOG1 suggests that lower birth weights are observed among vigorously exercising women, but there is no information linking exercise with an adverse outcome of the fetus. A number of studies have evaluated the influence of exercise during pregnancy and have found that the birth weights of the fetuses of exercising women are lower,2–4 the same weight as,5–7 or greater8 than the fetuses of women who are sedentary or not exercising as much as the studied women.
The effect of maternal exercise on prenatal complications is also unknown. A recent review from the Cochchrane Database9 was unable to demonstrate important benefits or risks to the mother or fetus with exercise in pregnancy. Two studies have reported correlations between endurance exercise and pregnancy outcome.2,3 In the initial study,2 Clapp and Dickstein reported that women who continued endurance exercise gained less weight and delivered earlier than women who stopped exercising before 28 weeks. In the follow-up study,3 women who continued to exercise in pregnancy had a lower incidence of operative abdominal and vaginal deliveries, shorter active labors, and fewer fetuses with acute fetal distress in labor than women who discontinued their exercise.
Small patient numbers or failure to take into consideration the confounding variables in larger samples hampers the studies assessing exercise with birth weight and pregnancy outcome. Because the labor force is composed of more working women, many who exercise at conception and continue during pregnancy, the impact of exercise on pregnancy outcome in working women is very important. The purpose of this investigation was to evaluate the influence of exercise, by level of activity, on maternal and perinatal outcome in a large low-risk healthy obstetric population of working women cared for by a single group of health care providers in one medical center.
MATERIALS AND METHODS
Healthy, low-risk, obstetric patients attending the prenatal clinic at the Naval Medical Center, San Diego, California were eligible to participate in this prospective observational investigation. Only active-duty women were recruited because they had all been screened for medical illness before enlistment in the military. They underwent mandatory periodic physical examinations, and if they developed significant illness or disabilities and could not perform the duties required of active-duty service persons, they were reevaluated upon completion of the pregnancy to determine if they could remain in the service. This insured that all studied patients would be healthy and low risk. Nearly all the active-duty women were primarily working in a supportive role to the “Combat Arms” of the military. Their military occupations included nursing, legal, religious, clerical, financing, and administrative job descriptions. These occupations made certain that the working environments were similar between groups and that the impact of exercise on preliminary outcome in working women could be evaluated. Other inclusion criteria were that the first and all prenatal visits along with the delivery took place at the Naval Medical Center in San Diego. Those excluded from the study were women not desiring to participate and women transferred from San Diego or who did not obtain all their prenatal care at the Naval Medical Center in San Diego. The Investigational Review Board at the Naval Medical Center in San Diego approved this study.
An extensive questionnaire, based on the Raine study questionnaire10 and modified for our patient population, was used to collect antepartum, intrapartum, and post-partum information on each patient. Information was collected on the first visit, during subsequent visits, and immediately after delivery on each participant. The women were divided into four groups based on their level of exercise during pregnancy. The types of exercise were defined for this investigation. Mandatory exercise was defined as participation in the regularly scheduled physical training of the Navy lasting at least 30 minutes for a minimum of three times per week. Voluntary exercise was defined as noncompulsory physical training for at least 30 minutes, three times per week. Aerobic exercise was defined as repeated exercise that depletes cellular oxygen and places the body in a state of stress. That level of exercise that needs to be attained is determined by the maternal heart rate. The simple formula is: (220–age) × 60–80% = target heart range.11
Group 1 (no exercise) women did no voluntary exercise, no mandatory exercise during the pregnancy, voluntary exercise that was not aerobic, or mandatory exercise less than three times per week, and stopped before 20 weeks' gestation. Group 2 (light exercise) women voluntarily exercised aerobically stopping at less than 20 weeks' gestation, did mandatory exercise less than three times per week, but stopped at less than 28 weeks, or exercised five times per week, but stopped at less than 20 weeks. Group 3 (moderate exercise) women voluntarily exercised aerobically, but stopped at less than 28 weeks, did mandatory exercise three times per week stopping after 28 weeks, or mandatory exercise five times per week stopping at less than 28 weeks. Group 4 (heavy exercise) women voluntarily exercised at more than 28 weeks, did mandatory exercise stopping after 28 weeks, or did not stop mandatory exercise during the pregnancy.
Women were evaluated for the confounding variables of maternal age, race, gravidity, parity, maternal illness, education, income after taxes, marital status, height, prepregnancy weight, weight gain during the pregnancy, prior preterm delivery, smoking, social support, and stress. The outcome variables assessed were the risk of anemia, preterm labor, preterm birth, preterm premature rupture of membranes, induction of labor, length of labor, abnormal fetal heart rate tracing during labor, amnioinfusion, meconium-stained amniotic fluid, mode of delivery, postpartum hemorrhage, birth weight, and intrauterine growth restriction. Specific outcome variables for this study were defined. Preterm labor was defined as regular uterine contractions with observed cervical dilatation, effacement, or descent of the presenting part before 36 completed weeks' gestation. The first stage of labor was defined as the time from the onset of labor to complete cervical dilatation. Intrauterine growth restriction was defined as neonatal weight below the 10th percentile by weight for gestational age. Decreased amniotic fluid volume was defined as an amniotic fluid index less than or equal to 5. Postdatism was defined as a pregnancy more than 42 weeks by reliable dating criteria. Prolonged second stage of labor was defined as labor exceeding 1 hour after complete cervical dilatation in a parous patient and more than 2 hours in a nulliparous patient. When an epidural was used, then 2-and 3-hour definitions were used for parous and nulliparous patients, respectively. Failure to progress was defined as a patient in the active stage of labor (4 cm) who had no further dilatation of the cervix or descent of the presenting part despite adequate uterine activity over a period of 2 hours. Meconium-stained amniotic fluid was defined as the presence of meconium in amniotic fluid and was further defined as thin or thick. Nearly all (680 of 750, 90%) delivery information was obtained from this medical record before maternal hospital discharge. All the delivery information was obtained within 1 week of maternal hospital discharge.
A convenience sample of 750 women was chosen for the study based on the results of our previous study.12 In that study, we observed that neonatal birth weight was influenced by daily kilocalorie expenditure. Univariate comparison among groups was made using the F test or χ2 test as appropriate for numeric or categoric outcomes, respectively. Multivariable comparisons were examined using analysis of variance techniques or logistic regression as appropriate controlling for the confounding variables listed above.
Between January 1995 and January 1998, 772 women were enrolled into this investigation. Twenty-two women transferred from San Diego to other military commands during their pregnancy and were not in the analysis, leaving 750 evaluated women. Any military woman developing a complication during the pregnancy, which would disqualify her from the active-duty military, was medically evaluated upon completion of the pregnancy. If found to be unqualified for military service, she would receive a service-connected disability and be discharged from the service. None of the 772 participating women were discharged during the pregnancy. By level of exercise, 217 women (28.9%) were categorized into group 1 (no exercise), 222 (29.6%) into group 2 (light exercise), 73 (9.7%) into group 3 (moderate exercise), and 238 (31.7%) into group 4 (heavy exercise). There were no differences among groups in the maternal demographics for height, prepregnancy weight, gravidity, parity, first- or second-trimester abortions, antenatal maternal illnesses, smoking, stress, and family support. Group 4 women were older (P = .042), were exercising more at conception (P = .001), had higher incomes (P = .001), and continued to work full time longer in pregnancy (P = .006, Table 1). Active-duty women composed approximately 15% of the annual deliveries during the 3 years of the study.
The influence of exercise on weight gain in pregnancy, pregnancy loss (before 20 weeks, after 20 weeks, and neonatal death), the development of pregnancy-induced hypertension, gestational diabetes, anemia, and antenatal days hospitalized were similar between groups. The effect of exercise on pregnancy losses with exercise (P = .232) was not different among the groups. The number of antenatal colds and flu were increased in the groups as they exercised more in pregnancy (Table 2).
The influence of exercise on the labor and delivery outcomes of preterm labor, gestational age at the onset of preterm labor, preterm delivery, preterm premature rupture of membranes, onset of labor, reason for labor induction, length of second and third stages of labor, abnormal fetal heart rate tracing and its influence on delivery, mode of delivery, reason for operative delivery, trauma at vaginal delivery, meconium-stained amniotic fluid, amnioinfusion, and postpartum hemorrhage were similar between groups (Tables 3 and 4). Likewise, the influence of exercise on the neonatal outcomes of the gestational age at delivery and the risk of intrauterine growth restriction were similar between groups (Table 4). Women who did more exercise were more likely to need an induction of labor (P = .033, relative risk 1.84, 95% confidence interval 1.05, 3.20), induction or augmentation of labor with oxytocin (P = .015, relative risk 1.53, 95% confidence interval 1.19, 1.97), and had longer first-stage labors (P = .032) resulting in longer total labors (P = .011). The difference in the length of the first stage of labor was even greater if the no-exercise group was compared with the strongly exercising women (P = .009, relative risk 1.38, 95% confidence interval 0.16, 2.60). A trend toward fewer abnormalities of the fetal umbilical cord (fetal cord abnormalities were defined as true knots in the cord, velementous insertion of the cord, abnormally short cords, and nuchal cords, P = .051) was observed with increasing exercise. Although this effect of exercise on cord abnormalities did not reach significance, a comparison of the women who did no exercise with the most vigorously exercising women demonstrated a significant difference in the risk of cord abnormalities with the exercising women having fewer abnormalities (P = .034). After controlling for the effect on birth weight of gestational age, sex of the infant, parity, race, prepregnancy weight, height and pregnancy weight gain of the mother, maternal smoking, and diabetes, there was a significant contribution of exercise on neonatal birth weight. Compared with the infants of mothers who did no exercise, infants of the most strenuously exercising mothers were 86.5 g (standard error 43.7, P = .048) lighter at birth. There was a trend for this to be apparent in the women who exercised lightly also (group 2, P = .068, Table 4).
Influences of exercise, stress, and occupation on pregnancy outcome are difficult to examine because of confounding variables not taken into account in large investigations and the lack of an adequate sample size in smaller studies.13 Active-duty women present a unique group for examination because all have been screened for medical illnesses before their enlistment into the service. Also, these women undergo periodic examinations, and any women with serious illness are discharged from the service. They are also required to participate in mandatory exercise programs and to successfully pass a physical readiness test twice a year. The identification of a low-risk cohort of women without medical illness who are physically fit permits the assessment of individual variables with minimal influence of confounding variables.
Early pregnancy loss is worrisome in exercising women. Not all are planned, and the woman may be 4–6 weeks pregnant before she realizes she is pregnant and has the pregnancy confirmed. Early pregnancy loss was investigated in a study by Clapp,14 and he observed that the risk of first-trimester pregnancy loss was not altered by activity. We confirmed his findings that pregnancy loss is not correlated with the level of exercise.
A surprising finding in our low-risk working women was the increased risk of developing flu and colds with more exercise. This observation is unexplained and has not been previously reported. Reduced immunity has been observed in strenuously exercising athletes, and this may explain this observation. However, this finding needs to be evaluated in future studies.15 With increasing numbers of women strenuously exercising in pregnancy, the effect on pregnancy outcome is important. Two worrisome antenatal complications of pregnancy are intrauterine growth restriction and preterm labor. Strenuous exercise in pregnancy has been assessed and is not harmful to the mother or the fetus16 in healthy women if it is of limited duration. Heavy work or exercise also does not appear to increase the risk of a preterm delivery or intrauterine growth restriction17 in women at low risk for an adverse pregnancy outcome. Our findings are consistent with these investigations. We observed no increased risk of preterm labor or intrauterine growth restriction. Because our patient population included only low-risk working women, these results cannot be extrapolated to other groups of working women and may not apply to women with high-risk pregnancies.
The course of labor and pregnancy outcomes has also been evaluated in exercising women. In a study of 131 well-conditioned women, Clapp3 discovered that continuation of exercise in pregnancy had beneficial effects compared with women who discontinued exercise in pregnancy. He observed earlier labors, lower incidence of operative abdominal and vaginal deliveries, and acute fetal distress (meconium, fetal heart pattern, and Apgar scores). Minor discomforts of pregnancy including leg cramps, swelling, fatigue, and shortness of breath were reported as being decreased in women who engaged in exercise during the last trimester of pregnancy.18 On the contrary, Kardel and Kase6 observed no differences in the duration of labor or 1- and 5-minute Apgar scores in comparing women with medium- and high-intensity exercise in pregnancy. In our cohort, women in the heavy exercise group were more likely to need an induction or augmentation of labor and had longer first stages of labor resulting in total longer labors. Operative vaginal and abdominal deliveries were not influenced by the amount of exercise in pregnancy. The incidence of variable and late decelerations influencing delivery, meconium-stained amniotic fluid, and neonatal Apgar scores were also not affected by the level of exercise.
Maternal exercise has been reported to increase,8 decrease,2–4 and have no effect on newborn birth weight.5–7 In our investigation, the women who continued moderate or heavy exercise during the pregnancy (groups 3 and 4) delivered infants who were on average 86 g smaller than those of the women who did not exercise (group 1). No adverse consequences were observed in this group of smaller neonates. Smaller neonates were also observed in one of our earlier investigations.12 Women expending the greatest number of calories had the smallest neonates. Clapp and Capeless,19 using neonatal morphometrics, detected that the smaller neonates in exercising women had restricted growth limited to a restriction of neonatal fat mass. These authors suggested that this restriction of fat mass may have preventative value in reducing the risk of obesity in later life. Despite the statistically smaller neonates, we observed there were no clinically significant adverse consequences.12
The findings of this study are validated by the sample size of 750 working women, extensively evaluated, and a post hoc analysis that shows the investigation had a power of 82.8% to demonstrate an effect of exercise on birth weight after controlling for the factors included in the model. Exercise in low-risk working women does not appear to affect antenatal, intrapartum, or postpartum complications of pregnancy. The neonates of exercising women are affected by the level of exercise and are smaller but without adverse consequences. The reported benefits of earlier and shorter labors, less operative deliveries, and less evidence of fetal stress during labor are not supported by our data.
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© 2002 The American College of Obstetricians and Gynecologists
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