Materials and Methods

The current study is part of a larger investigation of exercise during pregnancy; results regarding fetal responses to current exercise recommendations for moderate and vigorous intensity exercise have been published. The present study includes unpublished data on fetal well-being and uterine artery Doppler data in response to strenuous exercise.

Participants included healthy women with low-risk, accurately dated (last menstrual period confirmed by first or second trimester ultrasound scans) pregnancies. Exclusion criteria included multiple gestation, body mass index of >35 kg/m ² , smoking, history of preterm delivery at <34 weeks’ gestation, cervical insufficiency or cerclage in place, placenta previa, any chronic medical condition, gestational diabetes mellitus or hypertension, or a fetus with known structural or chromosomal abnormalities or with growth restriction. Testing was performed between 28 weeks and 32 weeks 6 days’ gestation. This gestational age range was chosen because fetal well-being tests, particularly umbilical artery Doppler measurements, are generally more informative at >28 weeks’ gestation.

Women were classified into 1 of 3 groups according to self-reported physical activity during the 6 months before and continuing into pregnancy: (1) nonexercisers who did not perform regular physical activity (defined as >20 minutes per session for >3 times per week), (2) regularly active women who described their activity as mild to moderate for at least 20 minutes per session ≥3 days per week, and (3) highly active women who were predominantly runners who described their activity as vigorous >4 days per week. The Johns Hopkins University School of Medicine Institutional Review Board approved the protocol, and all participants provided written informed consent.

All women underwent a peak treadmill test to volitional fatigue according to a modified Balke protocol. In the current study, this is defined as strenuous exercise . After a warm-up at 3.0 mph and 0% grade, treadmill speed was maintained at 3.0 mph, and the incline was increased 2% every 2 minutes. After the incline reached 12%, it remained at this level, and speed was increased 0.2 mph every 2 minutes. Volitional fatigue was defined as the limit beyond which a participant no longer desired to continue the protocol. Treadmill time was recorded in minutes, excluding warm-up. Exercise capacity, which can be quantified by the measurement of oxygen consumption at maximal exercise (ie, V o ₂ ), is considered the best measure of cardiovascular fitness. Because it was not measured in this study, V o ₂ peak was estimated with the use of a validated prediction equation for pregnant women and is expressed as milliliters of oxygen used per kilogram of body weight per minute.

Maternal electrocardiograms were continuously recorded. Peak heart rate that was achieved during the test was recorded. Percent of predicted maximum heart rate that was achieved was calculated with the typical equation for the estimation of maximum heart rate (220 minus age). Rating of perceived exertion with the 0-10 point scale was obtained at the end of the test. This scale has been validated as an effective means to monitor exercise intensity.

Fetal well-being measures that were obtained at rest and immediately after exercise included umbilical artery Doppler data, fetal heart tracing, fetal heart rate (FHR), and biophysical profile (BPP). Uterine artery Doppler measures evaluated maternal blood flow. All testing was performed in the afternoon, starting between 3:30 and 7:30 pm . Women were instructed not to eat or drink anything except water for 1 hour before arrival. On arriving to the Fetal Assessment Center, the women lay in a semirecumbent position with a leftward tilt, and a fetal heart tracing was recorded. Fetal heart tracings were evaluated for “reactivity” according to established criteria for gestational age and were classified by the 3-tier interpretation system. Blood pressure and maternal resting heart rate were taken after 15 minutes of rest. After resting umbilical and uterine artery Doppler measures were obtained, participants performed the exercise test. Immediately after exercise, they returned to the semirecumbent position with a leftward tilt. Ultrasound scanning was performed to acquire umbilical and uterine artery Doppler measures followed by BPP and then fetal heart tracing.

Ultrasound scanning was performed by 1 researcher (L.M.S.), who is an obstetrician trained in maternal-fetal medicine. Umbilical artery flow velocity waveforms were assessed with the use of color Doppler imaging in a free loop of umbilical cord. Three to 5 time points, each of which contained a minimum of 3 sequential uniform waveforms, were recorded. Uterine artery Doppler measures were obtained from the maternal right side. Color flow Doppler data were used to assist in the identification of the uterine artery at the point of crossover with the (external) iliac artery, and velocimetry measurements were obtained approximately 1 cm distal to the crossover before branching of the uterine artery. The angle of insonation was always less than 50 degrees. Again, 3-5 time points, each of which contained 2-3 sequential uniform waveforms, were recorded for later analysis. Built-in software calculated the systolic-to-diastolic (S/D) ratio, resistance index, and pulsatility index (PI). Mean values were calculated for each frame and averaged over several time points. FHR was calculated from umbilical Doppler data. The immediate FHR after exercise was determined from the first Doppler measure.

Gestational age at delivery, mode of delivery, birthweight, and Apgar scores were obtained from delivery records and have been previously reported.

Sample size was calculated to achieve 80% power at the .05 level of significance with the use of umbilical artery S/D ratios, which was our primary outcome measure for fetal well-being. This variable was chosen because it can be measured precisely, can be reproduced, and has been used as a primary outcome variable in existing studies, providing data to perform a power analysis. Two analyses were performed. First, existing data that measured umbilical artery S/D ratios after exercise in pregnant women at 32 weeks gestation indicated that 12 women per group would be sufficient. Second, reference data that attempted to detect a change from the 50th percentile to the 75th percentile indicated that 11-13 women per group, depending on gestational age, would be sufficient. These percentiles were chosen to allow the detection of smaller differences among groups. Although a change from the 50th-90th percentile would likely be more clinically significant, this would have significantly decreased the number of women needed per group.

Shapiro-Wilkes tests were performed to evaluate for normality. One-way analysis of variance (ANOVA) was used to compare descriptive variables among groups. Bonferroni post-hoc analyses were used to probe significant differences among groups. Differences in FHR and Doppler indices before and after the exercise tests in the 3 groups were analyzed with a 2-way (group × time) ANOVA with repeated measures. Post-hoc comparisons evaluated significant differences with the use of Bonferroni’s method to correct for multiple comparisons. Subgroup analyses evaluated potential differences between those in the highly active group who experienced FHR decelerations after exercise to those in the highly active who did not. Because of the small sample sizes, the Kruskal-Wallis test was used to compare descriptive variables. Because of the unbalanced design in the subgroup analyses, FHR and Doppler indices were analyzed with a mixed effects regression analysis that examined the main effects of activity group and time (before-after) and accounted for within subject correlation and group by time interaction. Delivery data were analyzed by either 1-way ANOVA or chi-square (categoric variables). Statistical significance was reached at a probability value of < .05. Statistical analyses were performed with Stata software (version 12.1; StataCorp, College Station, TX).

Results

Forty-five healthy pregnant women were divided into 3 groups by physical activity level. Subject characteristics have been reported previously. Briefly, groups were similar in age, body mass index, and gestational age ( P > .05). Mean ages for the women who were nonexercisers, regularly active, and highly active were 32.9, 34.3, and 32.9 years, respectively. All women were normal weight before pregnancy. Gestational age at the time of testing was 30.7 ± 1.1, 30.2 ± 0.9, and 30.3 ± 1.0 weeks for women who were nonexercisers, regularly active, and highly active, respectively. As expected, maternal resting heart rate was lower in the highly active group (61.6 ± 7.2 beats/min) compared with the nonexercise (79.0 ± 11.6 beats/min) and regularly active (71.9 ± 7.4 beats/min) groups ( P < .001). Maternal heart rate at peak exercise (nonexerciser group: 163.0 ± 18.8 beats/min; regularly active group: 163.3 ± 8.9 beats/min; highly active group: 172.4 ± 11.7 beats/min) and percent of predicted maximum heart rate that was achieved (nonexerciser group: 87% ± 10.8%; regularly active group: 87.9% ± 4.8%; highly active group: 92.1% ± 5.7%) were similar ( P > .05). Predictably, treadmill time (nonexerciser group: 12.1 ± 3.6 min; regularly active group: 16.6 ± 3.4 min; highly active group: 22.3 ± 2.9 min) and predicted V o ₂ peak (nonexerciser group: 21.3 ± 2.5 mL/kg/min; regularly active group: 23.8 ± 2.2 mL/kg/min; highly active group: 27.7 ± 1.4 mL/kg/min) increased with increasing activity status ( P < .001). Rating of perceived exertion was similar among the groups (non-exerciser group: 8.0 ± 1.6; regularly active group: 8.3 ± 1.3; highly active group: 9.1 ± 0.6).

All variables were distributed normally, except uterine artery S/D ratio and PI. These data were then analyzed with log transformations, which normalized the distributions. Nontransformed means and standard deviations are reported.

FHR and Doppler indices before and after exercise by activity group are shown in Table 1 . There were significant group differences in FHR ( P = .017) and a statistically significant group by time interaction ( P = .033), which indicated that the groups responded differently to the exercise. Planned comparisons revealed no group differences between FHR at rest; however, after exercise FHR in the highly active group was lower than the other groups ( P < .05). Further evaluation of the data indicated that, after exercise, FHRs for the highly active group included 5 subjects with FHR decelerations after exercise. When the data were reanalyzed excluding these 5 subjects, the mean FHR after exercise in the highly active group was 149.3 ± 10.6 beats/min, and there were no group differences ( P = .714). Additionally, after exercise FHRs increased ( P < .001), and the group by time interaction was no longer significant ( P = .553), which suggests that FHR responses were similar in the 3 groups.

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