Safety and efficacy of supervised strength training adopted in pregnancy.

OBJECTIVE: Describe safety and efficacy of a supervised, low-to-moderate intensity strength training program adopted during pregnancy among women at increased risk for back pain. METHODS: 32 women adopted strength training twice per week for 12 weeks. Data on musculoskeletal injuries, symptoms, blood pressure, and the absolute external load used for 5 of 6 exercises were obtained during each session. A submaximal lumbar extension endurance exercise test was performed at weeks 5, 10, and 13. RESULTS: The mean (± SD) exercise session attendance rate was 80.5% (± 11.3%). No musculoskeletal injuries occurred. Potentially adverse symptoms (eg, dizziness) were infrequent (2.1% of sessions). Repeated-measures ANOVA showed large increases in the external load across 12 weeks (all P values < .001) and the percentage increases in external load from weeks 1 to 12 were 36% for leg press, 39% for leg curl, 39% for lat pull down, 41% for lumbar extension and 56% for leg extension. Training was associated with a 14% increase in lumbar endurance. Blood pressure was unchanged following acute exercise sessions and after 12 weeks of exercise training. CONCLUSION: The adoption of a supervised, low-to-moderate intensity strength training program during pregnancy can be safe and efficacious for pregnant women.

Aerobic exercise during pregnancy influences fetal cardiac autonomic control of heart rate and heart rate variability.

BACKGROUND: Previous studies using ultrasound technology showed that fetal heart rate (HR) may be responsive to maternal aerobic exercise. Although it is recognized that cardiac autonomic control may be influenced by the intrauterine environment, little is known about how maternal exercise affects fetal heart development. AIMS: This study tested the hypothesis that regular maternal exercise throughout gestation influences fetal cardiac autonomic control of HR and heart rate variability (HRV) when compared to fetuses of non-exercising women. STUDY DESIGN: Magnetocardiograms (MCGs) were recorded using a dedicated fetal biomagnetometer at 28, 32 and 36 weeks gestational age (GA) from 26 regularly exercising (>30 min of aerobic exercise, 3x per week) and 35 healthy, non-exercising pregnant women. Fetal MCG was isolated and normal R-peaks were marked to derive fetal HR and HRV in the time and frequency domains. We applied a mixed-effects model to investigate the effects of exercise, GA and fetal activity state. RESULTS: At 36 weeks GA, during the active fetal state, fetal HR was significantly lower in the exercise group (p=<0.0006). Post-hoc comparisons showed significantly increased HRV in the exercise group during the active fetal state at 36 weeks GA for both time and frequency domain measures. CONCLUSION: These results indicate that regular maternal exercise throughout gestation results in significantly lower fetal HR and increased HRV.

Long-term outcome after exercising throughout pregnancy: fitness and cardiovascular risk.

OBJECTIVE: The objective of the study was to test the null hypothesis that continuing vigorous weight-bearing exercise throughout pregnancy has no discernible long-term effect on indices of fitness and/or cardiovascular risk. STUDY DESIGN: This was a follow-up observational study of the fitness and cardiovascular risk profile of 39 women conducted on the General Clinical Research Center at the University of Vermont. Data were analyzed using the paired Student t test, analysis of variance, and linear regression. RESULTS: Women who voluntarily maintain their exercise regimen during pregnancy continue to exercise over time at a higher level than those who stop. Over time they also gain less weight (3.4 vs 9.9 kg), deposit less fat (2.2 vs 6.7 kg), have increased fitness, and have a lower cardiovascular risk profile than those who stop. CONCLUSION: Women who continue weight-bearing exercise during pregnancy maintain their long-term fitness and have a low cardiovascular risk profile in the perimenopausal period.

A foetal energy balance equation based on maternal exercise and diet.

Empirical data indicate that the maternal diet composition has a direct impact on foetal fat mass and resulting birth weights. Weight-bearing maternal exercise influences the placental volume, which has also been correlated to birth weight. A foetal energy balance equation, based on the first law of thermodynamics, that incorporates maternal diet and exercise is developed. Model parameters and validity are evaluated using published data.

Low-versus high-glycemic index diets in women: effects on caloric requirement, substrate utilization and insulin sensitivity.

BACKGROUND: Lowering dietary glycemic index appears to have positive health effects in obese and/or insulin resistant individuals. However, detailed studies in lean young men show no effect. METHODS: This study was designed to test the null hypothesis that a diet rich in low-glycemic carbohydrate has no effect on lipid profile, caloric requirements, fat oxidation, or insulin sensitivity in adult women when compared to one rich in high-glycemic carbohydrate. The metabolic feeding protocol used was conducted in both a free-living and in-patient setting using a randomized crossover design. Seven women were studied on each of 2 diets in which 60% of the calories were from either high- or low-glycemic carbohydrate sources. Each diet lasted 20 days with measurements of caloric requirement, resting metabolic rate, glucose and insulin responses to diet and activity, insulin sensitivity, and lipid profile over the last 7 days. Caloric requirement was determined by bomb calorimetry. Other techniques included indirect calorimetry, hydrodensitometry, stable isotope tracers, and the euglycemic clamp. RESULTS: On the low-glycemic index diet the women's caloric requirements were 11% +/- 1% higher, fat oxidation at fasted rest supplied an average of 45% +/- 4% versus 28% +/- 5% of oxidative requirements, average glucose and insulin levels were approximately 40% lower, low density lipoproteins (LDL) and leptin concentrations were lower, and various indices of insulin sensitivity were > 20% higher. CONCLUSIONS: In this group of adult women, a diet that lowered glycemic index well below that typically found in western diets increased both daily caloric requirement and fat oxidation, decreased insulin and glucose concentrations and increased insulin sensitivity.

Effects of Diet and Exercise on Insulin Resistance during Pregnancy.

Current evidence suggests that both diet and exercise can alter the usual increase in insulin resistance seen in Western societies during mid and late pregnancy. A low-glycemic diet combined with a low-volume exercise regimen during pregnancy decreases the glucose and insulin response to both mixed caloric intake and exercise, and probably lowers both 24-h blood glucose concentrations and the maternal substrate utilization ratio of carbohydrate/fat. The end result is a marked decrease in both maternal weight gain and size at birth. Regular weight-bearing exercise alone lowers markers of insulin resistance and lowers blood glucose concentration during and immediately after exercise during pregnancy. Changes in diet and/or physical activity appear to prevent the onset of gestational diabetes mellitus in at-risk women and may be of value in the treatment of those who develop gestational diabetes.

Maternal insulin-like growth factor-I levels (IGF-I) reflect placental mass and neonatal fat mass.

OBJECTIVE: This study was undertaken to test the null hypothesis that serial changes in maternal insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding-protein-1 (IGFBP-1) levels during pregnancy do not reflect differences in either corrected birth weight or placental mass at term. Study design Serial blood samples were obtained before pregnancy and at 8, 16, 24, 32, and 38 weeks from 56 healthy women enrolled in various exercise training regimens. Maternal, placental, and fetal morphometric parameters were monitored throughout. Enzyme-linking immunosorbent assays were used to determine IGF-I and IGFBP-1 levels. RESULTS: IGF-I and IGFBP-1 levels varied widely among the subjects at all time points, but there was a consistent fall in IGF-I levels in early pregnancy, followed by a rapid increase between 16 and 36 weeks' gestation, whereas IGFBP-1 levels rose in the first 16 weeks and were unchanged thereafter. The strongest linear correlations with morphometric outcome were between the increase in maternal IGF-I levels from 16 to 32 weeks and corrected birth weight (r(2)=0.27), neonatal fat mass (r(2)=0.65), and placental mass at term (r(2)=0.50). These were improved when maternal glucose level was included in a stepwise regression analysis (r(2)=0.50-0.70). CONCLUSION: There is a robust relationship among the rate of increase in individual maternal IGF-I levels after 16 weeks, placental mass, and neonatal fat mass. This does not imply causality but does indicate that midpregnancy changes in IGF-I levels may be a valuable marker for anomalous fetal growth.

The effect of nuchal cord on amniotic fluid and cord blood erythropoietin at delivery.

OBJECTIVE: We investigated the effect of a nuchal cord on fetal hypoxia by using amniotic fluid and cord blood erythropoietin as markers of chronic and acute hypoxia, respectively. METHODS: A total of 167 full-term pregnancies without maternal complications or fetal prelabor complications except fetal growth restriction of unknown cause were studied prospectively. Of these, 47 had a nuchal cord at delivery, and 62 had one or more complications during labor and delivery (nonreassuring fetal heart rate pattern, birth weight less than 2500 g, Apgar score at 1 minute less than 7, presence of meconium-stained amniotic fluid, oligohydramnios), and 26 had both nuchal cord and at least one of the intrapartum complications. RESULTS: Erythropoietin levels (mean +/- standard error of the mean) were not significantly different between the nuchal cord group (n = 47) and the no nuchal cord group (n = 120) in either amniotic fluid (19.3 +/- 4.1 mU/mL versus 13.7 +/- 1.1 mU/mL) or cord blood (57.9 +/- 10.3 mU/mL versus 52.1 +/- 4.9 mU/mL). Similarly, in the 62 fetuses with intrapartum complications, there were no significant differences in amniotic fluid (14.3 +/- 2.0 mU/mL versus 18.8 +/- 2.9 mU/mL) or cord blood erythropoietin (66.9 +/- 16.8 mU/mL versus 72.6 +/- 12.6 mU/mL) levels between those with (n = 26) or without a nuchal cord (n = 36). Among the 107 uncomplicated cases, however, amniotic fluid erythropoietin was significantly elevated in the nuchal cord group (25.5 +/- 8.7 mU/mL, n = 21) compared with that in the no nuchal cord group (11.5 +/- 0.9 mU/mL, n = 84) (P <.05), whereas there was no significant between-group difference in cord blood erythropoietin levels between nuchal cord and no nuchal cord groups (46.8 +/- 10.0 mU/mL versus 43.3 +/- 4.1 mU/mL). Tightness of the nuchal cord did not affect amniotic fluid or cord blood erythropoietin concentrations. CONCLUSION: Although nuchal cord may not significantly increase the risk of acute or labor-associated fetal hypoxia, it appears to be an independent risk factor of mild, chronic, prelabor fetal hypoxia.

The natural history of antenatal nuchal cords.

OBJECTIVE: The purpose of this study was to test the null hypotheses that nuchal cords occur with equal frequency throughout gestation, the presence of an antenatal nuchal cord is usually a random or chance event, nuchal cords do not usually persist, and nuchal cords do not usually produce evidence of acute fetal compromise before labor. STUDY DESIGN: A prospective subject-, examiner-, and physician-blinded study design was used. The fetuses were examined serially at 24 to 26, 30 to 32, and 36 to 38 weeks' gestation and during labor and delivery. Measurements included: ultrasonic or visual detection of nuchal cords and assessment of fetal and neonatal well-being by using standard clinical techniques. RESULTS: The incidence of nuchal cord rose with advancing gestation from 12% at 24 to 26 weeks to 37% at term. However, at each gestational age, its occurrence was a random or chance event and was not associated with clinical evidence of fetal compromise before labor. CONCLUSION: Antenatal nuchal cords usually occur randomly with increased frequency in late gestation and appear to be a normal part of intrauterine life that is rarely associated with perinatal morbidity and mortality.

The effects of maternal exercise on fetal oxygenation and feto-placental growth.

Sustained bouts of maternal exercise during pregnancy cause an acute reduction in oxygen and nutrient delivery to the placental site. The decreased flow also initiates a slight fall in intervillous and fetal pO2 which initiates a fetal sympathetic response. This, coupled with hemoconcentration and improved placental perfusion balance, maintains fetal tissue perfusion and oxygen uptake. Exercise training during pregnancy (regular bouts of sustained exercise) increases resting maternal (and perhaps fetal) plasma volume, intervillous space blood volume, cardiac output and placental function. These changes buffer the acute reductions in oxygen and nutrient delivery during exercise and probably increase 24 h nutrient delivery to the placental site. Thus, the effect of any given exercise regimen on fetal growth and size at birth is dependent on the type, frequency, intensity and duration of the exercise as well as the time point in the pregnancy when the exercise is performed. Maternal carbohydrate intake is yet another modifying factor. Beginning a moderate exercise regimen increases both anatomic markers of placental function and size at birth while maintaining a rigorous exercise regimen throughout pregnancy selectively reduces growth of the fetal fat organ and size at birth. Likewise, decreasing exercise performance in late-pregnancy increases size at birth while increasing exercise performance decreases it. Finally, the infants born of exercising women who eat carbohydrates which elevate 24 h blood glucose levels are large at birth irrespective of exercise performance.

Effect of maternal exercise and fetoplacental growth rate on serum erythropoietin concentrations.

OBJECTIVE: This study was undertaken to test the null hypotheses that neither weight-bearing exercise nor fetoplacental growth has a short- or long-term effect on the maternal serum erythropoietin level. STUDY DESIGN: Serial blood samples were obtained before and after exercise from seven women who exercised regularly and seven physically active controls before pregnancy and at 8, 16, 24, 32, and 38 weeks' gestation. Fetoplacental growth was assessed both in midpregnancy (ultrasound) and at birth (morphometry). RESULTS: Maternal serum erythropoietin levels rose with advancing gestation in both groups. Individual patterns, however, were quite variable and not related to differences in fetoplacental growth. There were no significant between-group differences at any time point, but levels rose after exercise in mid and late pregnancy. CONCLUSION: The highly variable, pregnancy-associated changes in maternal serum erythropoietin were unrelated to variability in fetoplacental growth or maternal hematocrit. Absolute levels of erythropoietin are not influenced by regular exercise before or during pregnancy, but small acute elevations are seen after exercise in mid and late pregnancy.

Maternal carbohydrate intake and pregnancy outcome.

Experimental evidence indicates that the primary maternal environmental factor that regulates feto-placental growth is substrate delivery to the placental site, which is the product of maternal substrate levels and the rate of placental-bed blood flow. Thus, maternal factors which change either substrate level or flow alter feto-placental growth rate. The best-studied substrate in human pregnancy is glucose, and there is a direct relationship between maternal blood glucose levels and size at birth. Altering the type of carbohydrate eaten (high- v. low-glycaemic sources) changes postprandial glucose and insulin responses in both pregnant and non-pregnant women, and a consistent change in the type of carbohydrate eaten during pregnancy influences both the rate of feto-placental growth and maternal weight gain. Eating primarily high-glycaemic carbohydrate results in feto-placental overgrowth and excessive maternal weight gain, while intake of low-glycaemic carbohydrate produces infants with birth weights between the 25th and the 50th percentile and normal maternal weight gain. The calculated difference in energy retention with similar total energy intakes is of the order of 80,000 kJ. Preliminary information from subsequent metabolic studies indicates that the mechanisms involved include changes in: daily digestible energy requirements (i.e. metabolic efficiency), substrate utilization (glucose oxidation v. lipid oxidation), and insulin resistance and sensitivity. Thus, altering the source of maternal dietary carbohydrate may prove to be a valuable tool in the management of pregnancies at risk for anomalous feto-placental growth and for the prevention and/or treatment of obesity and insulin resistance in the non-pregnant state.

Continuing regular exercise during pregnancy: effect of exercise volume on fetoplacental growth.

OBJECTIVE: The purpose of this study was to test the null hypothesis that the volume of exercise at different times during pregnancy has no effect on fetoplacental growth. STUDY DESIGN: Seventy-five women who exercised regularly were evaluated before pregnancy and randomly assigned at 8 weeks' gestation to one of 3 exercise regimens for the remainder of pregnancy. Primary outcome variables included placental growth rate, birth weight, and placental volume at term. RESULTS: The offspring of the women who were randomly assigned to a high volume of exercise in mid and late pregnancy were significantly lighter (3.39 kg vs 3.81 kg) and thinner (8.3% fat vs 12.1% fat) than those offspring born of women who were randomly assigned to reduce their exercise volume after the 20th week. Maternal weight gain, fresh placental volumes, and histomorphometric indices of placental function were greater in the high-low group. CONCLUSION: These data indicate that a high volume of moderate-intensity, weight-bearing exercise in mid and late pregnancy symmetrically reduces fetoplacental growth, whereas a reduction in exercise volume enhances fetoplacental growth with a proportionally greater increase in fat mass than in lean body mass.