Special Feature

Physical Activity in Pregnancy Improves Maternal Well Being, Decreases Complications, Improves Fetal Outcome

By John C. Hobbins, MD, Professor, Department of Obstetrics and Gynecology, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.

Dr. Hobbins reports no financial relationship relevant to this field of study.

Synopsis: Studies have shown that exercise in pregnancy enhances perinatal outcomes as long as the guidelines described below are adhered to in individuals with various starting weights and levels of baseline fitness.

Over the last 30 years, recommendations regarding weight gain and exercise in pregnancy have been inconsistent. With obesity rates skyrocketing, it is important to encourage pregnant women to be active and to set expectations for pregnancy weight gain at the first prenatal visit. This special feature has been drafted to help providers deliver the most up-to-date recommendations and to address additional issues like exercising at altitude or in the heat.

Obesity

Less than 10 years ago, no state had an obesity rate of greater than 30%. Centers for Disease Control (CDC) statistics in 2009 indicate that 60% of women in the United States are overweight (body mass index [BMI] > 25 kg/m2). In 19 states the rate of obesity (BMI > 30 kg/m2) exceeded 30%. The obesity winner (or loser might be the more apropos) was Mississippi at 34%. Happily, my home state of Colorado has the lowest obesity rate of 21%. Obesity and pregnancy are a troublesome combination, as rates of macrosomia, still birth, cesarean section, shoulder dystocia, maternal and childhood diabetes, and maternal cardiovascular compromise are increased appreciably. Also, with each pregnancy, a woman's baseline weight stair-steps upward due to the tendency not to return to her baseline pre-pregnant weight.

Ideal Pregnancy Weight Gain

Average weight gain in pregnancy is about 25 pounds. The fetus is responsible for about 7 pounds, the placenta for 1 to 2 pounds, the amniotic fluid for about 3 pounds, breast engorgement for 4 pounds, and, most importantly, an increased blood volume accounts for about 10 pounds. However, there is great variability in these individual contributions, even in the average healthy woman.

It is difficult to nail down how much weight gain best suits each woman as starting weights vary significantly. Two studies involving Missouri women have shed some light on this by correlating weight gain and pregnancy outcomes. One study showed that patients with "normal" BMIs (18-25 kg/m2) who gained less than 25 pounds had significantly lower rates of cephalo-pelvic disproportion (odds ratio [OR] = 0.64), failed induction (0.68), cesarean section (0.82), and large for gestational age (LGA) babies (0.40).1 On the other hand, they did have a higher rate of small for gestational age (SGA) (2.14). Those mothers with normal BMIs who gained more than 35 pounds had significantly higher rates of preeclampsia (OR = 1.88), LGA (2.43), and failed inductions (1.35), but fewer SGA babies (0.48).

Another study from the same state showed that obese women with BMIs > 30 kg/m2 who gained fewer than 15 pounds had the lowest rates of LGA, preeclampsia, and cesarean section, compared with obese women gaining more weight.2

Last, a study from Sweden involving 30,000 patients showed that those whose BMIs were between 25-30 kg/m2 (overweight) had the best composite pregnancy outcomes when gaining fewer than 20 pounds, and if BMIs exceeded 30 kg/m2 (obese), the lowest rate of composite adverse outcome was in those who gained fewer than 13 pounds.3

Current advice emanating from the National Research Council4 adds a bit of cushion to the above data, but the recommendations are reasonable and, importantly, well accepted (See Table 1). Most importantly, exercise during pregnancy can help women limit their pregnancy weight gain.

Table 1
Body Mass Total Weight 2nd and 3rd  
Index Gain Trimester lb/week
Underweight (< 18) 28-40 lbs 1.0
Normal (18-25) 25-35 1.0
Overweight (25-30) 15-25 0.6
Obese (> 30) 10-20 0.5

Exercise in pregnancy

First (and foremost), with the exception of a few women, exercise is GOOD in pregnancy. Studies have shown that a regimen of appropriately tailored exercise diminishes the need for cesarean section,5 decreases fetal distress,6 and reduces maternal weight gain.7 Study results are conflicting regarding the effect of exercise on birth weight, but no studies have shown a detrimental effect if the pregnant woman adheres to reasonable guidelines. Some studies have shown an increase in overall maternal well-being, with fewer musculoskeletal complaints,8 better emotional stability,9 and fewer muscle cramps.10

Exercise physiologists generally employ a measure of exercise tolerance and overall fitness, which quantifies a woman's capacity for maximal oxygen extraction (VO2max). Using the Fick equation one can calculate the volume of oxygen extracted (VO2) by multiplying the cardiac output (Q) by the difference between oxygen content in arteries and veins (Ca-Cv). This is expressed in mL/min/kg. With this in mind, during exercise the estimated VO2max has been used as a measure of fitness, and has been employed to fashion guidelines for pregnant women. In general, pregnancy increases the baseline V02 by about 20%, but there is little difference in V02max in exercising pregnant vs non-pregnant individuals except in very fit athletes.

The average baseline maternal heart rate increases by about five beats per minute in pregnancy. The diastolic blood pressure decreases by 10 to 15 mmHg, while the systolic pressure barely budges. The maximum heart rate in exercising pregnant patients is about 4 to 5 beats lower than in non-pregnant individuals, but, as indicated above, the resting heart rate is higher — resulting in reduced heart rate reserve.11

Exercise Recommendations

In little more than 50 years, there has been a huge change in the philosophy regarding physical activity in pregnancy. In the 1950s, the paternalistic concept was that a pregnant woman's physical activity was limited to household chores and a one-mile walk a day, broken up into 15-minute segments. This concept changed in 1985 to a more liberal approach to exercise, as long as the heart rate did not exceed 140 for 15 minutes. The latest recommendations by the American Academy of Sports Medicine and the CDC12 are that women should be encouraged to engage in moderate physical activity for 30 or more minutes a day for most days of the week. For example, this would translate into a brisk walk at 3-4 mph. For women who have been previously inactive, they should start at 15 minutes per day, four days a week, and gradually increase this to 30 minutes four times a week.

A study involving 5700 Danish women assessed whether there was an association between the degree of physical activity and preterm birth (PTB).13 Sedentary women had a rate of PTB of 4.3% vs 1.9% in the "moderate/heavy" group. Those who were involved in competitive sports had the lowest rate of PTB.

The timing of exercise seems to be important. For example, Clapp et al found that in generally fit women the largest babies were born to those whose treadmill exercise went from "heavy" (60 minutes) in the second trimester to either light (20 minutes) or moderate (40 minutes) in the third trimester.14 Birth weight was lowest in those who started "light" and increased their activity in the second half of pregnancy. The concept of maintaining a high degree of fitness at the start and the backing off somewhat later in pregnancy is in sync with Clapp's concept that exercise in the second trimester stimulates branching angiogenesis in the placenta, while heavy exercise in the third trimester may discourage terminal villus development.

Exercises that are encouraged include walking, swimming, stationary cycling, and water aerobics. Activities that are less enthusiastically endorsed are water skiing, skiing, indoor racquet sports, scuba diving, and mud wrestling (just kidding). Runners need to be a bit more careful once in the second trimester as the extra weight and change in the weight-bearing puts more strain on the joints. Because of the tendency for the uterus to compress the inferior vena cava, pregnant women are discouraged from exercising in a supine position once in the second trimester.

The concept of using the heart rate as a guideline for monitoring the intensity and duration of exercise has swung back and forth. Initially, the dictum was that the heart rate should not exceed 120. Then there was a swing toward using maternal perception of fatigue as a gauge. Now, there is enough evidence from studies using VO2max in patients with normal and high BMIs to estimate heart rate levels that would best represent any pregnant woman's ability to exercise at the suggested 60% to 80 % of VO2max (but initially at 20%-39% in previously sedentary women).

Currently, the recommendation is that heart rate be used, but with more liberal thresholds. Some have advocated using both methods. One can roughly gauge their heart rate by how they feel — through a rating of perceived exertion (RPE).11 For example, "very light" would be analogous to a heart rate of 70-90; "fairly light," 100-120; "somewhat hard," 120-140; "hard," 140-160; "very hard," 160-180; and "very, very hard," 180-200. In most cases, the recommendation would be for pregnant women to back off once "hard" (140) is attained, or, at least, to check their heart rate at that point.

Table 2 represents recommendations regarding target heart rate for those with various starting body weights.

Table 2    
  HR for those with HR for sedentary women
Age BMIs < 2515 with BMIs > 2516
< 20 140-155  
20-29 135-150 102-124
30-39 130-145 101-120
> 39 125-140  

Exercising at Altitude

Compared with sea level, birth weights are about 5% less in Denver, which is at 5000 feet above sea level. We also have noted a higher rate of preeclampsia and SGA births in high mountain towns, such as Leadville, Colorado (> 10,000 feet). Also, anecdotally, we have seen improvement in Doppler profiles in some IUGR fetuses when their mothers come down to Denver from higher altitudes in late pregnancy. However, in general, mothers and fetuses adapt extremely well to altitude as long as it is not suddenly forced upon them. Therefore, although women are certainly not discouraged from exercising at altitude, it has been suggested that there should be an allowance of 4 to 5 days of acclimation before exercising at > 8000 feet.

Rise in Maternal Core Temperature

Since fetal heart rate increases during and after exercising, it has been thought that this results from a competition for blood flow between mother and fetus. Also, the idea emerged that fetuses might tolerate poorly any increase in exercise-induced maternal core temperature. After a study surfaced that possibly linked fetal neural tube defects with temperature rise,17 there has been an off and on concern about an adverse effect of exercise and/or hot tubs on neural tube closure. However, the link has never been validated, and since the neural tube closes between days 22 and 29 post-conception, any event raising maternal core temperature by more than 1°C would have to occur during a narrow time window.

Conclusion

In summary, pregnant women should be encouraged to exercise in pregnancy as this can improve maternal well being, decrease pregnancy complications, and improve fetal outcome. Nevertheless, exercise is best undertaken according to recommended guidelines which tailor the activity according to each woman's body type and fitness level. It is underscored that adequate hydration is of utmost importance in the exercising pregnant woman, especially at altitude, since any drop in blood volume could have an adverse effect on uterine blood flow.

References

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  4. Data from the Institute of Medicine National Research Council Committee to re-examine weight gain during pregnancy. Washington DC: National Academies Press; 2009.
  5. Beckmann CR, Beckman CA. Effect of a structured antepartum exercise program on pregnancy and labor outcome in the primiparas. J Reprod Med 1990;35:704-709.
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