Alright time to get down to business with a question that I've been wondering about lately. Does exercise induce preterm labor? Bear with me, this one takes a long path to get to the point but it's necessary to fully understand the point. Again I'm siting this information from James F. Clapp's book "Exercising Through Your Pregnancy." Research in industrial medicine has shown that "several types of on-the-job physical stress increase the incidence of labor starting more than three weeks before the baby is due (premature). The research also shows that these stresses increase the number of infants who weigh less than they should for the time they are born." In addition, it has also been found that when these workplace stresses are reduced these two problems are alleviated. Thus people assumed that physical stress is physical stress no matter where you find it and vigorous recreational activities (such as running) should be avoided in pregnancy. "However, the types of physical stress in the workplace that have had significant effects are much different physiologically from those of recreational exercise." These include:
- quiet standing for four or more hours a shift
- walking for protracted periods
- long work shifts
- frequent heavy lifting
It is understandable how this kind of job would produce feelings of extreme fatigue in a pregnant woman, and we can see how if she is regularly experiencing this stress it could result in an under-grown baby. Let's look at this scenario that Dr. Clapp uses as an example in his book:
- The woman has been busy or is allowed infrequent breaks so she doesn't drink because she doesn't have time.
- If she can drink, she doesn't because it will mean many trips to the ladies' room.
- This dehydration is compounded by her being on her feet for a long time, which causes blood to collect and pool in her relaxed leg veins.
- Swelling of the lower leg and ankle occurs because the back pressure from distended veins causes fluid to leak out of capillary vessels into the tissues.
- If she doesn't eat frequently, her blood sugar level falls.
- Fatigue sets in.
The reduced blood flow (caused by dehydration) along with low blood sugar, reduces oxygen availability and decreases nutrient supply to the baby, in turn slowing its growth and increasing irritability of the uterine muscle and risk of premature labor. "Once it was recognized that recreational exercise usually does not produce most of these effects - symptoms of severe fatigue, pooling of blood in the legs, or low blood pressure - investigators began to separately quantify physical stress on the job and physical stress from recreational exercise to determine if either is associated with premature labor or smaller than average babies." Clapp goes on to say that all studies show that recreational exercise not only does not increase the incidence of smaller babies or premature labor, but it might actually decrease in the incidence of both. So all of that was Dr. Clapp looking at the information that was available to him, now we can look at the study that he designed. The two questions he asked:
- Does continuing a regimen of sustained, weight-bearing exercise (running or aerobics as opposed to biking or swimming) throughout pregnancy increase the risk for premature labor?
- Do either sudden foot strike shock or bouncing, ballistic motions cause the membranes that surround the baby to burst before they should?
To answer these questions they "established an accurate due date for each woman who enrolled in the study...then monitored exercise performance throughout the pregnancy in the regularly exercising women who continued to perform sustained types of weight-bearing exercise at or above the basic fitness level throughout pregnancy." The timing of their deliveries were compared to a control group (women with active lifestyles who did not maintain a regular exercise regimen).
Exercise group criteria:
- Maintain exercise regimen at least 50% of prepregnancy level. (Example: if she ran 6 miles before becoming pregnant, she could remain in the exercise group as long she continued running at least 3 miles throughout pregnancy)
- Her level of exercise also had to remain above that required to maintain basic fitness.
Over 70% continued to meet these criteria and in most cases, performance was above 50% of prepregnancy level. "Over the last three months, their average exercise intensity fell a little bit (from 66 to 59 percent of maximal aerobic capacity), but, in many cases, the time spent each week in exercise increased."
Of the women who stopped or cut back after the 30th week, we ask why did they stop?
- No longer had enough time once they started preparing for the birth.
- Pressure from people around them who were concerned that the exercise might hurt the baby or initiate preterm labor.
- Becoming concerned themselves.
"Physical discomfort was way down on the list."
They followed all of the women on to the timing of delivery and therefore have information on 3 rather than two groups.
- Over 200 who continued vigorous exercise to within a week of delivery
- Over 80 who exercised vigorously throughout early and midpregnancy but later cut back or stopped
- 250 physically active controls who rarely engaged in sustained exercise but occasionally played tennis, walked, gardened, etc.
The RESULTS!
Running and aerobics did not cause the water to break before the onset of labor at term (the end of the 40th week). "The chance that this would occur before the beginning of the 37th week was low and was the same in all three groups. This was also true at the end of pregnancy. Even after the cervix had begun to dilate, women could continue to run or participate in aerobics without increasing the chance that the membranes surrounding the baby would burst before the onset of labor." More women in the exercise group actually delivered earlier than the control group but were still at term. In other words, exercising women didn't necessarily deliver earlier than 40 weeks, but most of the controls delivered 5 to 7 days later than the exercising mothers. "The timing of women who stopped exercising midpregnancy was no different than the controls."
As always I think this is good information, it gives me hope that I will be able to continue running for the next 13 weeks, but always listen to your Doctor or other healthcare provider as sometimes complications do arise. Listen to your body and rest when you need to, as in any stage in life there needs to be a good balance between rest and exercise. Drinking water to stay hydrated is always important but it is even more so while pregnant. One of my friends recently said it best when she was expressing that as pregnant women we aren't handicapped but we don't have to be super heroes either.
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