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Abstract & Commentary
Synopsis: 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.
Source: Clapp JF, et al. Am J Obstet Gynecol. 2002; 186:142-147.
A recent article from clapp and collaborators has shed some important light on placental development and its relationship to fetal growth. Clapp et al took 75 healthy, regularly exercising pregnant women and randomized them into 3 weight-bearing exercise regimens that they pursued 5 days a week through pregnancy. The intensity of the exercises was gauged according to the time spent: 20 min (low), 40 min (moderate), and 60 min (high). One group consisted of women doing high-intensity exercise up until the 24th week, then diminishing their exercise to low intensity thereafter. Using the same 24-week division, a low/high group also was randomly created along with a third moderate/moderate group.
Clapp et al found that the low/high group delivered babies that were lighter and had less fat than either the moderate/moderate or high/low group. The most striking difference occurred in the high/low group, in which participants who had babies who were on average 3900 g compared with the low/high group of 3300 g, and had 12% body fat compared with an average of about 8% between the other 2 groups. Also, placental volumes, ultrasonically measured, were consistently larger in the high/low group.
Comment by John C. Hobbins, MD
This comprehensively designed and beautifully implemented study is one of many carried out throughout the years by Clapp and various colleagues. At first glance, one might consider the clinical connotations of the study to be less than earth shaking, since none of the infants was clinically compromised by any of the exercise schedules. Although the low/high babies were lighter and scrawnier than the others, none had a weight below the 10th percentile. However, this should not be surprising since the study group was chosen for its normalcy, and one would expect little pathology to be generated from this healthy, active, pregnant population.
Clapp et al set out to test a hypothesis that evolved from their earlier work, which suggested that high intensity exercise stimulated stem villous development in early pregnancy and discouraged terminal villous development in late pregnancy. Therefore, an optimal exercise regimen would be to exercise early and often up to 20 weeks and to lay off somewhat in the third trimester. The fact that the high/low babies were bigger, fatter, and had larger placentas proved their point.
So what clinically relevant information can we take away from this study? New techniques of placental casting have yielded exciting information regarding placental development. Basically, there are 3 stages of placental development. During the first stage, primitive primary stem villi extend downward from the fetal surface to provide a structural framework for later activity. The second stage occurs during the second trimester when the initial villi will branch into 10-15 immature stem villi. Not surprisingly, this has been entitled the "branching angiogenesis" stage. The third stage involves the budding out of terminal villi from the existing intermediate villi. Transfer of oxygen and nutrient is dependent upon the amount of terminal villi. This stage has been labeled "nonbranching angiogenesis."
Any process that interferes with either or both of these 2 stages can result in compromise of fetal growth and well being. Smoking, altitude, and maternal anemia can affect these phases of placental development. In many cases of IUGR, where almost uniformly fewer terminal villi are found in the placentas studied, a single cause cannot be identified.
I have taken away (with some embellishment) the following messages from this study:
1. In normal, healthy pregnancy exercise is not overtly dangerous and may even enhance fetal placental growth if undertaken in early pregnancy.
2. If a patient is at risk for IUGR (having delivered a growth-restricted fetus in a previous pregnancy) or has a condition that would not contra-indicate exercise, one might be able to optimize placental development by a regimen of exercise in the first and second trimester of pregnancy.
3. If IUGR is diagnosed in mid-to-late pregnancy, pursuing high intensity exercise is not a good idea (since the fetus can use any extra terminal villi that can be recruited). In fact, we have had many patients diagnosed to have IUGR in the third trimester, with no obvious cause who have responded with increased fetal growth and improved Dopplers by simply limiting maternal activity. Often these are upwardly mobile types with high-pressure jobs requiring high-energy expenditure. If the placenta has a marginal capability to feed a fetus who is becoming more demanding of nutrients, one can optimize placental perfusion by stopping the maternal competition for the blood supply to the fetus.
Dr. Hobbins is Professor and Chairman, Department of OB/GYN Tufts University School of Medicine, Boston, Massachusetts.