DON'T WALK AWAY WITH CONTRACTIONS AND UNKNOWNS
to the editor: Amber Miller gave birth to a healthy girl 8 h after completing a marathon. Zavorsky and Longo (5) gave no details on when labor started. The effect of exercise-induced uterine contractions on fetal safety was not discussed.
Studies in healthy chronically instrumented sheep demonstrated that even exhaustive exercise does not cause fetal hypoxia or stress as a result of effective compensatory mechanisms (2, 3). During exercise uterine blood flow (UBF) decreases in proportion to maternal heart rate, and uterine oxygen consumption is relatively maintained through hemoconcentration and increased oxygen extraction. As a result fetal Po2 stays within the normal range. Although unverified, most likely these mechanisms also apply to humans.
In pregnant women with a cervix ready for delivery, exercise induces uterine contractions that may not be perceived by the exercising woman (4). During uterine contractions, UBF decreases in proportion to the intrauterine pressure increase (1), not negligible in humans. The combined reduction in UBF, induced by contractions and exercise, has not been quantified.
Not all factors relevant to fetal wellbeing are known in a pregnant woman participating in a marathon. Factors to be considered include reduction in uteroplacental blood flow reserve near term, hemoconcentration by prehypertension, increased fetal oxygen demand with maternal temperature increase, and further reduction in UBF with relative dehydration.
Amber Miller has shown that, 1 day near term, she could complete a marathon and deliver a healthy baby. This remarkable feat should be taken as proof of concept rather than as proof of safety.
COMMENTARY ON VIEWPOINT: ARE THERE VALID CONCERNS FOR COMPLETING A MARATHON AT 39 WEEKS OF PREGNANCY?
to the editor: Zavorsky and Longo (4) indicate that Amber Miller was cleared medically to complete the Chicago marathon. Amber said that her doctor gave her permission to do the first half, but she decided to complete the rest. What should be emphasized is that Amber had a very trusting and open relationship with her doctor, which is important for any pregnant woman and absolutely necessary for one who performs significant vigorous exercise throughout gestation.
Concerns would not be for overheating and subsequent neural tube defects (which occur during the first trimester) or low birth weight (Amber was term) (1, 3). Amber's primary concerns would likely have been her physiological response to heat production and hypoglycemia. According to newspaper reports, Amber did not walk briskly for the entire race, but rather, ran much of the first half where core temperature likely increased (2). Without temperature and sweat rate/evaporation monitoring, it is not possible to tell how well she dissipated heat. With respect to hypoglycemia, unless Amber was dizzy and/or lightheaded, blood glucose was likely adequate.
Although I applaud the effort of this remarkable woman, the take home message should not be, completing a marathon at 39 wk gestation is “no big deal.” It is a big deal. It is something that very few women can or should do during pregnancy. Although I have always been a proponent of exercise during pregnancy, if even one woman reads this article and subsequently exceeds her physical limitations during pregnancy, that would be one too many.
- Copyright © 2012 the American Physiological Society