30(+) years of exercise in pregnancy

In 1980 I came to Loma Linda to study maternal exercise, with Dr. Longo as my mentor. For millennia strenuous exercise was considered harmful for the fetus. Early studies reinforced that idea, by showing that exercise reduced uterine blood flow and fetal PO2 by up to 40 and 29 %, respectively. But utero-placental reserve is ~50 %. So why was fetal PO2 so much reduced during exercise?Methods proved to be important. It took chronically instrumented animals accustomed to the laboratory environment, experiments standardized to fitness of the individual (%VO2max), measurement of total uterine blood flow, and blood gas values corrected for body temperature. The results were simple and hold till this day. Uterine blood flow decreases linearly with maternal heart rate increase, which depends on exercise intensity and duration. Maximal reduction in uterine blood flow is ~20 % and uterine O2-uptake remains unaltered because blood flow reduction is compensated by increases in hematocrit and uterine O2-extraction. Fetal body temperature increases with that of the mother by ~2 °C at maximal exercise and fetal blood gas values are little affected by exhaustive maternal exercise, if properly corrected for temperature. So I left Loma Linda knowing that pregnant sheep can exercise to exhaustion without harm to the fetus, thanks to effective compensatory mechanisms.After returning to Erasmus University Rotterdam further studies in humans showed that physical fitness is unaffected by pregnancy, weight-gain affects performance, and strenuous exercise in healthy pregnant women does not harm the fetus. Thus, the millennia-old perspective has changed.