Third Trimester
By the third trimester, most physiological adaptations have peaked. Running will be impacted by the decrease in these adaptations and by your enlarging uterus, weight gain and the hormones that prepare your body for delivery. Although total blood volume continues to expand, by the third trimester the increase in heart rate has plateaued and stroke volume and cardiac output begin to decrease.
The enlarging uterus continues to push on the diaphragm, causing a decrease in vertical chest height. As a result, the volume of air that remains in the lungs after each exhalation falls considerably in late pregnancy. This causes a decrease in oxygen reserve. Higher intensity and prolonged running are more challenging.
Only some women are able to run through the end of their pregnancy. Any running you do in the third trimester should be done in moderation. It's important to stay hydrated and cool. While the fetus will be buoyed by the movement of the uterus and amniotic fluid with running, consider reducing the impact if you're experiencing low back, hip or other joint pain.
Research indicates that continuing with some form of exercise throughout pregnancy is beneficial. The better shape you're in before delivery, the easier it will be to get back in running shape after your baby is born. However, the third trimester is often a time to alter your running program. Recognize that every runner is different and alterations that you make to your running depend on how you feel. Many women runners cut down on the frequency and duration of their running. Others substitute non weight-bearing activities. If you're able to run during the third trimester, limit your runs to 30 to 60 minutes at a moderate intensity.
If you can't run, stationary cycling or swimming are good alternatives and can be substituted using the same frequency, intensity and duration. Swimming is a particularly good alternative to running, as the buoyancy of the water decreases forces on joints, minimizes heat stress and controls leg swelling.
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