Mothers in Motion


By Lisa Unger


Tiny Cells…Enormous Changes

Robin Nagle is a writer for Running Times magazine who also runs and is the mother of a 5-year old. Robin recalls that in the days before she learned that was pregnant she noticed her resting pulse was slightly elevated and then she was not able to perform as anticipated in the 1998 NYC Marathon. Even though she was only 10 days pregnant when she ran the marathon, she had already started to notice changes in her body that effected her pulse, breathing, oxygen delivery efficiency and her emotions. (She notes an unexpected outburst of tears upon completing the marathon in what was to her a disappointing time.) There are sound medical reasons for the feelings that Robin experienced.

Changes that happen to a pregnant runner are intense, on-going and begin at the moment of conception. As soon as the egg implants on the uterine wall, hormones are released that cause the blood vessels to begin to expand and relax. This results in less blood moving into and out of the heart. The heart and adrenal glands respond by sending different hormones to the kidneys instructing them to take less salt and water out of the blood than usual which helps to restore blood volume. This process continues throughout pregnancy until the mother’s blood volume will have increased by up to 50%. This greater quantity of blood requires the heart rate to increase as the amount of blood pumped per heart beat increases. Primarily effected by these changes are the heart, skin, kidneys and reproductive organs. Thermoregulation is aided as the skin receives greater blood flow. This means that a pregnant woman will start to sweat sooner and in greater volume than pre-pregnancy as her body regulates its core temperature. Oxygen exchange efficiency also increases which helps to further regulate body temperature. The growing fetus and placenta will eventually crowd the bladder, diaphragm and lungs, however the body will compensate and by the last weeks of pregnancy a woman may have up to 30% greater aerobic capacity than normal.

Pregnant runners may find themselves heating up faster than usual because of their growing weight and increased sensitivity to environmental stimuli like temperature changes. Early tests on animals and experience with human mothers who had suffered high fevers during pregnancy suggest that early utero exposure to extreme heat can cause neural tube defects, leading to serious problems like spina bifida. Dr. James Clapp, III, professor of reproductive biology at Case Western Reserve and his colleague Raul Artral found that pregnancy provided a remarkable counter-adaptation. While pregnant women heat more quickly, they also dissipate heat more efficiently. In fact, Dr. Clapp and Mr. Artral found that core body temperatures actually declined during pregnancy and that exercise does not heat a mother or her baby to a danger zone.

Another hormonal change that occurs in pregnant women is the release of relaxin. This hormone causes ligaments and tendons to become more elastic to prepare the body for bearing the extra weight of the baby as well as the pelvic stretching that will happen during childbirth. Relaxin will make the joints loser and can also cause balance issues, though the effects may not become noticeable until later in pregnancy. A pregnant runner feeling the effects of relaxin may benefit from prenatal massage or use an elastic maternity belt for abdominal and back support.

It’s amazing that these monumental body changes begin with the merging of two tiny cells and keep going even after the baby is born. A pregnant woman should embrace these changes, for they happen only during this very special time.