How does the body change in pregnancy?

Early in pregnancy, women are often surprised to learn of the variety of physiologic changes which take place almost immediately after a women becomes pregnant. The hormones increased during pregnancy, primarily Progesterone and Estrogen, cause a variety of physiologic changes which become increasingly more prominent as the pregnancy continues.

One of early changes is that of water retention and venous dilation. The body, in preparation for the need of extra blood volume to the growing uterus, begins to retain water and make more red cells. The general water retention leads to breast tenderness and dilation of all veins. Some patients can develop significant discomfort from the dilated veins in the lower half of the body, interfering with exercise and even normal daily activity. These women may benefit significantly from hydro-therapy. Swimming and gentle exercises in a swimming pool helps reduce the side effects of vein dilatation by pushing fluids back into the body's trunk and core. Many patients will notice dramatic improvement in the swelling of the feet, legs, and perineal areas after even short periods of pool therapy.

Another early change is an often dramatic drop in blood pressure and increase in basal heart rate. Women whose baseline pulse may be 70-80 when not pregnant may find themselves with a pulse of over 100 when at rest during pregnancy. Again, the drop in vascular resistance caused by progesterone leads to an increase in pulse, and decrease in blood pressure. A woman now becomes increasingly more sensitive to dehydration, and lack of proper fluid replacement can lead to dizziness and fainting. These feelings of "light headedness" can be further aggravated by dropping blood sugars, and carrying around a supply of water and quick snacks during the hot summer months is a good habit for the pregnant woman.

As the pregnancy continues, blood pressures continue to drop until weeks 32-34. At this gestational age the blood pressure reaches its lowest point, then pressures typically begin to rise slowly until delivery.

Another normal condition in the pregnant woman is anemia. Although the body will attempt to fill the new and dilated vessels with blood, the maternal bone marrow can never generate enough red cells to meet the needs of the increasing blood volume required. As a result, there is a normal "physiologic anemia" which all pregnant women experience. The body is able to "fill the blood vessels" with fluid, but can not quite meet the demands for red cell generation. This results in a dilutional effect with blood tests in pregnancy always showing an "anemia" when compared to non-pregnant values.

As the weight of the fetus increases, this has two important effects on the physically fit patient. First, the woman's center of balance will be dramatically changed. The weight increasing in the front of the spine causes muscles along the lateral borders of the spine to contract to compensate, often leading to back pain and back tension. A gentle program of stretching of the back muscles and massage therapy can often help alleviate this discomfort. Activities that rely heavily on balance often must be decreased or abandoned. This makes sports like tennis extremely difficult for pregnant women approaching the third trimester.

What about running and jogging? Many patients that are avid runners before becoming pregnant would like to continue running through their pregnancy. In the majority of cases, some running can be continued throughout part of the pregnancy. Concerns arise because the arteries that carry blood to the muscles of the lower extremities also carry the blood to the developing fetus. Extremes such as marathon running can lead to insufficient blood flow to the fetus, sometimes causing growth restriction and significantly affecting the pregnancy. The fetus essentially receives the "left over" blood flow, and extreme exhaustion or "pushing" the limits is not recommended. In summary, a moderate exercise program is encouraged. Professional athletes who become pregnant can sometimes continue fairly aggressive training schedules with the close supervision of a physician and frequent fetal surveillance. These are rare cases that require very special attention. In general, because there is so much variation in what patients consider a "tough workout", it is recommended that each individual decrease what their pre-pregnancy workout routine included by about 50%. Monitoring pulse levels is a useful guide in gauging the degree of exercise, and is addressed below. It is interesting to know that because the estrogens of pregnancy help loosen the ligaments of the body, many professional runners will find their times significantly improved when they return to competitive running post-partum!

Why am I not supposed to lie on my back when I'm pregnant?

Another commonly known precaution for the pregnant woman is that of taking care to not lay on her back for prolonged periods of time. As a woman enters the last half of her pregnancy, she will begin to feel increasingly more uncomfortable when laying on her back or reclined without turning to the side.

The physiologic explanation for this is straight forward. The major blood vessels running to and from the heart (the Vena Cava and Aorta) run along the right side and just anterior to the spine. As the weight of the growing uterus and fetus increase, the pregnancy will begin to compress on these vessels whenever a pregnant woman lays on her back. This has several direct effects. First, the weight compresses the large vein returning blood to the heart (the Vena Cava). This can partially or completely occlude the Vena Cava, thereby halting all return blood flow to the heart. The heart, as an empty pump, can not produce any blood pressure, and suddenly the woman will feel extremely faint and nauseated. These are direct effects of the hypotension produced by phenomenum.

As the weight of the fetus continues to increase, the Aorta may also be compressed, thereby occluding the outflow. This suddenly and artificially increases the blood pressure sensed by the heart, and the body responds by slowing down the heart. This again leads to extreme "light headedness", and can produce syncope (fainting).

Turning to one side or the other immediately relieves this compression and restores normal blood pressure. Pregnant women are usually told to lay on their "left side" because this offers more protection against this phenomenum. The reason the left is better than the right side is because the vessels are primarily on the right, and because gas in the colon on the left side helps keep the pressure of the uterus off the underlying structures!

Exercise goals in pregnancy

The goal when exercising in pregnancy should be to do enough to produce a feeling of "well being" in the mother, but not so much so as to detract from normal fetal growth. Although this sounds simple, the exact amount of activity tolerated by each maternal-fetal couple, will vary tremendously.

Some general recommendations

Renal Disease, Pulmonary Disease, Heart Disease, or any major medical problem.

Of course, if you are unsure about the safety of any planned activities, always consult a physician for your own specific recommendations. Remember, every pregnant woman is different, and every pregnancy is different!

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