Bodily Changes During Pregnancy Health Article

The hormonal and physiologic changes during pregnancy are unique in the life of women.

Hormonal Changes

Estrogen and progesterone are the chief pregnancy hormones. A woman will produce more estrogen during one pregnancy than throughout her entire life when not pregnant. Her progesterone levels also are extraordinarily high. The changes in progesterone cause a laxity or loosening of ligaments and joints throughout the body. In addition, high levels of progesterone cause internal structures to increase in size, such as the ureters (which connect the kidneys with the maternal bladder). Progesterone also is important for transforming the uterus from the size of a small pear in its non-pregnant state to a uterus that can accommodate a full-term baby.

While these hormones are absolutely critical for a successful pregnancy, they also can make exercise more difficult. Pregnant women may be at greater risk for sprains and strains because the ligaments are looser (however, no studies have documented an increased rate in injury during pregnancy). A pregnant woman's entire posture changes. Her breasts are larger, and her abdomen transforms from flat or concave to very convex, increasing the curvature of her back. The combined effect shifts the center of gravity forward and may lead to changes in one's sense of balance.

The weight gain in pregnant women increases the workload on the body from any physical activity. This additional weight and gravity slow down the circulation of blood and body fluids, particularly in the lower limbs. As a result, pregnant women retain fluids and experience swelling of the legs, hands, and even face. This water weight is another limitation to exercise.

The increase in weight is usually the primary reason that the body cannot tolerate pre-pregnancy levels of exercise, even for the seasoned, elite, or professional athlete. Round ligament strain, increased size of the uterus, and pelvic instability from laxity of the ligaments may lead to increased discomfort during exercise in the pregnant woman.

Tip: For fun, take a photograph of yourself from the side profile early in your pregnancy, using your best posture. Take another photo near your due date and compare these side profiles. The changes are remarkable, aren't they?

Circulatory System Changes

During the second trimester of pregnancy, the mother's heart at rest is working 40% harder. Most of this increase results from a more efficiently performing heart, which ejects more blood at each beat. The heart rate does not generally increase and remains stable during pregnancy.

But, there are two types of circulatory changes that may have an impact on exercise during pregnancy. Pregnancy hormones can suddenly affect the tone in blood vessels. A sudden loss of tone may result in the feeling of dizziness and perhaps even a brief loss of consciousness (passing out), because the loss of pressure sends less blood to the brain and central nervous system. Additionally, vigorous exercise may lead to decreased blood flow to the uterus while diverting blood to muscles; however, this has not been shown to have a long-term impact on the baby. Furthermore, there is evidence to suggest that individuals who exercise have improved blood supply to the placenta at rest, which may be beneficial to placental and fetal growth.

Another form of dizziness can result from lying flat on the back. This dizziness is more common after 24 weeks, but can happen earlier during multi-fetal pregnancies or conditions that increase amniotic fluid. Lying flat on the back compresses the large blood vessel (vena cava) leading from the lower body to the heart, decreasing blood flow to and from the heart. This leads to a sudden and dramatic decline in blood pressure-and possibly dizziness or loss of consciousness. Understandably, after the first trimester, it is not recommended to do exercises where you are lying on your back due to the impact from blood vessel compression.

Women experiencing any of these conditions, particularly during exercise, should consult their doctor.

Respiratory Changes

During pregnancy, the amount of air moved in and out of the lungs increases by nearly 50% due to two factors. Each breath has more volume of air, and the rate of breathing increases slightly. As the uterus enlarges, the movement of the diaphragm may be limited; therefore, some women report the feeling of increased difficulty in taking deep breaths. Even without exercise these changes may cause shortness of breath or the feeling of being "air hungry." Exercise programs may increase these symptoms.

Overall, pregnant women have higher blood oxygen levels. Studies have shown that pregnant women consume 10 to 20% more oxygen at rest. This does not seem to have an impact on the amount of oxygen available for exercise-or work-during pregnancy.

Body Temperature Changes

Heat stress during exercise creates concern for two reasons. First, an increase in the mother's core temperature (hyperthermia) can be harmful to the baby’s development. Second, loss of water in the mother (dehydration) can decrease the amount of blood available to the fetus and lead to increased risk of preterm contractions.

Hyperthermia

In non-pregnant women, moderate aerobic exercise causes significant increases in core body temperature. Pregnant women, whether they exercise or not, experience an increase in base metabolic rate and core temperature. Fortunately, pregnant women regulate their core temperature very efficiently. Increased blood flow to the skin and the expanded skin surface release increase body heat. It has been shown that pregnant women do not have as much or an increase in body temperature during exercise as those who are not pregnant. However, it is recommended that pregnant women avoid exercising in non-breathable clothing and to avoid exercising when it is very hot or humid since the impact of hyperthermia can be severe. Using fans during indoor activity, swimming, and wearing light colored, loose fitting clothing may all help in dissipating heat during exercise.

Dehydration

Most women who exercise for 20 to 30 minutes or who exercise during hot and humid weather will sweat. In pregnant women, loss of body fluids from perspiration can decrease the blood flow to the uterus, the muscles, and some organs. Because the developing fetus needs a constant supply of oxygen and nutrients carried through the blood, increased injury may result.

In most conditions, uterine oxygen consumption is constant during exercise and the fetus is safe. However, exercising can be dangerous for women with pregnancy-induced hypertension, which limits uterine blood volume as the vessels clamp down and deliver less blood to the area.

If you are cleared for exercise during pregnancy, be sure to use common sense:

Indoors/Outdoors

• Avoid outdoor exercise during excessive heat and humidity; go indoors to a controlled environment.
• Workout in areas with excellent air circulation; position a small fan near your equipment.
• Swimming is an excellent form of exercise since body heat is easily dissipated.

Fluids

• Drink plenty of fluids during exercise, both indoors and outdoors.
• Always have handy a bottle of water or containers of decaffeinated beverages, diluted fruit juices, or diluted sports drinks.
• Drink and rehydrate even if you are not thirsty.

Clothing

• Wear light clothing that breathes.
• Always wear a good exercise support-bra.

General

• Avoid saunas and steam rooms.
• Limit hot tubs and whirlpools to only a few minutes-and only after you've cooled down from your primary exercise routine.
• Don't exercise when you are sick or have a fever.

Go to Who Shouldn't Exercise in Pregnancy, Create a Program in Pregnancy, Postpartum Care: Tips for After Delivery, and Endurance Exercises in Pregnancy.