Pregnancy is the unique vital process in which a woman’s body control systems are modified to preserve the life of the fetus during its growth and development. Throughout 38 weeks of gestation, a women’s body constantly changes to maintain body balance and biological homeostasis and deliver the baby without maternal or fetal complications. Any disarrangement or imbalance can adversely affect pregnant outcome.

But why should we exercise during pregnancy? Is there scientific evidence for a beneficial effect of regular physical activity during the course of pregnancy?

Despite pregnant woman being denied the possibility of an active gestation period in the past, there is growing evidence from observational and intervention studies that a woman with no obstetric complications may exercise during pregnancy without any detrimental impacts on maternal–fetal outcome. Fortunately, there is a current tendency toward regular physical activity during pregnancy, and more importantly, physical activity is today considered an important part of routine daily life and the pregnancy period is no exception.

Today’s concept of health has changed from the old idea of “absence of disease” to a wider concept that, besides the proper functioning of vital organs (cardio-circulatory, metabolic and hormonal functions), also considers psychological well-being and affective-social human behavior.

A sedentary lifestyle can lead to many health complications, and this also applies to pregnant women. Even Aristotle mentioned that “Difficult deliveries are due to a sedentary lifestyle” (Aristotle, C. III b.c.). Indeed, exercise is now known to benefit many aspects of a pregnant woman’s health.

For a new pregnancy not to reduce quality of life, we therefore need to find (among the many options that physical activity offers) a way of avoiding a sudden decline in physical activity.

Two basic facts should be taken into account:

• increased requirements due to fetal development; and
  
• the upward movement of certain structures as a consequence of the increased size of the uterus.

From the start of the first trimester until the end of pregnancy, cardiac output (the product of stroke volume and heart rate) increases by 30% to 40% as a result of an increased heart rate (e.g., from 70 beats/min before pregnancy to 85 beats/min in late pregnancy) and a slight increase in stroke volume.

Peripheral vascular resistance diminishes, and this leads to a slight drop in blood pressure. Diastolic blood pressure decreases in the first and second trimester and returns to prepregnancy values in the third trimester. Systolic blood pressure is modified slightly, with a tendency to decrease in the first and second trimester.

Blood volume increases by 45% (∼1800 mL), and this is accounted for by both an increased plasma volume (∼1500 mL) and an increased red cell volume (∼350 mL). This “heme dilution” maintains adequate uteroplacental flow.

Changes in the respiratory system cause anatomical and functional alterations. These modifications occur early during pregnancy owing to hormonal effects and the above-mentioned increments in blood volume, and include changes in lung dimensions and capacity as well as inspiratory/expiratory mechanisms.

Reduced functional residual capacity accompanied by increased oxygen consumption diminishes the oxygen reserve. The oxygen cost of ventilation also rises owing to an increased workload on the diaphragm. Estrogens and progesterone cause an increase in lung ventilation which gives rise to respiratory alkalosis. However, the acid–base balance is maintained by a compensatory metabolic acidosis mechanism and blood pH values remain around 7.44.

Most of these maternal breathing mechanisms are targeted at reducing arterial PCO₂ to create a mild alkalosis. This ensures maternal placental gas exchange and prevents fetal acidosis.

During pregnancy, normal metabolic processes are altered to accommodate the needs of the developing fetus. The protein contents of the body tissues increase. Carbohydrates accumulate in the liver, muscles, and placenta. In addition, fat deposits appear under the skin, especially affecting the breasts and buttocks, and this is frequently accompanied by increased blood lipid levels, including cholesterol. The pregnant woman’s body accumulates the salts of several minerals that are essential for normal fetal development, that is, calcium, phosphorus, potassium and iron. In addition, hormonal changes promote water retention in the tissues.

The heart rate in pregnant women is significantly higher during exercise compared with nonpregnant women. For stroke volume, similar results have been reported in pregnant and nonpregnant women. Cardiac output increases peaking between 20 and 24 weeks of gestation and thereafter dropping gradually until the end of pregnancy. This decline in the later stages of pregnancy may be due to changes in peripheral vascular resistance and mechanical obstruction of venous return generated by the pressure of the pregnant uterus.

Exercise modifies cardiac output by increasing blood flow to active muscle areas at the expense of decreasing flow to other areas such as the uteroplacental unit. Moderate-intensity physical activity causes a reduction in uteroplacental blood flow of ∼25%, the magnitude of this reduction being more marked at higher intensities. However, the hypothetical risks posed by exercise blood redistribution are offset by maternal–fetal mechanisms, which ensure maternal and fetal well-being during moderate-intensity aerobic exercise. Particularly, the greatest blood flow reduction occurs in the uterine wall so that the flow of oxygen and nutrients through the placenta is adequate, thus maintaining fetal homeostasis.

In summary, cardiovascular changes caused by moderate-intensity physical activity during pregnancy do not pose a health risk to the mother and fetus in a healthy pregnancy.