(Extract from the Level 3 Pre and Post Natal Exercise Course)

“The effects on the body’s systems;

The mother and baby’s circulations are separate but inter-related through the placenta; therefore, anything affecting the mother’s circulation will in turn affect that of the fetus. This includes lifestyle factors such as smoking, alcohol, drugs and exercise.

Cardiorespiratory adaptations to exercise

Many of the changes that occur within the cardiovascular system during pregnancy are almost identical to many of the training effects incurred through elite athletic training. The efficiency of the cardiovascular system is actually enhanced naturally through pregnancy, and it has been known for athletes to capitalize on these benefits by returning to intense training early in the postnatal period. 

Although this is certainly not recommended for the general exerciser, it is valuable to note that the cardiovascular system is actually increasing its capacity during pregnancy, and not, as many women feel, contributing to a reduction in their overall fitness. To verify this information, studies have reported that for the unfit, pregnancy may be viewed as a useful form of progressive resistance training (Sharp, 1993).

The cardiovascular system

The main physiological adaptations to the cardiovascular system are:

• Mother and baby circulation
• Blood volume

During pregnancy, maternal blood volume increases by approximately 30% near term(however, these increases can range between 15 and 50%). The water (plasma) content of the blood increases and the blood becomes more diluted with fewer red blood cells, resulting in less oxygen being carried around the body. This is known as physiological (ie, a natural) anaemia and partly accounts for the tiredness many women feel in pregnancy. Red blood cell counts are generally well monitored during pregnancy, and any problems of anaemia (in addition to the natural physiological anaemia) are generally addressed through the prescription of iron

• Stroke volume

Oestrogen encourages the laying down of some extra muscle tissue within the heart, increasing its strength and its ability to pump out more blood at each stroke. Stroke volume and consequently cardiac output gradually increase during pregnancy until the third trimester, while the heart can increase in size by a third.

• Cardiac output

Although cardiac output generally increases in pregnancy, changes in the position of the body (standing still or lying on back [supine]) may markedly affect blood circulation, and hence cardiac output, by interfering with venous return.

The respiratory system

• Carbon dioxide

The changes in the respiratory system during pregnancy are related to the effect of progesterone on the respiratory centre in the brain, which becomes more sensitive to carbon dioxide. Carbon dioxide influences the respiratory centre, which controls ventilation (ie, breathing rate and depth). This could be one of the reasons for the breathlessness often experienced by women on exertion from early pregnancy onwards.

• Oxygen uptake

The oxygen requirement gradually increases by 20% to accommodate the additional needs of the pregnant woman and the growing fetus.

The effects of exercise on the fetus

There are still many unanswered questions relating to the effects of exercise on the fetus. However, it is important to understand some general facts about fetal development and the implications there may be for exercise. In this way, we can use our present knowledge to give safe advice to a mother—to—be about exercise during pregnancy.

Fetal blood flow

There have always been reservations that blood flow to the uterus could be reduced during exercise because of the vascular shunt that redistributes blood to the working muscles and away from the abdominal organs. However, it is now generally accepted that during moderate exercise the oxygen supply to the fetus is not restricted. The intensity and duration of the exercise are both significant factors. Yoga practices that are correctly modified should not be a factor in reduced fetal blood flow. Gentle postures that flow in and out of preparatory to modified full versions are prescribed here.

Fetal heart rate

The fetal heart rate is approximately 140bpm, and it is generally felt that exercise at a moderate pace has no significant effect on this. However, it has been shown that during prolonged exercise (over 30 minutes), fetal heart rate begins to rise, possibly indicating that oxygen supply to the fetus is being affected. This also may be a protective mechanism, as the increase in heart rate allows the fetus to circulate more blood and thereby increase oxygen availability. To compensate for any slight reduction in blood flow to the uterus, the pregnant uterus is known to increase the extraction of oxygen from the blood. During prolonged exercise, it has been found that in a few women bradycardia occurs. This would only become a serious concern if the fetal heart rate fell to 100 bpm or below. Conclusions drawn from studies undertaken are to avoid high-intensity or prolonged cardiovascular training sessions.”