怀孕期间的身体变化

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Changes within the body during pregnancyThe Musculoskeletal System:

During pregnancy there is an evident change in the physical appearance of the women’s body. The most evident change is the substantial weight gain, anywhere from twenty to forty pounds depending on the individual (Artal and O'Toole, 2003; Paisley et al, 2003). During the conception period the uterus can expand up to 1000 times, causing an anterior orientation of the uterus (Hartmann and Bung, 1999). The development of the uterus leads to the visible ‘waddling’ way of walking during the last trimester. The growth of the uterus causes the body to respond with a shift in the center of gravity. The center of gravity is shifted back over the pelvis in response, preventing women from falling, the body's safety mechanism. Additionally the center of gravity usually shifts higher, with this there is an increased strain on the muscles and ligaments supporting the vertebral column (Wang and Apgar, 1998). The change in the uterus size, leads to the change in center of gravity, leads to in an increased strain on the back muscles, hence, the cause of low back pain experienced by most pregnant women. This musculoskeletal change means pregnant women face difficulty with balance throughout term. Hyperlordosis is seen resulting in progressive lumbar lordosis and rotation of the pelvis on the femur (Hartmann and Bung, 1999). This causes an increase in the anterior flexion of the cervical spine (a hunchback appearance) and adduction of the shoulders (rounding of the shoulders). There is also an increase in the laxity of ligaments throughout the body, causing a decreased stability of the joints (Hartmann and Bung, 1999; Wang and Apgar, 1998).

With these changes in mind it is important pregnant women are aware their balance can be compromised. Care should be taken when choosing the exercises to include in a pregnant women's exercise protocol. Taking into account the increased strain from the musculoskeletal changes taking place due to the growing uterus, the exercises chosen should not add additional unneeded stress on the low back.

Cardiovascular Adaptations

There are changes with the circulatory system apparent at rest during pregnancy: increases in blood volume, cardiac output, heart rate, and stroke volume, as well as a decrease in arterial blood pressure (Hartmann and Bung, 1999; O'Toole, 2003; Wang and Apgar, 1998). The increase in blood volume, as much as 45%, allows adequate blood to be transported to the uterus for proper development of the fetus (Paisley et al., 2003). Heart rate increases during pregnancy; there is as much as a 20% increase in heart rate during the second and third trimesters; early on there is an increase of around 7 beats per minute, later the increases reaches up to a 15 beat per minute increase (Wang and Apgar, 1998; O'Toole, 2003; Paisley et al., 2003). Stroke volume increases as much as 10% by the end of the first trimester. With the increase in heart rate and stroke volume there is an increase in cardiac output, reaching as high as 30 to 50% (Lumbers, 2002; Paisley et al., 2003). Cardiac output and heart rate begin to slowly increase in the sixth week and peak at the end of the second trimester.

The knowledge of the cardiovascular adaptations is especially important when dealing with body position during exercise. The supine position should be avoided after the first trimester; this position results in a relative obstruction of venous return and therefore a decrease in cardiac output (Hartmann and Bung, 1999; Wang and Apgar, 1998; Clark et al., 1991; ACOG, 2002). Additionally, continuous motionless standing should be avoided; motionless standing results in blood remaining in the legs, resulting in a lower blood pressure and a significant decrease in cardiac output (Hartmann and Bung, 1999; Wang and Apgar, 1998; Clark et al., 1991; ACOG, 2002). These changes are important to understand and keep in mind during exercise prescription design as well as daily living. A pregnant women should avoid standing for extended periods during exercise, leisure, as well as on the job.

Respiratory Adaptations

There are profound changes with the respiratory system in women during pregnancy. The adaptations are controlled primarily by progesterone and take place in the early stages of pregnancy (during the 7 th week) (Hartmann and Bung, 1999). The vital capacity remains normal. There is an increase in tidal volume (almost 50%), which causes an increase in minute ventilation throughout pregnancy (21% and 50% in the second and third trimesters respectively) (Artal et al., 1986; O’Toole, 2003). The enlarged uterus occurring later in gestation increases the pressure on the diaphragm during pregnancy causing an increase in resting oxygen requirements and an increase in the work required for breathing (Hartmann and Bung, 1999; Wang and Apgar, 1998). This happens because the diaphragm is a key muscle responsible for proper respiration, it helps to inflate the lungs; with extra pressure from the uterus it is harder for the diaphragm to contract, using more energy to bring in the same amount of air. This means there is a decreased amount of oxygen available for performance during aerobic exercise.

These changes are important to understand while starting an exercise protocol during pregnancy. With the increased resting oxygen requirements, which will increase throughout term, it is harder to perform at the same level pre-gestation. Towards the end of term, one may not be able to maintain the same amount of intensity or work level as during the start of gestation.

Temperature Adaptations

With pregnancy comes and increase in the metabolic rate resulting in greater heat production. The greater metabolic rate also calls for a higher need for energy, or higher food consumption. The fetus metabolic rate generates additional heat; the fetus is thus at a temperature 0.5 to 1.0 ºC (0.9 to 1.8 ºF) above the mother’s levels (Hartmann and Bung, 1999; Wang and Apgar, 1998). The mother’s temperature decreases 0.3 ºC (0.5 ºF) in the first trimester and decreases 0.1 ºC (0.2 ºF) per month through gestation helping to prevent a mother’s core temperature from rising with the increased metabolic rate (Wang and Apgar, 1998). The physiologic changes (increased skin blood flow) occurring during pregnancy help to produce the reduced maternal body temperature (Hartmann and Bung, 1999; Wang and Apgar, 1998).

With this increase in the resting metabolic rate of women during pregnancy there comes an increased need for energy to allow for proper growth of the fetus throughout term; after the 13 th week of pregnancy roughly 300 calories extra are needed per day to meet the needs of pregnancy (Wang and Apgar, 1998). Additionally with the increased resting body temperature of the mother caused by the fetus, it is important to exercise in a well ventilated or air-conditioned area. It might be in the interest of the mother to place a fan in the room during exercise to provide for extra air circulation; exercising outside during the hottest part of the day should also be avoided. With the increased temperature, it is crucial to the mother stay hydrated throughout gestation (Soultanakis-Aligianni, 2003; Wang and Apgar, 1998). This means consuming water throughout the day and not using thirst as sign to drink fluids; thirst is a sign you are already dehydrated! Also remember caffeine (cokes and coffee), is a diuretic.

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