The hormonal and physiologic changes during pregnancy are unique in the life of women.
Pregnant women experience sudden and dramatic increases in estrogen and progesterone, as well as changes in the amount and function of a number of other hormones. These changes can not only affect mood, they can also create the “glow” of pregnancy, significantly aid in the development of the fetus, and alter the physical impact of exercise and physical activity on the body.
Estrogen and Progesterone 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. The increase in estrogen during pregnancy enables the uterus and placenta to improve vascularization, transfer nutrients, and support the developing baby. In addition, estrogen is thought to play an important role in helping the fetus develop and mature. Estrogen levels increase steadily during pregnancy and reach their peak in the third trimester. The rapid increase in estrogen levels during the first trimester may cause some of the nausea associated with pregnancy and, during the second trimester, plays a major role in the milk duct development that enlarges the breasts.
Progesterone levels also are extraordinarily high during pregnancy. 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.
Pregnancy Hormones and Exercise Injuries
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 ankle or knee 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.
Weight Gain, Fluid Retention, and Physical Activity
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, feet, hands, and even face. This water weight is another limitation to exercise.
Many women begin to notice slight swelling during the second trimester, which often continues into the third trimester. This increase in fluid retention is responsible for about 25% of the increase in weight women experience during pregnancy. It can be alleviated in some cases through rest, avoiding long periods of standing, avoiding caffeine and sodium, and increasing dietary potassium.
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?
Because of the hormonal changes that begin in the first trimester, many physiological changes take place throughout the body. These changes help prepare the mother’s body for pregnancy, childbirth, and breastfeeding. In addition, they can dramatically alter how a woman experiences the world through sight, taste, and smell.
Hair and Nail Changes
Many women experience changes in hair and nail growth during pregnancy. In some cases, hormone changes can cause excessive hair shedding or hair loss (especially in women with a family history of female alopecia). Many women experience hair growth and thickening during pregnancy, however, and may even notice hair growth in unwanted places. Hair growth on the face, arms, legs, or back can occur and can be removed cosmetically. Most changes in hair growth return to normal after the baby is born; it is common, however, for hair loss or increased shedding to occur in the first few months or first year postpartum as hair follicles and hormone levels regulate themselves without the influence of pregnancy hormones.
Many women also experience faster nail growth during pregnancy. Although some women may find the change desirable, many may notice increased nail brittleness, breakage, grooves, or keratosis. Healthy dietary changes to increase nail strength can help prevent breakage without the use of chemical nail products.
Pregnant women’s breasts often undergo a series of significant changes during pregnancy as their bodies prepare to supply milk to the newborn baby. Pregnancy hormones that affect skin pigmentation often darken the areola, or the darker skin around the nipple. As the breasts grow, pregnant women may experience tenderness or sensitivity and notice that the veins are darker and the nipples protrude more than before pregnancy. Some women may develop stretch marks on the breasts, particularly if they undergo rapid growth, and many women will notice an increase in size of the nipple and areola.
Small bumps on the areolas often appear, and most women will begin producing (and sometimes “leaking”) small amounts of a thick, yellowish substance known as colostrum during the second trimester. In addition to producing the colostrum for the baby’s first feeding, milk ducts in the breasts expand to prepare to produce and store milk. Some women may notice small lumps in the breast tissue, which can be caused by blocked milk ducts; if the lumps do not disappear after a few days of massaging the breast and warming it with water or a washcloth, a doctor should examine the lump at the next prenatal visit.
The cervix, or the entry to the uterus, undergoes physical changes during pregnancy and labor. In many women, the tissue of the cervix thickens and becomes firm and glandular, and the cervix produces a thick mucus plug to seal off the uterus. The plug is often expelled in late pregnancy or during delivery. Up to a few weeks before giving birth, the cervix may soften and dilate slightly from the pressure of the growing baby. Prior to delivery, the cervix dilates significantly, softens, and thins, allowing the baby to pass through the birth canal.
Some women experience vision changes during pregnancy, characterized by increased myopia (nearsightedness). Although researchers do not know the precise biological mechanisms behind changes in vision, most women return to pre-pregnancy vision after giving birth. Common changes in vision during pregnancy include blurriness and discomfort with contact lenses. Pregnant women often experience an increase in intraocular pressure, and women experiencing preeclampsia or a diabetic pregnancy may be at an elevated risk of rare eye problems, such as retinal detachment or vision loss.
Taste and Smell Changes
Most women experience changes in their sense of taste during pregnancy, preferring saltier foods and sweeter foods than non-pregnant women and having a higher threshold for strong sour, salty, and sweet tastes. Dysgeusia, a decrease in the ability to taste, is most commonly experienced during the first trimester of pregnancy. Certain taste preferences may vary by trimester, and although many women experience a dulled sense of taste for a short period of time postpartum, women typically regain full gustatory capability after pregnancy. Some women also experience a metallic taste in the mouth during pregnancy, which can aggravate nausea and may indicate a nutrient imbalance.
At times, pregnant women also report changes in their sense of smell, describing a heightened awareness and sensitivity to a variety of odors. Although there is little consistent and reliable data indicating that pregnant women actually notice and identify certain odors and intensity of odors more than non-pregnant counterparts, the vast majority of pregnant women report a perceived increase in their own sensitivity to odors.
Many women will experience changes in the physical appearance of their skin during pregnancy. Although most are temporary, some – like stretch marks – can leave permanent changes in the skin. In addition, women who experience some of these skin changes during pregnancy are more likely to experience them again in future pregnancies or even while taking hormonal contraceptives.
Mask of Pregnancy and Hyperpigmentation
The vast majority of pregnant women experience some type of hyperpigmentation during pregnancy, which consists of a darkening in skin tone on body parts such as the areolas, the abdominal linea alba, genitals, and scars. Hyperpigmentation can occur in women of any skin tone, although it is more common in women with darker complexions.
In addition, up to 70% of pregnant women experience a darkening of skin on the face, known as the “mask” of pregnancy, or melasma. This skin condition can be worsened by sun exposure and radiation, so a broad spectrum UVA/UVB sunscreen should be used daily during pregnancy. In most cases, melasma resolves after pregnancy.
Perhaps the most well known skin change of pregnancy, stretch marks (or striae gravidarum) are caused by a combination of physical stretching of the skin and the effects of hormone changes on the skin’s elasticity. Up to 90% of women develop stretch marks by the third trimester of pregnancy, often on the breasts and abdomen. Although the pinkish-purple stretch marks may never fully disappear, they often fade to the color of surrounding skin and shrink in size postpartum.
Mole and Freckle Changes
The hyperpigmentation caused by changes in hormones during pregnancy can cause changes in the color of moles and freckles. Although some darkening of moles, freckles, and birthmarks can be totally benign, it is always a good idea to see a dermatologist or physician about changes in size, color, or shape. Pregnancy hormones can also cause the appearance of dark patches of skin that are often unpreventable. Although most skin pigmentation changes will fade or disappear after pregnancy, some changes in mole or freckle color may be permanent. Regardless of the type of change in skin pigmentation, it is a good idea to have a skin check for potential skin cancer or pregnancy-specific skin conditions during pregnancy if any changes occur.
Pregnancy-Specific Rashes and Boils
Small percentages of women may experience skin conditions that are specific to pregnancy, such as PUPPP (pruritic urticarial papules and plaques of pregnancy) and folliculitis. Most conditions involve pustules and red bumps along the abdomen, legs, arms, or back. Although most rashes are harmless and resolve quickly postpartum, some skin conditions (such as intrahepatic cholestasis and pemphigoid gestationis) may be associated with premature delivery or problems for the baby.
Circulatory System Changes
Huffing and puffing while climbing stairs, feeling dizzy after standing quickly, and experiencing changes in blood pressure are common during pregnancy. Because of rapid expansion of the blood vessels and the increased stress on the heart and lungs, pregnant women produce more blood and have to utilize more caution with exercise than non-pregnant women.
Heart Beat and Blood Volume During Pregnancy
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. Heart rate may increase up to 15% during pregnancy. Blood volume increases progressively during pregnancy, beginning in weeks 6-8 and continuing until weeks 32-34. The volume of plasma increases 40-50% and red blood cell mass 20-30%, creating a need for increased iron and folic acid intake.
Blood Pressure and Exercise
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.
Dizziness and Fainting
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 that involve lying on the back due to the impact from blood vessel compression. Lying on the left side may help relieve dizziness.
Women experiencing any of these conditions, particularly during exercise, should consult their doctor.
Respiratory and Metabolic Changes
Because of increased demand for blood and the dilation of blood vessels, pregnant women experience increases in the amount of oxygen they transport in their blood. This growth also forces increases in metabolic rates during pregnancy, requiring women to up energy intake and use caution during periods of physical exertion.
Breathing and Blood Oxygen Levels
During pregnancy, the amount of air moved in and out of the lungs increases by nearly 50% due to two factors. Each breath has a greater 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 other physical work during pregnancy.
Basal or resting metabolic rate (RMR), the amount of energy the body expends while at rest, increases significantly during pregnancy. This is measured by the amount of oxygen used during periods of total rest and helps estimate the amount of energy intake required to maintain or gain weight. Changes in metabolic rates explain the need to increase calorie consumption during pregnancy – the body of a pregnant woman slowly increases its energy requirements to help fuel the changes and growth taking place in both the mother and baby.
Metabolic rates increase substantially by just 15 weeks gestation and peak in the third trimester. This increased metabolic rate may put pregnant women at a higher risk of hypoglycemia, or low blood sugar. Although the metabolic rate may drop slightly as the pregnancy reaches term, it remains elevated over pre-pregnancy levels for several weeks postpartum (and will remain elevated for the duration of breastfeeding in women producing milk).
Body Temperature Changes
An increase in basal body temperature is one of the first hints of pregnancy, and a slightly higher core temperature will be maintained through the duration of pregnancy. Women also have a greater need of water during pregnancy, and can be at higher risk of hyperthermia and dehydration without caution to exercise safely and remain hydrated.
Hyperthermia – Overheating During Pregnancy
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.
In non-pregnant women, moderate aerobic exercise causes significant increases in core body temperature. Pregnant women, whether they exercise or not, experience a general 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 increased body heat.
It has been shown that pregnant women do not have as much of 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 in very hot or humid conditions, 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.
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, injury may result from a lack of fluid.
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:
- 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.
- 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.
- Wear light clothing that breathes.
- Always wear a good exercise support-bra.
- 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.
- Do not exercise when you are sick or have a fever.