Maternal physiological changes in pregnancy are the normal adaptations that a woman undergoes during pregnancy to better accommodate the embryo or fetus. They are physiological changes, that is, they are entirely normal, and include cardiovascular, hematologic, metabolic, renal and respiratory changes that become very important in the event of complications. The body must change its physiological and homeostatic mechanisms in pregnancy to ensure the fetus is provided for. Increases in blood sugar, breathing and cardiac output are all required. Levels of progesterone and estrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and subsequently the menstrual cycle. The woman and the placenta also produce many hormones.

The body must change its physiological and homeostatic mechanisms in pregnancy to ensure the fetus grows properly and receives adequate nutrition. Increases in blood sugar, breathing and cardiac output are all required.

Hormonal

Pregnant women experience adjustments in their endocrine system.

Levels of progesterone and estrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and subsequently the menstrual cycle. Estrogen is mainly produced by the placenta and is associated with fetal well–being. Women also experience increased human chorionic gonadotropin (β-hCG); which is produced by the placenta. This maintains progesterone production by the corpus luteum. The increased progesterone production, first by corpus luteum and later by the placenta, mainly functions to relax smooth muscle.

Prolactin levels increase due to maternal pituitary gland enlargement by 50%. This mediates a change in the structure of the mammary gland from ductal to lobular-alveolar. Parathyroid hormone is increased which leads to increases of calcium uptake in the gut and reabsorption by the kidney. Adrenal hormones such as cortisol and aldosterone also increase.

Human placental lactogen (hPL) is produced by the placenta and stimulates lipolysis and fatty acid metabolism by the woman, conserving blood glucose for use by the fetus. It can also decrease maternal tissue sensitivity to insulin, resulting in gestational diabetes.

Physical

Illustration of fundal height at various points during pregnancy.

One of the most noticeable alterations in pregnancy is the gain in weight. The enlarging uterus, the growing fetus, the placenta and liquor amnii, the acquisition of fat and water retention, all contribute to this increase in weight. The weight gain varies from person to person and can be anywhere from 5 pounds (2.3 kg) to over 100 pounds (45 kg). Doctor-recommended weight gain range is 25 pounds (11 kg) to 35 pounds (16 kg), less if the woman is overweight, more (up to 40 pounds (18 kg)) if the woman is underweight.

Breast size

A woman’s breasts grow during pregnancy sizes. Once the baby is born and about 50 to 73 hours after birth, the mother will experience her breasts filling with milk (sometimes referred to as “milk coming in”) and at that point changes in the breast happen very quickly. Once lactation begins, the woman’s breasts swell significantly and can feel achy, lumpy and heavy (which is referred to as engorgement). Her breasts may increase again in size and individual breast size can vary daily or for longer periods depending on how much the infant nurses from each breast. A regular pattern of nursing is usually established after 8–12 weeks, and a woman’s breasts will usually reduce in size, perhaps to about 1 cup size larger than prior to her pregnancy.

Cardiovascular

The woman is the sole provider of nourishment for the embryo and later, the fetus, and so her plasma and blood volume slowly increase by 40-50% over the course of the pregnancy to accommodate the changes. The increase is mainly due to an increase in plasma volume through increased aldosterone. It results in an increase in heart rate (15 beats/min more than usual), stroke volume, and cardiac output. Cardiac output increases by about 50%, mostly during the first trimester. The systemic vascular resistance also drops due to the smooth muscle relaxation and overall vasodilation caused by elevated progesterone, leading to a fall in blood pressure. Diastolic blood pressure consequently decreases between 12–26 weeks, and increases again to pre-pregnancy levels by 36 weeks. If the blood pressure becomes abnormally high, the woman should be investigated for pre-eclampsia and other causes of hypertension. This is due to an increase in plasma volume through increased aldosterone. Progesterone may also interact with the aldosterone receptor, thus leading to increased levels. Red blood cell numbers increase due to increased erythropoietin levels.

Hematology

During pregnancy the plasma volume increases by 50% and the red blood cell volume increases only by 20-30%. Consequently, the hematocrit decreases on lab value; this is not a true decrease in hematocrit, however, but rather due to the dilution. The white blood cell count increases and may peak at over 20 mg/mL in stressful conditions. Conversely, there is a decrease in platelet concentration to a minimal normal value of 100-150 mil/mL.

A pregnant woman will also become hypercoagulable, leading to increased risk for developing blood clots and embolisms, due to increased liver production of coagulation factors, mainly fibrinogen and factor VIII (this hypercoagulable state along with the decreased ambulation (exercise involving legs) causes an increased risk of both DVT and PE). Women are at highest risk for developing clots, or thrombi, during the weeks following labor. Clots usually develop in the left leg or the left iliac venous system. The left side is most afflicted because the left iliac vein is crossed by the right iliac artery. The increased flow in the right iliac artery after birth compresses the left iliac vein leading to an increased risk for thrombosis (clotting) which is exacerbated by the aforementioned lack of ambulation following delivery. Both underlying thrombophilia and cesarean section can further increase these risks.

Edema, or swelling, of the feet is common during pregnancy, partly because the enlarging uterus compresses veins and lymphatic drainage from the legs.

Metabolic

During pregnancy, both protein metabolism and carbohydrate metabolism are affected. One kilogram of extra protein is deposited, with half going to the fetus and placenta, and another half going to uterine contractile proteins, breast glandular tissue, plasma protein, and hemoglobin.

An increased requirement for nutrients is given by fetal growth and fat deposition. Changes are caused by steroid hormones, lactogen, and cortisol.

Maternal insulin resistance can lead to gestational diabetes. Increased liver metabolism is also seen, with increased gluconeogenesis to increase maternal glucose levels.

Nutrition

Nutritionally, pregnant women require a caloric increase of 300 kcal/day and an increase in protein to 70 or 75 g/day. There is also an increased folate requirement from 0.4 to 0.8 mg/day (important in preventing neural tube defects). On average, a weight gain of 20 to 30 lb (9.1 to 14 kg) is experienced.

Renal

A pregnant woman may experience an increase in kidney and ureter size. The glomerular filtration rate (GFR) commonly increases by 50%, returning to normal around 20 weeks postpartum. Plasma sodium does not change because this is offset by the increase in GFR. There is decreased blood urea nitrogen (BUN) and creatinine and glucosuria (due to saturated tubular reabsorption) may be seen. Persistent glucosuria may suggest gestational diabetes. The renin-angiotensin system is upregulated, causing increased aldosterone levels.

Gastrointestinal

During pregnancy, woman can experience nausea and vomiting (morning sickness); which may be due to elevated B-hCG and should resolve by 14 to 16 weeks. Additionally, there is prolonged gastric empty time, decreased gastroesophageal sphincter tone, which can lead to acid reflux, and decreased colonic motility, which leads to increased water absorption and constipation.

Immune Tolerance

The fetus inside a pregnant woman may be viewed as an unusually successful allograft, since it genetically differs from the woman. In the same way, many cases of spontaneous abortion may be described in the same way as maternal transplant rejection.

Musculoskeletal

Neuro-mechanical adaptations to pregnancy refer to the change in gait, postural parameters, as well as sensory feedback, due to the numerous anatomical, physiological, and hormonal changes women experience during pregnancy. Such changes increase their risk for musculoskeletal disorders and fall injuries.

Musculoskeletal disorders include lower-back pain, leg cramps, and hip pain. Pregnant women fall at a similar rate (27%) to women over age of 70 years (28%). Most of the falls (64%) occur during the second trimester. Additionally, two-thirds of falls are associated with walking on slippery floors, rushing, or carrying an object. The root causes for these falls are not well known. However, some factors that may contribute to these injuries include deviations from normal posture, balance, and gait.

The body’s posture changes as the pregnancy progresses. The pelvis tilts and the back arches to help keep balance. Poor posture occurs naturally from the stretching of the woman's abdominal muscles as the fetus grows. These muscles are less able to contract and keep the lower back in proper alignment. The pregnant woman has a different pattern of gait. The step lengthens as the pregnancy progresses, due to weight gain and changes in posture. On average, a woman’s foot can grow by a half size or more during pregnancy. In addition, the increased body weight of pregnancy, fluid retention, and weight gain lowers the arches of the foot, further adding to the foot’s length and width. The influences of increased hormones such as estrogen and relaxin initiate the remodeling of soft tissues, cartilage and ligaments. Certain skeletal joints such as the pubic symphysis and sacroiliac widen or have increased laxity.

Lumbar Lordosis

To positionally compensate the additional load due to the pregnancy, pregnant mothers often extend their lower backs. As the fetal load increases, women tend to arch their lower backs, specifically in the lumbar region of their vertebral column to maintain postural stability and balance. The arching of the lumbar region is known as lumbar lordosis, which recovers the center of mass into a stable position by reducing hip torque. According to a study conducted by Whitcome, et al., lumbar lordosis can increase from an angle of 32 degrees at 0% fetal mass (i.e. non-pregnant women or very early in pregnancy) to 50 degrees at 100% fetal mass (very late in pregnancy). Postpartum, the angle of the lordosis declines and can reach the angle prior to pregnancy. Unfortunately, while lumbar lordosis reduces hip torque, it also exacerbates spinal shearing load, which may be the cause for the common lower back pain experienced by pregnant women.

Gait

Gait in pregnant women often appear as a “waddle” – a forward gait that includes a lateral component. However, research has shown that the forward gait alone remains unchanged during pregnancy. It has been found that gait parameters such as gait kinematics, (velocity, stride length, and cadence) remain unchanged during the third trimester of pregnancy and 1 year after delivery. These parameters suggest that there is no change in forward movement. There is, though, a significant increase in kinetic gait parameters, which may be used to explain how gait motion remains relatively unchanged despite increase in body mass, width and changes in mass distribution about the waist during pregnancy. These kinetic gait parameters suggest an increased use of hip abductor, hip extensor, and ankle plantar flexor muscle groups. To compensate for these gait deviations, pregnant women often make adaptations that can result in musco-skeletal injuries. While the idea of “waddling” cannot be dispensed, these results suggest that exercise and conditioning may help relieve these injuries.

Written under the guidance of Associate Prof. Dr. M. A. Randive, Sharir Kriya Dept., Tilak Ayurved Mahavidyalaya, Pune.

References

Text book of medical physiology − by Gyton and Hall. 12th edition
Essentials of Medical physiology − by K Sembulingam , Prema Sembulingam.
Wikipedia

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