Respiratory System

The physiology of breathing in pregnancy is altered by both anatomic and hormonal changes. Anatomic changes include an increase in chest diameter and circumference, as well as a rise in the level of the diaphragm. The result is a reduction in total lung capacity by 5% and functional residual capacity by 20%. In contrast, vital capacity does not change.⁴ The respiratory rate also remains constant, but because of a progesterone-mediated increase in both tidal volume and minute ventilation, PaCO₂ decreases to an average of 30 mm Hg. The sensation of dyspnea is increased during pregnancy.

Hematologic and Immunologic Systems

Blood volume increases during pregnancy by an average of 40% to 50% secondary to both plasma volume expansion and increased erythrocyte mass. Plasma volume increases approximately 50%, with a plateau reached at 30 weeks of gestation. Erythrocyte mass increases 20% to 30% over prepregnancy levels and peaks near term, with greater increases associated with iron supplementation. Asymmetric expansion of the plasma and erythrocyte mass results in a relative anemia, referred to as the physiologic anemia of pregnancy. Plasma expansion begins earlier than erythropoiesis but then stabilizes, with the nadir in hemoglobin concentration occurring between weeks 16 and 28.⁵ Hemoglobin levels normally do not drop below 10.5 g/dL during the nadir period and should measure 11 g/dL or higher during the remaining pregnancy.

Procoagulation factors are increased during pregnancy, whereas inhibitors of coagulation are reduced or unchanged. These changes in the coagulation cascade may serve to protect the mother against peripartum hemorrhage, but when combined with venous stasis and vessel wall injury, they predispose a patient to thromboembolic disease.⁶

Pregnancy has been described as a state of immunodeficiency but is more accurately described as a period of modified immune response.⁷ The peripheral white blood cell count is elevated during pregnancy; it ranges from 5110/mm³ to 12,200/mm³ during gestation and rises even higher during labor. Additionally, changes in both chemotaxis and adherence of neutrophils occur during pregnancy, as well as a shift by the immune system away from the cell-mediated immune response toward antibody-mediated immunity. This altered immune focus allows tolerance of the maternal immune system to paternal antigens but increases susceptibility to pathogens and variation in the activity of autoimmune diseases.⁸

Cardiovascular System

Changes in diaphragm position and rib cage dimension cause the heart to be displaced to the left and upward and rotated on its long axis. On radiographic studies these changes are manifested as an increase in heart silhouette in the absence of actual cardiomegaly. Likewise, this change in position is responsible for apparent left axis deviation on an electrocardiogram.

Cardiac output consistently and dramatically increases during pregnancy, with a 37% to 53% increase over prepregnancy values.⁹ This change is driven by increases in both heart rate and stroke volume. The heart rate increases 15 to 20 beats/min over pregravid rates, and stroke volume increases by 20% to 30%. In the later stages of pregnancy, decreased venous return as a result of compression of the inferior vena cava leads to decreased cardiac output. The highest levels of cardiac output occur in the right and left lateral positions; the lowest levels occur in the supine, sitting, and standing positions. Supine hypotension with symptoms such as dizziness, nausea, and syncope develop in a small number of pregnant patients (5% to 10%).⁶ Despite the increase in cardiac output, the pregnancy-associated reduction in systemic vascular resistance causes an overall reduction in maternal blood pressure. Blood pressure, like cardiac output, is dependent on position, highest when sitting and lowest in the lateral recumbent position.

Dermatologic System

Striae gravidarum (“stretch marks”) occur in 50% to 90% of pregnancies during the late second or third trimester. Mechanical stretching of the skin along with hormonal changes affecting the dermis are implicated. Hormonal stimulation of melanocyte activity is responsible for the hyperpigmentation seen in as many as 90% of pregnancies.

Genitourinary System

Changes in the urinary system during normal pregnancy include increases in kidney size, glomerular filtration rate, and renal blood flow. Additionally, dilation of the ureters, renal pelvis, and renal calices occurs as a result of mechanical compression by the expanding uterus and ovarian structures, as well as progesterone-mediated smooth muscle relaxation. The right ureter is more commonly affected than the left one, probably secondary to anatomic susceptibility. The glomerular filtration rate increases 50% over nongravid values by the end of the first trimester and is maintained throughout pregnancy. This increase and the subsequent increase in filtration fraction leads to decreases in both creatinine and blood urea nitrogen levels. The dependent edema accumulated during the day is mobilized at night, which results in increased nocturia during pregnancy. An increased incidence of glycosuria is common during pregnancy and may be unrelated to blood glucose levels or kidney dysfunction, but hematuria or proteinuria before labor should be considered pathologic.

The weight of the uterus increases from 70 to 1100 g, and intrauterine volume increases from 10 to 5000 mL. The gradual change in color of the vaginal walls to dark blue or black (Chadwick sign) is secondary to venous congestion.

Gastrointestinal System

Changes in the gastrointestinal system during pregnancy include an overall decrease in gastric tone and motility. There is also a decrease in lower esophageal sphincter tone caused by altered hormone levels. This muscle relaxation, coupled with compression of the stomach by the expanding uterus, leads to an increase in gastroesophageal reflux disease. Paradoxically, a concurrent decrease in peptic ulcer disease takes place.

Liver anatomy and function are essentially unchanged. However, serum alkaline phosphatase levels are elevated during the third trimester secondary to placental production of the isoenzyme. Gallbladder emptying is slower and less efficient during pregnancy. In addition to stasis, changes in the chemical composition of bile lead to increased formation of cholesterol crystals. The result is an increase in symptomatic cholelithiasis.

Musculoskeletal System

To counterbalance the expanding uterus and prevent an anterior shift in the center of gravity, the degree of lordosis of the lumbar spine increases throughout pregnancy, which often results in low back pain. Increased laxity of the ligament of the pubic symphysis and sacroiliac joints can also cause pain. Increasing tenderness of the breasts is common in pregnancy.

Endocrine System

Enlargement of the breasts and nipples is normal, and discharge of colostrum from the nipples during the later stages of pregnancy is not uncommon. Despite histologic changes, women remain euthyroid during pregnancy, and the slight increase in thyroid size that does occur is not clinically detectable. A palpable increase in the thyroid during pregnancy is pathologic and must be further evaluated. The results of laboratory tests commonly used to evaluate thyroid function should be within normal limits. Significant hypothyroidism during pregnancy is associated with fetal neurologic defects such as mental retardation and lower IQ scores.^10,11

The pituitary gland increases 136% in comparison with its prepregnancy size. This increase in size makes the gland more susceptible to infarction with hemorrhage (Sheehan syndrome) but does not impair the optic chiasm.

Some level of glucose intolerance is associated with pregnancy. Normal pregnancy is marked by hyperglycemia after eating, followed by hypoglycemia when fasting. Postprandial hyperglycemia ensures adequate delivery of nutrients to the fetus.

Diabetes

Diabetes occurs in approximately 3% to 5% of all gestations.¹² Three types of diabetes affect pregnancy: type 1, type 2, and gestational diabetes. Gestational diabetes accounts for 90% of cases. Fetal risks during pregnancy in women with diabetes include congenital malformation, intrauterine growth retardation, macrosomia, fetal hypoglycemia, fetal respiratory distress syndrome, neonatal hypocalcemia, hyperbilirubinemia, polycythemia, intrauterine demise, and neonatal jaundice.¹³ These risks are greatest for women with type 1 diabetes, although type 2 and gestational diabetes are also associated with a significant increase in fetal mortality.

With tight glucose control, the perinatal mortality rate of diabetic pregnancies can approach that of uncomplicated pregnancies. However, maintaining normal blood sugar levels is extremely difficult because of the changing degree of insulin resistance throughout pregnancy.

Patients who are unable to achieve glucose control with diet and exercise require insulin therapy. The side effects of oral hypoglycemic agents in pregnant patients have not been studied extensively. Some agents may be associated with an increased rate of congenital malformation, and frequently they do not provide adequate glucose control.

Diabetic ketoacidosis occurs in as many as 10% of patients with type 1 diabetes. Treatment during pregnancy is the same as for nongravid patients but should include assessment of fetal status and supportive measures such as oxygen and use of the left lateral decubitus position to maximize fetal blood flow.

Urinary Tract Infections

The incidence of acute cystitis or acute pyelonephritis is approximately 1% during pregnancy. Asymptomatic bacteriuria is a related condition in which urine culture is positive in an asymptomatic patient. The incidence of asymptomatic bacteriuria is similar in both gravid and nongravid populations, with 2% to 10% of the population being affected.^14,15