Cardiovascular system and blood
The primary adaptation mechanism is a decrease in total peripheral vascular response driven by nitric oxide. In response, cardiac output compensatorily increases by 40–50%. In early stages, this occurs via stroke volume, while by weeks 20–24, an increased heart rate (physiological tachycardia) joins in.
Physiological anemia of pregnancy develops. Circulating blood volume increases sharply, but plasma arrives faster than erythrocytes. As a result, hemoglobin concentration drops, although the need for iron and folic acid increases manifold.
Pregnancy is a temporary prothrombotic state. Fibrinogen levels rise (up to 50%) along with clotting factors, while fibrinolytic activity decreases. This protects against postpartum hemorrhage, but combined with venous stasis, increases the risk of thrombosis.
Endocrine remodeling and metabolism
A complex maternal-placental-fetal system is formed.
- Pituitary gland: the anterior lobe enlarges, and prolactin levels increase 10-fold. The posterior lobe serves as a depot for oxytocin, the synthesis of which peaks during labor.
- Adrenal glands: cortical hyperplasia occurs. Cortisol levels increase 2–3 times, and the renin-angiotensin-aldosterone system is sharply activated (aldosterone rises up to 10-fold by the third trimester).
- Thyroid gland: operates under conditions of relative iodine deficiency. Bound fractions of thyroid hormones compensatorily increase.
- Pancreas: in the second half of pregnancy, placental hormones induce physiological insulin resistance. Glucose is less readily utilized by maternal tissues to provide energy for the fetus. Lipolysis and ketogenesis intensify—ketone bodies freely cross the placenta to fuel the fetal brain.
Respiration, kidneys, and digestion
Under the influence of progesterone, the respiratory center becomes more sensitive to carbon dioxide. Hyperventilation occurs (due to increased vital capacity), partial pressure of $CO_2$ decreases, and a mild compensatory alkalosis may develop.
Renal blood flow and glomerular filtration rate increase by 50%. Blood urea and creatinine levels drop, but protein and glucose may appear in the urine due to incomplete tubular reabsorption. Renal pelves and ureters dilate, predisposing to pyelonephritis.
In the GI tract, progesterone reduces motility—hence early nausea and constipation. In the liver, synthesis of specific proteins (fibrinogen, ceruloplasmin) increases, and alkaline phosphatase levels rise 2–4 times due to placental production.
Uterine and musculoskeletal changes
The uterus adapts to fetal growth through muscle fiber hypertrophy and connective tissue hyperplasia.
- Dimensions: mass reaches 1000 g, capacity increases 500–1000 times (cavity volume may exceed 5 L). Growth is asymmetric—the uterine fundus grows most actively.
- Walls: initially thicken, but thin to 1.5 cm by the end of pregnancy, turning the uterus into a soft "muscular pouch."
- Musculoskeletal system: ligaments, cartilage, and the pubic symphysis soften, increasing the anteroposterior diameter of the pelvic inlet. Due to the shift in the center of gravity, physiological lumbar lordosis intensifies.
The 50% rule: during pregnancy, cardiac output, circulating blood volume, and glomerular filtration rate increase by approximately 50%.
Class and exam questions
Why is it difficult for pregnant women to lie on their back in late stages?
The heavy uterus compresses the inferior vena cava. This disrupts venous return to the heart, reduces cardiac output by 25%, and can cause fetal hypoxia (supine hypotension syndrome).
Is a decrease in hemoglobin normal during pregnancy?
Yes, physiological anemia develops. The fluid portion of the blood (plasma) increases faster than new red blood cells are produced, so the blood becomes diluted.
Why do pregnant women often experience mood swings and drowsiness?
In early stages, the tone of the autonomic nervous system changes. Additionally, a gestational dominant forms in the cerebral cortex—all interests focus on carrying the child, and cortical excitability temporarily decreases.
What else to study on this topic
- Characteristics of the fetal immune system and hemostasis
- Metabolic acidosis as a norm for intrauterine development
- Skin changes: chloasma, striae, and spider angiomas
- Preparation of the mammary glands for lactation and colostrum secretion
- Hormonal function of the corpus luteum and placenta
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