Physiological Changes in Pregnancy

Cardiovascular System

• Cardiac output increases up to 40-50% by term, driven by both increased heart rate (+15-20 bpm above baseline) and increased stroke volume (~30%). The rise begins as early as 6 weeks' gestation.
• Peripheral vascular resistance falls early in pregnancy (nadir mid-second trimester), causing a drop in blood pressure of 5-15 mm Hg (diastolic falls more than systolic) until BP returns toward baseline in the third trimester.
• Blood volume expands by 35-45%, reaching maximum around 32-34 weeks.
• Cardiac chambers enlarge with mild myocardial hypertrophy; exam findings can mimic heart disease - collapsing pulses, laterally displaced apex, systolic ejection murmur, exaggerated S1 splitting, audible S3, and occasionally a small pericardial effusion.
• Aortocaval compression: after ~20 weeks, the gravid uterus can compress the IVC and aorta in the supine position, causing "supine hypotension syndrome" (~5% of women at term) - hypotension with pallor, sweating, nausea. Left lateral tilt relieves this.
• Cardiac output rises further during labor and immediately postpartum (60-80% above pre-labor values) due to catecholamines, autotransfusion from uterine contractions, and relief of caval compression. Full hemodynamic resolution can take up to 6 months postpartum.

Respiratory System

• Minute ventilation increases up to 50% (mainly via increased tidal volume, +40%), driven by progesterone-mediated respiratory center stimulation.
• PaCO2 falls to 28-32 mm Hg with a compensatory decrease in plasma bicarbonate, preventing significant alkalosis; this facilitates CO2 transfer from fetus to mother.
• Functional residual capacity (FRC) decreases ~20% at term (mainly reduced expiratory reserve volume) due to diaphragmatic elevation, while vital capacity is largely preserved.
• Oxygen consumption rises 20-50%. Combined with reduced FRC, this predisposes pregnant patients to rapid desaturation during apnea - hence the importance of preoxygenation before anesthesia.
• Airway resistance decreases (progesterone-mediated bronchodilation); capillary engorgement of the airway mucosa can cause nasal congestion and increase intubation difficulty.

Hematologic System

• Plasma volume increases more (~45-55%) than RBC mass (~17-45%), producing a physiologic dilutional ("physiologic") anemia, maximal between 28-34 weeks. Hemoglobin usually stays above 11 g/dL. MCV/MCHC remain unchanged in dilutional anemia (unlike true iron-deficiency anemia, where indices fall progressively).
• Hypercoagulable state: fibrinogen and factors VII, VIII, IX, X, and XII all rise (factor XI may fall slightly); this limits blood loss at delivery but raises venous thromboembolism risk.
• Mild leukocytosis (up to 21,000/uL, especially in labor) and a ~10% fall in platelet count by the third trimester are common.
• Iron and folate requirements rise substantially for fetal use; deficiency anemias readily develop without supplementation.

Renal System

• Renal plasma flow and glomerular filtration rate (GFR) increase by roughly 50%, so serum creatinine and BUN normally fall (creatinine as low as ~0.5 mg/dL, BUN ~9 mg/dL) - values considered "normal" outside pregnancy may indicate renal impairment in pregnant patients.
• A reduced renal tubular threshold for glucose and amino acids commonly causes mild glycosuria and proteinuria (<300 mg/day) even without pathology.
• Plasma osmolality decreases by 8-10 mOsm/kg, and the ureters/renal pelvis dilate (more on the right) due to progesterone effects and mechanical compression.

Gastrointestinal & Hepatic System

• Gastric motility slows and the gastroesophageal sphincter becomes less competent as the enlarging uterus displaces the stomach, promoting reflux and esophagitis and increasing aspiration risk (though gastric acidity/volume themselves aren't significantly altered).
• Overall hepatic blood flow is unchanged; mild rises in transaminases and LDH can occur in the third trimester. Alkaline phosphatase rises due to placental secretion.
• Serum albumin falls (diluted by expanded plasma volume), reducing colloid oncotic pressure.
• Pseudocholinesterase activity drops 25-30% at term (clinically usually insignificant for succinylcholine duration).
• Reduced gallbladder emptying (progesterone inhibits cholecystokinin release) plus altered bile composition predispose to cholesterol gallstones.

Metabolic / Endocrine System

• Pregnancy is a diabetogenic state: insulin resistance rises progressively (mediated by human placental lactogen/human chorionic somatomammotropin, cortisol, and progesterone), while fasting glucose and amino acids trend low and free fatty acids, ketones, and triglycerides rise - a pattern resembling accelerated starvation that spares glucose/amino acids for the fetus.
• Thyroid-binding globulin increases (estrogen-driven), raising total T4/T3, though free hormone levels remain largely stable in normal pregnancy.

Central Nervous System

• Minimum alveolar concentration (MAC) for general anesthetics falls progressively, by up to 40% at term (returning to normal by day 3 postpartum), attributed to markedly elevated progesterone and a labor-related beta-endorphin surge.
• Sensitivity to local anesthetics is enhanced, and neural blockade occurs at lower doses (epidural local anesthetic requirements may fall up to 30%), partly hormonal and partly due to epidural venous engorgement from IVC compression, which also increases cephalad spread of spinal/epidural anesthetics.

Summary Table (approximate maximum changes at term)

Response of the Mother's Body to Pregnancy

• Uterus: grows from ~50 g to ~1100 g
• Breasts: double in size, preparing for lactation
• Vagina: enlarges, introitus widens
• Some women develop edema, acne, or mild masculinizing/acromegalic features due to hormonal effects

Weight Gain

• Average gain: 25-35 lb (about 11-16 kg), mostly in the last two trimesters
  • Fetus: ~8 lb
  • Amniotic fluid, placenta, membranes: ~4 lb
  • Uterus: ~3 lb; Breasts: ~2 lb
  • Extra blood/extracellular fluid: ~5 lb
  • Maternal fat stores: ~3-13 lb
• The extra fluid is excreted as urine in the first few days postpartum once placental fluid-retaining hormones are lost.

Metabolism During Pregnancy

• Basal metabolic rate (BMR) rises ~15% in the latter half of pregnancy due to increased thyroxine, adrenocortical hormones, and sex hormone secretion, plus the added fetal/placental tissue mass.
• This is why pregnant women often feel overheated, and they need extra energy for the added muscular load of carrying the fetus.

Nutrition During Pregnancy

• Fetal growth is fastest in the last trimester (weight nearly doubles in last 2 months), so maternal stores of protein, calcium, phosphate, and iron built up earlier in pregnancy are drawn upon.
• Iron: fetus needs ~375 mg, mother needs an extra ~600 mg for her own expanded blood volume - inadequate dietary iron causes hypochromic anemia.
• Vitamin D is needed for calcium absorption; Vitamin K is often supplemented near term to ensure adequate fetal prothrombin and prevent hemorrhage (especially intracranial) during birth.

Changes in the Maternal Circulatory System

• Cardiac output rises 30-40% above normal by the 27th week (due to increased placental blood flow, ~625 mL/min through the placenta by the last month, plus the general rise in metabolism), then falls back to only slightly above normal in the last 8 weeks.
• Maternal blood volume increases ~30% by term, mainly in the second half of pregnancy - attributed to increased aldosterone and estrogen causing renal fluid retention, plus increased bone marrow activity producing more RBCs. This gives the mother a "safety reserve" of 1-2 liters of extra blood, of which only about a quarter is typically lost during delivery.

Maternal Respiration

• Oxygen consumption rises ~20% (due to higher BMR and larger body mass), with proportional CO2 production.
• Minute ventilation increases ~50% - more than needed just for the extra oxygen use, because progesterone increases the sensitivity of the respiratory center to CO2.
• Net result: arterial PCO2 falls a few mm Hg below non-pregnant levels.
• The growing uterus pushes the diaphragm upward, reducing its excursion, so respiratory rate increases to compensate and maintain the higher ventilation.

Maternal Kidney Function

• Urine formation increases slightly (higher fluid intake and excretory load).
• Tubular reabsorption of sodium, chloride, and water increases up to 50%, driven by increased placental and adrenocortical steroid hormones (salt- and water-retaining).
• Renal blood flow and GFR increase up to 50% due to renal vasodilation (exact mechanism still not fully clear).

Hormonal Factors in Pregnancy

1. Human Chorionic Gonadotropin (hCG)
   • Secreted by syncytial trophoblast cells; detectable in blood 8-9 days after ovulation
   • Peaks at 10-12 weeks, declines by 16-20 weeks, then plateaus
   • Function: maintains the corpus luteum, preventing menstruation and keeping progesterone/estrogen secretion going until the placenta itself takes over (~7th-12th week); if the corpus luteum is removed before week 7, abortion usually occurs
   • Also stimulates the fetal testes to secrete testosterone, driving male sexual differentiation
2. Estrogens - promote growth of maternal reproductive organs and breast tissue
3. Progesterone - maintains the decidual endometrium, suppresses uterine contractility, and prepares breasts for lactation
4. Human Chorionic Somatomammotropin (human placental lactogen) - contributes to breast development and has metabolic effects (mobilizing maternal fat for energy, sparing glucose for the fetus)