Physiologic Changes in Pregnancy

Written by: Khalil Abualhumos, M.D.

Keywords: Physiologic changes, pregnancy, cardiovascular adaptations, blood volume, cardiac output, pulmonary changes, renal adaptations, gastrointestinal changes, hematologic changes, red blood cell mass, thrombosis risk, skin changes, hyperpigmentation, striae gravidarum, varicosities, anemia, clotting factors, edema, melasma.

Overview

Pregnancy induces various physiologic changes to support fetal development. Key adaptations occur in multiple systems, including the cardiovascular, pulmonary, renal, gastrointestinal, and hematologic systems. The cardiovascular system experiences increased blood volume, cardiac output, and vascular resistance changes, which prepare the body for labor. Pulmonary changes involve increased ventilation and mild respiratory alkalosis. Renal adaptations include enhanced blood flow, elevated glomerular filtration rate, and increased urine production. Gastrointestinal alterations, such as reflux and slowed motility, may lead to constipation and heartburn. Hematologic changes include increased red blood cell mass and white blood cell count, with a higher risk of thrombosis. Skin changes like hyperpigmentation and stretch marks are common, while vascular changes may lead to varicosities. These physiological modifications help ensure maternal and fetal health, though they may also predispose women to complications like edema, infections, and thromboembolic events.

Buzzwords Scenario: A 30-year-old pregnant woman at 28 weeks gestation presents with mild shortness of breath, leg swelling, and heartburn. On examination, she has a split S1, loud S3, and visible spider angiomas. Lab results show hemoglobin of 11.5 g/dL and mild leukocytosis.

Definition

Physiologic changes in pregnancy are natural body adaptations that support fetal development and prepare for labor, affecting multiple organ systems.

Maternal Physiology During Pregnancy

Pregnancy causes significant anatomical, physiological, and biochemical changes to support fetal development.

●     Understanding these changes is crucial for:

○     Interpreting normal lab values during pregnancy.

○     Adjusting medication dosages appropriately.

○     Identifying high-risk pregnancies and managing complications.

NOTE: These adaptations can strain maternal reserves, making some women more prone to complications.

Cardiovascular System in Pregnancy

Anatomic changes:

●     Heart shifts leftward & upward due to uterine enlargement.

●     Apical beat moves laterally as heart size increases (~12%).

●     Vascular changes: Include smooth muscle hypertrophy & reduced collagen.

Blood volume:

●     Increases by ~50%, peaking at 30 weeks.

●     Driven by pregnancy hormones (estrogen, aldosterone, progesterone).

●     Red cell mass increases by ~30%, supporting oxygen delivery.

●     Prepares for blood loss at delivery (500-600 mL vaginal, 1000 mL C-section).

Cardiac output:

●     Increases ~40%, peaking at 20-24 weeks.

●     Stroke volume rises by 25-30%, then plateaus after 20 weeks.

●     Heart rate increases (~15 bpm higher at term).

NOTES: Cardiac output

●       Further increases during labor & delivery due to contractions & sympathetic activation.

●     Sensitive to maternal position: Supine position reduces stroke volume & cardiac output.

Blood pressure:

●     Slight decline during pregnancy, lowest at 24-28 weeks, returning to normal by 36 weeks.

●     Diastolic pressure drops more than systolic, widening pulse pressure.

●     Venous pressure rises in lower limbs → risk of edema & varicose veins.

Peripheral vascular resistance:

●     Decreases by ~34%, lowest by 14-20 weeks, then slightly rises near term.

●     Mediated by vasodilators (nitric oxide, prostacyclin).

Blood flow distribution:

●     Increases to uterus, kidneys, skin, & breasts to support pregnancy.

●     Uterine blood flow can reach 800 mL/min (~4x nonpregnant level).

●     Renal blood flow increases by 400 mL/min.

●     Exercise may reduce uteroplacental perfusion, but adapted women can continue moderate exercise.

Heart murmurs & rhythm:

●     Systolic ejection murmurs common (~90%) due to increased cardiac output.

●     Split first heart sound & louder third heart sound may be heard.

●     Increased risk of supraventricular tachycardia (SVT).

●     ECG changes: Leftward axis shift (15-20°), mild ST depression, T-wave flattening.

Figure 1. An image showing breast changes during pregnancy, which include increased size plus darker and larger areola.

Hughey, M. (2011, September 2). Breast changes during pregnancy [Image]. Brookside Associates, Ltd. https://commons.wikimedia.org/wiki/File:Breast_changes_during_pregnancy_1.png

Pulmonary System in Pregnancy

Anatomic changes:

●       Increased capillary dilation → engorgement of nasopharynx, larynx, trachea, and bronchi.

●       More prominent pulmonary vascular markings on X-ray due to increased pulmonary blood volume.

●     Diaphragm elevated by 4 cm as uterus enlarges.

●     Rib cage displaced upward → increased lower thoracic diameter (+2 cm) and thoracic circumference (+6 cm).

●       Reduced abdominal muscle tone → increased diaphragmatic breathing.

Lung volumes and capacities:

●     Increased: Dead space volume, tidal volume, inspiratory capacity.

●     Decreased: Total lung capacity, functional residual capacity (includes expiratory reserve & residual volumes).

Respiration:

●       Respiratory rate remains unchanged.

●     Minute ventilation ↑ by 50% (due to increased tidal volume).

●     Mild respiratory alkalosis (pH 7.4-7.5) due to ↓ maternal arterial CO₂ (27-32 mmHg).

●     Progesterone stimulates hyperventilation by increasing chemoreflex sensitivity to CO₂.

●     Maternal hyperventilation protects the fetus from high CO₂ levels.

●     Peak expiratory rate declines throughout pregnancy, worsens in supine position.

Renal System in Pregnancy

Anatomic changes:

●     Kidneys enlarge: Increase in length (1-1.5 cm) and weight.

●     Urinary tract dilation:

○       Renal calyces, pelves, and ureters dilate (more on the right side).

○     Renal pelvis volume increases up to 6 times the nonpregnant size.

○     Ureters widen, elongate, and curve, leading to urine retention (risk of infections).

○     Urinary tract changes resolve by postpartum day 4.

Renal function:

●     Increased blood flow:

○     Renal plasma flow rises by 50-85% (peaks in first trimester).

○     Glomerular filtration rate (GFR) increases by 40-65% and remains high.

●     Hormonal influences:

○     Progesterone & relaxin lower renal vascular resistance.

○     Nitric oxide & endothelin contribute to vasodilation.

●     Urine production & electrolyte balance:

○     Increased GFR leads to lower serum creatinine and blood urea nitrogen (BUN).

○     Glucosuria is common due to increased GFR.

○     Renin-angiotensin system activated, but pregnant women are resistant to hypertension.

○     Sodium retention occurs, lowering plasma sodium by ~5 mEq/L.

○     Plasma osmolality decreases by ~10 mOsm/kg.

○     Risk of diabetes insipidus-like condition due to vasopressin metabolism by placenta.

Bladder changes:

●     Bladder is displaced upward & flattened due to the growing uterus.

●     Urinary frequency increases early due to hormones, later due to mechanical pressure.

●     Bladder capacity increases up to 1500 mL due to reduced muscle tone.

Gastrointestinal System in Pregnancy

●     Anatomic changes:

○       Uterus growth shifts stomach upward and bowels laterally.

○       Organs return to normal postpartum.

●     Oral cavity:

○       Increased salivation (partly due to nausea-related swallowing issues).

○       Gums become hypertrophic, hyperemic, spongy, and prone to bleeding (due to increased estrogen).

●     Esophagus & stomach:

○     Reflux (heartburn) affects 30-80% of pregnancies.

○       Increased gastrin → higher stomach volume & acidity.

○       Decreased esophageal peristalsis & lower esophageal sphincter relaxation → worsened reflux.

○       Delayed gastric emptying of solids (not liquids) → higher risk of regurgitation & aspiration.

●     Intestines:

○     Slower transit time in 2nd & 3rd trimesters, normal postpartum.

○       Reduced motility linked to estrogen (via nitric oxide release) and progesterone.

○     Slow movement increases water absorption → predisposes to constipation.

●     Gallbladder:

○     Slowed & incomplete emptying, commonly appears dilated.

○     Bile stasis increases gallstone risk.

●     Liver:

○       No structural changes.

○       Lower plasma albumin → mimics liver disease pattern.

○     Serum alkaline phosphatase doubles due to placental production.

Hematologic System in Pregnancy

●       Red blood cells:

○       Red cell mass increases by ~33% (~450 mL).

○       Greater plasma volume expansion → physiologic anemia.

○       Hemoglobin averages 10.9 g/dL (2nd trimester), 12.4 g/dL (term).

●       Iron:

○       Increased erythropoiesis → iron needs 6-7 mg/day in late pregnancy.

○       Many women start pregnancy iron-deficient → common need for iron supplementation.

○       Placenta actively transports iron → fetus usually not anemic even if the mother is.

●       White blood cells:

○       Leukocyte count increases (from 4300-4500/µL to 5000-12,000/µL in late pregnancy).

○       Up to 16,000/µL in third trimester, 20,000-25,000/µL in labor.

○       Impaired chemotaxis & adherence of polymorphonuclear leukocytes → higher infection risk.

○       Basophils decrease, eosinophils remain unchanged.

●       Platelets:

○       Increased thrombocytopoiesis but progressive platelet consumption.

○       Thrombocytopenia (<150,000/µL in 6% of pregnancies), resolves after delivery.

○       Increased prostacyclin (inhibits aggregation) & thromboxane A2 (induces aggregation & vasoconstriction).

●       Clotting factors:

○       Increased levels of fibrinogen (factor I), factor VIII, and other factors (VII, IX, X, XII).

○       Fibrinogen nearly doubles by late pregnancy (4-6.5 g/L).

○       Factor XI slightly decreases, factor XIII reduces up to 50% at term.

○       Protein S decreases early and stays low.

○       Depressed fibrinolysis despite increased plasminogen → net procoagulant effect.

○       Higher clotting tendency → increased risk of thromboembolic disease & hemorrhage management challenges.

Skin Changes in Pregnancy

●       Hyperpigmentation:

○       Common, seen in linea nigra and melasma (mask of pregnancy - on the face).

○       Melasma affects up to 70% of pregnancies, worsened by sun exposure.

○       Likely due to increased melanocyte-stimulating hormone, estrogen, and progesterone.

●       Striae gravidarum (Stretch marks):

○       Develop in 2nd trimester on the abdomen, breasts, thighs, and buttocks.

○       Due to decreased collagen adhesiveness & increased ground substance.

○       Genetic predisposition, no proven treatment.

●       Other common changes:

○       Spider angiomas, palmar erythema, and cutis marmorata (mottled skin) due to vasomotor instability.

○       Varicosities (40% of pregnancies) due to vena cava compression → affects legs, anus (hemorrhoids), vulva.

●       Nail & hair changes:

○       Nails: Brittle, may show Beau’s lines (horizontal grooves).

○       Hair: Thickens in pregnancy (increased anagen phase) but sheds excessively postpartum (telogen phase).

○       Hair growth returns to normal within 12 months postpartum.

Figure 2. The image shows multiple instances of melasma, characterized by dark, irregular patches on the face.

Elord. (n.d.). Melasma on the face [Photograph]. Wikidocs. https://commons.wikimedia.org/wiki/File:Melasmablemish.jpg

Figure 3. The image shows a dark, vertical line running from the pubic area to the upper abdomen, known as linea nigra.

Kemp, J. (2004, December 26). Linea nigra in Asian American woman at 22 weeks pregnant [Photograph]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Linea_nigra.jpg

Reference

Flick, A. A., & Kahn, D. A. (2019). Maternal physiology during pregnancy. In A. H. DeCherney, A. S. Roman, L. Nathan, & N. Laufer (Eds.), Current diagnosis & treatment obstetrics & gynecology (12th ed., pp. 164–171). McGraw-Hill Education.