Physiology · Reproduction, pregnancy and the fetus
Maternal physiological changes in pregnancy !
- Regulation
- Exam priority: high
In pregnancy the mother's blood volume, heart output, breathing and kidneys all step up to feed the baby, and several lab values change.
Updated 2026-11-01
Also called: Physiology of pregnancy
See it in 3D: Body of uterus · Left ventricle · Kidneys · Thyroid gland · Inferior vena cava · Mammary gland lobes
What it is
Placental hormones reset almost every maternal system. Blood volume and cardiac output rise, blood pressure dips in mid-pregnancy, ventilation increases, the kidneys filter more and the blood becomes more prone to clot. Knowing these normal changes avoids mistaking them for disease, and makes real disease (preeclampsia, anemia, thrombosis) stand out.
Key facts
- Blood pressure falls in the first and second trimesters (vasodilation from progesterone, relaxin and nitric oxide) and returns toward baseline at term.
- Aldosterone and renin rise, retaining sodium and water, yet natriuretic effects of progesterone balance them; the result is a net gain of about 1 kg of sodium-retaining fluid space.
- Human placental lactogen and other hormones cause insulin resistance in the second half: more glucose for the fetus, risk of gestational diabetes.
- Total T4 rises (more thyroxine-binding globulin) and hCG weakly stimulates the TSH receptor, so TSH may be low in the first trimester.
- Progesterone relaxes smooth muscle: heartburn, slow gastric emptying, constipation and ureter dilation (higher pyelonephritis risk).
- At altitude (Bogotá ≈2600 m) the hemoglobin cut-off for anemia is adjusted upward, because baseline hemoglobin is higher.
Typical changes at term versus non-pregnant
| Parameter | Value | Note |
|---|---|---|
| Plasma volume | ↑ ≈40–50% | peaks at 30–34 weeks |
| Cardiac output | ↑ ≈30–50% | ≈6–7 L/min |
| Heart rate | ↑ ≈10–20 beats/min | |
| Hemoglobin (anemia cut-off at sea level) | <11 g/dL | WHO; <10.5 g/dL in the 2nd trimester (CDC) |
| GFR | ↑ ≈50% | creatinine ≈0.4–0.8 mg/dL |
| PaCO₂ | ≈28–32 mmHg | compensated respiratory alkalosis |
| Functional residual capacity | ↓ ≈20% | diaphragm pushed up |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Regulation
- +Progesterone: Raises ventilation, relaxes smooth muscle, contributes to vasodilation.
- +Estrogen: Raises hepatic binding globulins and clotting factors, renin substrate and uterine blood flow.
- +Renin–angiotensin–aldosterone: Retains sodium and expands plasma volume despite low vascular resistance.
- +Human placental lactogen and placental GH: Insulin resistance and lipolysis that spare glucose for the fetus.
- +Relaxin and nitric oxide: Lower vascular resistance and raise renal plasma flow.
Normal change versus a warning sign
| System | Normal in pregnancy | Suggests disease |
|---|---|---|
| Blood pressure | Falls mid-pregnancy | ≥140/90 mmHg after 20 weeks (with proteinuria or organ damage: preeclampsia) |
| Blood | Mild dilutional anemia | Hemoglobin below cut-off, low ferritin: iron deficiency |
| Kidney | Creatinine ≈0.4–0.8 mg/dL | Creatinine ≥1.0 mg/dL or new proteinuria |
| Breathing | Dyspnea on exertion, PaCO₂ ≈30 mmHg | PaCO₂ ≥40 mmHg (impending respiratory failure) |
| Coagulation | High fibrinogen | Unilateral leg swelling: deep vein thrombosis |
Clinical notes
- Preeclampsia: abnormal placentation releases antiangiogenic factors (sFlt-1) that injure maternal endothelium: hypertension, proteinuria, HELLP, eclampsia.
- A PaCO₂ of 40 mmHg in a pregnant woman with asthma is not reassuring: her normal is near 30.
- Physiological murmur: a soft systolic flow murmur and a third heart sound are common; diastolic murmurs are not.
Test yourself
Think of the answer, then tap the question to see it.
Why does hemoglobin fall in a healthy pregnancy even though red cell mass rises?
Plasma volume expands more (≈40–50%) than red cell mass (≈20–30%), so the concentration falls: a dilutional anemia that also lowers blood viscosity for the placenta.
An ABG in late pregnancy shows pH 7.44, PaCO₂ 30 mmHg, HCO₃⁻ 20 mEq/L. How should it be interpreted?
Normal for pregnancy: progesterone-driven hyperventilation with renal compensation (compensated respiratory alkalosis). It also helps CO₂ move from fetus to mother.
A pregnant woman at 32 weeks feels faint lying on her back. What causes it?
The gravid uterus compresses the inferior vena cava, cutting venous return and cardiac output. Turning to the left side restores flow.
Why can a creatinine of 1.0 mg/dL be abnormal in pregnancy?
GFR rises about 50%, so normal creatinine falls to ≈0.4–0.8 mg/dL; 1.0 may already mean reduced kidney function.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The placenta and pregnancy hormones
- Parturition: onset and uterine contraction
- Cardiac output and what sets it
- The renin–angiotensin–aldosterone system
- High altitude and acclimatization
- Respiratory acidosis and alkalosis
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 28.4 Changes During Pregnancy, Labor, and Birth (OpenStax (Rice University), 2022); Preeclampsia (StatPearls (NCBI Bookshelf), 2024); Physiology, Placenta (StatPearls (NCBI Bookshelf), 2023); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 83). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.