ECG during pregnancy

What it looks like on ECG

In a healthy pregnant woman, the ECG generally remains normal, but regular shifts may occur that are not mandatory for every patient.

HR and rhythm. Sinus rhythm is preserved, HR gradually increases from the 1st to the 3rd trimester due to an increase in circulating blood volume and sympathetic activation — typically 10–20 bpm higher than the individual woman's baseline level. Absolute HR values vary widely, and there is no fixed upper limit of normal for all pregnant women; sinus tachycardia up to 100 bpm and higher at rest is not uncommon in the 3rd trimester, but persistent or disproportionate tachycardia requires clinical evaluation.

AAxis and heart position. The enlarged uterus displaces the diaphragm upward; in some pregnant women, the heart assumes a more horizontal position and shifts to the left and upward. On the ECG, this may manifest as a leftward axis deviation and a horizontal heart position, although in many women the axis does not change significantly — this is a possible, but not mandatory sign.

Waves and segments. Due to positional features, small Q waves and a flattened or mildly negative T wave sometimes appear in III and aVF. The variability of such a Q wave with respiration supports its positional nature, but this is only an auxiliary guide: one must evaluate the width and depth of the wave, the presence of Q waves in adjacent leads, ST-T dynamics, and clinical presentation. Minor nonspecific ST-T changes are also possible in the right precordial leads, especially near childbirth and in the early postpartum period; horizontal or downsloping ST depression, widespread or progressive changes do not fit this picture and require the exclusion of ischemia, overload, and myocarditis.

Intervals. QRS duration generally does not change significantly. The PR interval may shorten slightly due to autonomic and hemodynamic changes, usually remaining within the normal range. QTc should not normally show persistent pathological prolongation — any noticeable and sustained prolongation requires separate assessment rather than being interpreted as a "feature of pregnancy," while QTc measurement during sinus tachycardia should preferably be confirmed using formulas other than Bazett's alone.

Mechanism

During pregnancy, circulating blood volume increases by 30–50%, stroke volume and cardiac output rise, and the sympathetic nervous system tone increases — hence the increase in HR. In some women, the enlarged uterus elevates the diaphragm and shifts the heart upward and leftward, rotating it more horizontally within the chest — this is the mechanical cause of potential axis deviation and positional changes in the T and Q waves in the inferior leads. Hemodynamic, autonomic, and hormonal changes may contribute to the appearance or increased frequency of atrial premature beats and supraventricular tachycardia, although their frequency also depends on the baseline cardiac status and the presence of an arrhythmogenic substrate.

Causes of pathological findings in pregnant women

If ECG changes exceeding physiological limits are detected in a pregnant woman, look for:

• gestational hypertension, preeclampsia (LV hypertrophy, overload)
• peripartum cardiomyopathy (late pregnancy — first months postpartum)
• pulmonary embolism (increased risk due to pregnancy-related hypercoagulation)
• decompensation of a pre-existing heart valve disease or cardiomyopathy
• myocarditis
• electrolyte imbalances during severe toxicosis/vomiting
• arrhythmias newly manifesting or escalating against the background of pregnancy (SVT, extrasystole)
• congenital or acquired long QT syndrome (risk of arrhythmic events depends on genotype and period — see the "Differential diagnosis" section)

Differential diagnosis

• True ischemia/ACS — cannot be reliably excluded based on a single ECG. Isolated stable positional changes in III, aVF without typical ST dynamics, without reciprocal changes in I, aVL, and without clinical signs are more often positional in nature, but do not rule out ACS: ischemia may occur without typical pain and without diagnostic ST changes, and troponin reflects myocardial injury and may be normal in the early stage. In the presence of symptoms, suspicious ECG dynamics, or rising troponin, serial ECGs and high-sensitivity troponin interpreted according to the ACS algorithm are necessary, regardless of pregnancy.
• Peripartum cardiomyopathy — heart failure with LV systolic dysfunction (typically LV EF <45% on echocardiography) occurring toward the end of pregnancy or in the first months postpartum in the absence of another cause; this is a diagnosis of exclusion. ECG may show nonspecific ST-T changes, and sometimes signs of hypertrophy/dilation, but by themselves they are insufficient for diagnosis.
• Pulmonary embolism — sinus tachycardia per se is nonspecific; signs of right heart overload (SI-QIII-TIII, right bundle branch block, P pulmonale) have low sensitivity and specificity and do not confirm the diagnosis on their own. When pulmonary embolism is clinically suspected, such ECG findings increase vigilance, and the workup is performed according to an algorithm adapted for pregnancy, taking into account clinical probability, indications for D-dimer, and imaging.
• Long QT syndrome — physiological pregnancy should not cause sustained pathological QTc prolongation; if QTc exceeds normal limits, differentiation from congenital syndrome or drug-induced effects is necessary.

Clinical significance and management

Pregnancy itself is not a cause of structural heart disease, and most of the described ECG changes are a variant of normal requiring no intervention. The physician's task is not to mistake physiological ST-T lability and potential axis shifts for organic pathology, but also not to attribute findings to pregnancy when they cannot be explained by it.

If deviations are detected that do not fit the described physiological pattern (ST elevation, pathological Q, wide QRS, persistent QTc prolongation, sustained or symptomatic arrhythmias, signs of chamber hypertrophy/strain), further evaluation is required: echocardiography, laboratory markers (troponin, D-dimer taking into account the physiological increase during pregnancy), and a cardiology consultation. In case of newly onset symptomatic tachyarrhythmia or suspected structural heart disease, the pregnant woman is managed jointly by an obstetrician-gynecologist and a cardiologist, taking into account restrictions on diagnostic imaging modalities and the choice of drugs safe during pregnancy.

Common interpretation errors

• To interpret a positional Q in III, aVF as a pathological Q wave based solely on the respiratory test, without assessing width, depth, adjacent leads, and clinical presentation.
• To write off any ST depression or T inversion as "pregnancy" without ruling out ischemia, strain, or myocarditis, and to consider a normal or non-specific ECG sufficient grounds to rule out ACS.
• To ignore persistent or symptomatic resting tachycardia as "normal for a pregnant woman" without assessing for anemia, thyrotoxicosis, hypovolemia, or PE.
• To evaluate QTc during tachycardia using Bazett's formula alone without accounting for its systematic overestimation of the result.
• To fail to consider the postpartum period (especially the first months) as a time of increased risk for arrhythmias and cardiomyopathy when interpreting an ECG.

Frequently asked questions

Related terms

More in the section "Children, Athletes, Pregnant Women"

Entire section

Sources

• Hampton J.R. The ECG Made Easy
• Surawicz B., Knilans T.K. Chou's Electrocardiography in Clinical Practice
• Orlov V.N. Guide to Electrocardiography
• AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009

Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.