How does the shift in center of gravity affect maternal balance and gait during pregnancy?

Maternal Balance and Gait Changes During Pregnancy

Direct Impact of Center of Gravity Shift

The shift in center of gravity during pregnancy significantly impairs maternal balance and gait by increasing postural instability, reducing movement speed, and elevating fall risk, particularly in the third trimester. 1

The biomechanical changes are driven by:

• Increased abdominal mass that shifts the center of mass anteriorly and superiorly, creating additional load on the musculoskeletal system 1
• Altered center of mass position that increases anteroposterior postural sway and reduces postural stability 1
• Increased demand on stabilizing muscles due to weakened joint stability from pregnancy-related ligament laxity 1

Specific Gait Alterations

Pregnant women demonstrate measurable compensatory gait changes:

• Reduced walking speed by approximately 0.34 m compared to non-pregnant women 2
• Decreased stride length by 0.19 m on average 2
• Reduced stride frequency by 19.06 steps/min 2
• Decreased thigh acceleration and swing work during gait 2
• Slower completion times on mobility tests that increase progressively with gestational age 3

These changes represent adaptive compensatory strategies rather than impaired movement quality—pregnant women maintain movement quality by reducing speed to safely navigate despite increased mass and limited thoracopelvic rotations 3.

Balance Impairment by Trimester

The impact on postural stability follows a clear progression:

• First and second trimesters: No significant differences in overall, anterior-posterior, or medial-lateral stability compared to non-pregnant controls 4
• Third trimester: Significantly higher overall, anterior-posterior, and medial-lateral stability index scores compared to controls (p<0.05), indicating substantially impaired balance 4
• Fall risk scores: Significantly elevated in the third trimester compared to earlier trimesters and non-pregnant women (p<0.001) 4

Neuromuscular Adaptations

Pregnancy induces specific muscle recruitment changes:

• Decreased muscle-induced acceleration in late pregnancy 2
• Increased reliance on gluteus muscles rather than quadriceps for knee extension during walking—an adaptive change to accommodate altered body shape and mass 2
• Increased gastrocnemius contribution to hip and joint movement 2
• Higher ankle stiffness in pregnant non-fallers compared to pregnant fallers, suggesting ankle stiffness is a dominant protective strategy 5

Fall Risk Quantification

Approximately 25-27% of pregnant women experience falls during pregnancy 3, 5. The risk is highest during:

• Complex, dynamic movements rather than static standing 3
• Third trimester when center of gravity shift is maximal 4
• Activities requiring rapid postural adjustments to surface perturbations 5

Pregnant women who fall demonstrate significantly smaller peak differences between center of pressure and center of gravity compared to pregnant non-fallers and controls (p<0.01), indicating reduced postural control capacity 5.

Clinical Implications for Physical Activity

The center of gravity shift creates specific occupational hazards:

• Overhead lifting should be avoided due to increased postural instability and anteroposterior sway 1
• Floor-level lifting becomes impractical after 20 weeks' gestation as abdominal protrusion extends the distance objects must be handled 1
• Bending at the waist >1 hour/day increases preterm labor risk nearly 3-fold and spontaneous abortion risk nearly 3-fold 1
• Prolonged standing or stooping exacerbates venous insufficiency and may induce fetal hypoxia 1

Despite these limitations, regular moderate-intensity physical activity within guidelines does not increase adverse outcomes and may reduce risks of gestational diabetes, hypertension, and preterm birth in women with obesity 1.

Protective Interventions

Maternity support belts (MSB) significantly improve balance and reduce fall risk across all trimesters, with the greatest benefit in the third trimester 6. Specifically:

• MSB use lowers anterior-posterior stability index and fall risk scores in the first trimester (p<0.05) 6
• MSB use reduces medial-lateral stability index and fall risk scores in the second trimester (p<0.05) 6
• MSB use improves overall, medial-lateral, and fall risk scores in the third trimester (p<0.05) 6
• Fall risk test scores are consistently lower with MSB use across all pregnancy periods (p<0.05) 6

Common Pitfalls

• Underestimating third trimester fall risk: The dramatic increase in postural instability in late pregnancy requires heightened awareness and environmental modifications 4
• Assuming static balance tests are sufficient: Most pregnancy-related falls occur during dynamic movements, not static standing 3
• Ignoring ankle stiffness training: Increasing ankle stiffness may be the most important strategy to prevent falls, as it directly addresses the dominant biomechanical deficit 5
• Failing to modify work activities: Continued heavy lifting, overhead work, or prolonged stooping despite center of gravity changes increases both maternal musculoskeletal injury and adverse fetal outcomes 1