How Hormonal Changes in Pregnancy Cause Backache
Pregnancy hormones—primarily relaxin, progesterone, and estrogen—cause ligamentous laxity that reduces ligament rigidity throughout the body, particularly affecting the symphysis pubis and sacroiliac joints, which weakens joint stability and increases demand on stabilizing muscles, creating the primary mechanism for pregnancy-related backache. 1, 2
Primary Hormonal Mechanism
The hormonal changes during pregnancy trigger a cascade of musculoskeletal effects:
- Ligamentous laxity is the direct result of pregnancy hormones softening and relaxing the pelvic floor ligaments, symphysis pubis, and sacroiliac joints 1, 2
- This hormone-induced laxity represents an evolutionary adaptation that allows the rigid bony pelvis to expand slightly and accommodate passage of the fetal head during vaginal delivery 2
- The reduced ligament rigidity weakens joint stability throughout the body, forcing stabilizing muscles to work harder to maintain structural support 1, 3
Biomechanical Consequences of Hormonal Changes
The hormonal laxity combines with mechanical factors to produce pain:
- Increased joint instability from weakened ligaments places excessive demand on the paraspinal and pelvic stabilizing muscles, which must compensate for the loss of passive ligamentous support 1, 2
- Biomechanical overload occurs as the growing uterus creates increased abdominal mass that shifts the center of gravity forward, placing additional stress on already-weakened joints 1, 3
- Postural compensation develops as women adopt altered postures to accommodate the growing fetus, creating abnormal stress patterns on the spine and supporting structures 1, 3
- The combination of hormonal laxity and mechanical load particularly affects the sacroiliac joints and symphysis pubis, which experience both softening and increased mobility 2
Clinical Impact and Prevalence
The hormonal and biomechanical changes produce substantial clinical burden:
- Up to two-thirds of pregnancies are affected by low back pain, with prevalence peaking in months 6 and 7 of gestation 1, 2
- Nearly 20% of pregnant women experience pelvic girdle pain involving the anterior and/or posterior aspects of the pelvic ring 1, 2
- The severity ranges from mild discomfort to severe pain affecting 15-20% of pregnant women, with significant impacts on sleep, daily activities, and mental health 2
Risk Factors That Amplify Hormonal Effects
Certain factors worsen the impact of hormonal laxity:
- Heavy lifting (10-20 kg more than 20 times per week) significantly increases risk by placing excessive stress on hormonally-weakened joints 1, 2, 3
- Obesity creates chronic overloading of connective tissue structures that are already compromised by hormonal changes 1, 2, 3
- Women with prior back pain, early symptom onset, or both low back pain and pelvic girdle pain simultaneously face higher risk for persistent postpartum pain 1, 2
Common Clinical Pitfall
- Underreporting by patients is a major issue—women often don't report back pain to their prenatal providers, requiring active questioning to identify symptoms and prevent progression to severe, disabling pain 1, 2
- Failing to modify activities, especially heavy lifting, allows continued stress on hormonally-weakened structures and worsens symptoms 1, 2