The alteration in foot size and shape during pregnancy is a common experience, often resulting in a need for larger footwear. This physiological change stems from a complex interplay of internal hormonal shifts and external biomechanical forces. The structural changes involve adjustments to the body’s foundation, not merely temporary swelling. Understanding these mechanisms reveals why some women find their shoe size has permanently increased after childbirth.
The Role of Hormonal Changes
The body releases the peptide hormone Relaxin from the earliest stages of pregnancy. Produced by the corpus luteum and later the placenta, its primary function is to prepare the body for delivery by softening the ligaments and connective tissues around the pelvis. Its effects are systemic, impacting ligaments throughout the body, including those that support the arch of the foot.
These ligaments form a crucial arch that acts as a natural shock absorber. When Relaxin causes this connective tissue to become lax, the foot’s structural integrity is compromised. This makes the arch more susceptible to collapse under pressure. The hormonal softening of the ligaments is the internal trigger that allows the foot’s shape to change.
Impact of Increased Weight and Mechanical Stress
The hormonal loosening of the foot’s structure is compounded by the substantial increase in body mass during pregnancy. The average weight gain places a significant mechanical load onto the feet, particularly on the plantar fascia and the arches. This combined effect of lax ligaments and greater pressure causes the arch to flatten, a condition known as overpronation.
When the arch flattens, the foot lengthens and widens. Changes in posture and gait further contribute to this mechanical stress. The center of gravity shifts forward, altering how weight is distributed with each step and forcing the relaxed ligaments to stretch beyond their normal capacity. This sustained stretching under a heavier load is a primary cause of the lasting structural changes in the foot.
Fluid Retention and Temporary Swelling
Beyond the structural changes, a temporary increase in foot size is commonly observed due to fluid retention, or edema. During pregnancy, the body’s overall blood volume increases significantly to support the mother and the developing fetus. Additionally, the growing uterus places pressure on the large veins that return blood from the lower extremities to the heart.
This increased blood volume and restricted circulation cause fluid to pool and seep into the surrounding tissues of the ankles and feet. This swelling is typically more noticeable in the third trimester and often worsens toward the end of the day. While this temporary volume increase contributes to the feeling of tight shoes, it is distinct from the permanent changes caused by ligament laxity and mechanical stress.
How Permanent Are the Changes
The outcome of these combined factors is often a lasting alteration in foot size and shape for many women. Studies measuring women’s feet from early pregnancy to several months postpartum suggest that a permanent loss of arch height is common. Researchers found that for 60 to 70 percent of participants, their feet became both longer and wider following their first pregnancy.
The arch drop and resulting foot lengthening typically measure between 2 and 10 millimeters, which often translates to a half or full shoe size increase. While the temporary swelling from edema resolves shortly after delivery, the structural changes caused by the stretched ligaments and flattened arch remain. This permanent change in the foot’s geometry is most pronounced after the first pregnancy, though subsequent pregnancies may still contribute to a degree of arch laxity.