The study measured the arch height and foot length of 49 women during their pregnancy and five months after they had given birth.
Twenty-nine of the women in the study were first-time moms; 17 were second-time moms, and three were third-time moms.
On average, the women's arch height decreased, and in turn, their foot length increased about 0.1 to 0.4 inches during this period, MyHealthNewsDaily reported.
Overall, about 60 to 70 per cent of the women had longer feet and shorter arches after childbirth, the study found.
Eleven of the women reported changes in their shoe size, the researchers said.
"I heard so many women talking about having to go buy new shoes after pregnancy," said study researcher Neil Segal, an associate professor of orthopaedics and rehabilitation at the university.
The change in foot size may be due to the extra weight women carry around during pregnancy, which puts greater stress on the feet, and, thus, may flatten the arch, the researchers said.
Moreover, pregnant women produce hormones that increase the looseness of the joints and ligaments, possibly making the foot structure more malleable.
Most of the women involved in the study who experienced changes in their foot length and arch height were first-time mothers. Women had given birth to two or three children did not experience such significant changes.
This result suggests that a woman's first pregnancy may have the greatest impact on foot size, the researchers said. But a larger study will be needed to confirm this, Segal added.
Changes in the feet during pregnancy may explain why women are at increased risk for pain or arthritis in their feet, knees, hips and spine than men, Segal said.
A flattened foot can strain the ligaments in the foot's sole, causing changes in gait that put extra strain on the knees, Segal added.
The study will be published in the American Journal of Physical Medicine & Rehabilitation.