A new study has confirmed what many mothers already suspected - pregnancy can alter the size of your feet and these changes can be permanent.

US scientists followed the progress of 49 pregnant women, carrying out detailed measurements of their feet during the first trimester and again five months after the women had given birth.

They found that for at least six in 10 of the women, their feet had become longer and wider.

Specifically, the height of the arch of the foot decreased significantly. In other words, the arch of the foot flattened out, possibly as a result of the extra weight being placed on the feet during pregnancy and the increased looseness of the joints.

The rigidity of the arch also decreased. This led to increases in foot length of between two and 10mm.

The scientists from the University of Iowa noted that these changes tended to be most associated with first pregnancies. The structure of the foot did not appear to change with subsequent pregnancies, they said.

"I had heard women reporting changes in their shoe size with pregnancy, but found nothing about that in medical journals or textbooks. We found that pregnancy does indeed lead to permanent changes in the feet," explained lead investigator, Dr Neil Segal.

He noted that it is already known that women, especially those who have had children, are more affected by musculoskeletal disorders.

"It is possible that these foot changes that occur during pregnancy may help explain why, in comparison with men, women are at higher risk for pain or arthritis in their feet, knees, hips, and spines," he explained.

Details of these findings are published in the American Journal of Physical Medicine & Rehabilitation.

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