London - The challenge of losing the baby weight is one keenly undertaken by many new mothers.

But there is one part of their bodies no post-baby diet will help shrink - their feet.

Women’s feet really can increase in size when they are expecting - and the change can be permanent, a study has shown.

Researchers were testing the claim often made by women that they rise a shoe size during pregnancy, when flat feet are a common problem.

Actress Denise Van Outen revealed two years ago that during her pregnancy with daughter Betsy her feet grew from size five to six - and stayed that way.

Doctors believe the arch of the foot flattens out, possibly due to the extra weight and increased looseness of the joints associated with pregnancy.

And the new study, published in the March issue of the American Journal of Physical Medicine & Rehabilitation, suggests this loss of arch height is permanent.

The University of Iowa study followed 49 pregnant women and collected arch measurements, both at rest and when walking, during the first trimester of pregnancy, and again five months after childbirth.

The researchers found that 60 to 70 percent of the women in the study saw arch height and measures of arch rigidity decrease significantly from early pregnancy to five months after childbirth, causing corresponding increases in foot length - between 2mm and 10mm - and arch drop.

Professor Neil Segal, of the University of Iowa, 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.”

The study also suggested that first pregnancies may account for most of the observed changes, while second, third, or higher pregnancies may not further alter foot structure.

Professor Segal added: “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.”

Van Outen said her change in foot length meant she could no longer wear her collection of Christian Louboutin and Chanel shoes. The 38-year-old said at the time: “My feet have grown a size - I had to give away all my shoes!”

Van Outen, who married husband Lee Mead, 31, in April 2009, said that while she had swiftly returned to her size ten, her feet remained at their new larger size.- Daily Mail