It's one of those weird side effects of pregnancy - one that most women are aware of, but which has gone largely unnoticed by the scientific community. But a new study has backed up what many mums already know: foot size increases during pregnancy - and it's often a permanent change.   

Headed by Neil Segal, an associate professor of orthopaedics and rehabilitation at the University of Iowa, a team of researchers measured the arch height and foot length of 49 women during pregnancy, and then again five months later.

Researchers found that foot length increased by 2-10mm, while the average arch height fell.

In total, up to 70 per cent of the women had their feet grow longer but flatter over the pregnancy period, with 11 even permanently changing shoe size.

The researchers believe that the changes are caused by the extra weight carried in pregnancy, which could put more stress on the feet, therefore flattening the arch. The pregnancy hormone relaxin may also have an impact, as it increases the looseness of ligaments and joints, possibly making the foot more malleable. 

A flattened foot can strain the ligaments in the sole, changing the normal gait and putting extra strain on the knees, Segal said, explaining that this could be part of the reason why women are at higher risk of pain and arthritis in their feet, hips and spine than men. He plans to investigate whether modifying the shoes of pregnant women could help prevent the foot flattening.  

Interestingly, women involved in the study noticed the largest changes if it was their first pregnancy: those who already had two or more children didn't see such a difference. Segal believes this is because the first pregnancy has the biggest impact on foot size and shape, but says a larger study would be needed to confirm the idea.

The study will be published in the March issue of the American Journal of Physical Medicine & Rehabilitation.

- Essential Baby