Url Keywords In Subfolders

Hi

re my website www.only4mummy.co.uk I would like to change the subfolder urls to contain the keywords within the URL. I have been told this is not possible for my website due to the design?

for example this link

http://www.only4mummy.co.uk/products.html?cat_id=51

I would like to say

http://www.only4mummy.co.uk/pregnancy_Gifts.html?cat_id=51