The link above has more on multicultural health practices. Scroll down to bottom of page 16 (start of the Hispanic section which goes to page 20). Also see the section on India, where it describes the practices during the 40 day postpartum period. It seems these customs evolved over thousands of years. I didn't check the links to the other cultures, but I'm sure customs are similar. Still didn't find anything on the custom of tying a string around the pregnant woman's abdomen.