The Church of the Holy Sepulchre, the oldest building in the entire Basilica of Santo Stefano. 12 columns surround the shrine which held relics of San Petronio that had been recovered here in 1141. The little door of the tomb used to be opened for one week year during which time it was possible to crawl inside and pay one’s respects to the saint’s remains.

According to a curious custom, pregnant women in Bologna would circle the sepulchre thirty-three times, one for each year of Christ’s life. After each turn, they would crawl into the small sepulchre to pray. At the end of the ritual, they would go to the nearby church of Martyrium to pray in front of a fresco of the pregnant Madonna. In 2000 the body of San Petronio was moved to the basilica that bears his name, reunited with his head which was already there. From that point on, the sepulchre was never opened again.

http://www.italyguides.it/us/italy/emilia-romagna/bologna/piazza-santo-stefano/church-santo-stefano.htm