I've heard that it is better for a child to be born on his 7th month rather than his 8th month. Any idea why?

This is a myth. The more premature a baby is, the more immature he/she will be, leaving him/her at a higher risk of a number of neonatal complications that are extremely common and usually related between one another (Infections, respiratory distress syndrome, neurological, gastric and cardiovascular complications, ophthalmic and auditory problems, etc). No baby dies of prematurity. They die because of those complications prematurity brings.