The longer the baby stays in the mothers womb the more likely the placenta is to stop functioning normally.
When the placenta stops functioning normally it can lead to health problems with the baby and could result in a stillbirth, neurological disorders and cause the baby to actually weight too little.
If a baby starts to become overdue then the mother may need to have surgery to remove the baby before it leads to health problems and a possible stillbirth.
However sometimes a baby can still be healthy even if a little overdue but it's best if the baby is born before becoming overdue.