Conditions for the necessity of the funeral prayer and burial for a stillbirth or fetus?

Note: If it is necessary this post can be divided into two separate questions (I hope for some constructive comments). Basically what I want to know is: what are the conditions for a burial for the named cases ... or from what development stage or age of a fetus on is a burial necessary... And I do assume that the necessity of to bury a body imply the necessity of performing salat al-Janazah I just want to know whether my assumption is right. I hope for answers with evidences that cover an approved fiqh opinion on the matter.