http://www.what-buddha-taught.net/Books ... NATION.htm
And what, bhikkhus, is birth? The birth of the various beings into the various orders of beings, their being born, descent (into the womb), production (abhinibbatti= rebirth), the manifestation of the aggregates, the obtaining of the sense bases. This is called birth." The meaning of the term `various orders of beings', is fully brought out by a passage in another sutta specifically dealing with Dependent Origination, the Mahanidana Sutta (DN 15): "With birth as condition there is aging and death. How that is so, Ananda, should be understood in this way. If there were absolutely and utterly no birth of any kind anywhere - that is, of gods into the state of gods, of celestials into the state of celestials, of spirits, demons, human beings, quadrupeds, winged creatures, reptiles, each into their own state - if there were no birth of beings, of any sort into any state, then, in the complete absence of birth, with the cessation of birth, would aging and death be discerned?" "Certainly not, venerable sir."3 Again, it is quite clear here that birth means what we would normally consider it to be: the arising in the human realm of a being in the womb.