In the Bhagavatam, Krishna is the 8th son of Devaki. Why did he have to come as the eighth son?
Why not 1st? or 2nd? or 3rd? or 4th? or 5th? or 6th? or 7th? or 9th?
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Because the 6 dead sons of Devaki are reincarnation of Marichi's sons. They got cursed by Sage Devala for laughing on him or by Brahma ( because they have seen Brahma trying to copulate with his own daughter and hence derided at him. Indignant Brahma had then curse them to take birth in demon incarnation.)
So they therefore were born as the sons of Kaalanemi who was the son of Hiranyakashipu's brother Hiranyaksha. They used to remain in the water at Patala loka performing penance to obtain immortality. Brahma granted them immortality. However Hiranyakashipu became angry at this. He did not like his brother's grandsons obtaining a boon from Brahma since he considered himself as the only worshipable person. Therefore, he cursed them to be born again and be killed by their own father. Accordingly, they were born as Devaki's children and were killed by their own father. Kamsa as Kaalanemi was their father in their earlier birth.
To eliminate contradiction of curse and boon, Goddess Durga ( or Yogmaya in some versions) made the six of those sons of Kaalanemi to fall in deep sleep, retrieved their souls from those sleeping bodies and placed those souls in the womb of Devaki. Thus, King Kamsa killed six of his own sons of his earlier body.
The seventh one was Balram, because Rama promised Lakshmana to be his younger brother in his next incarnation. That's why he is 8th son of Devaki.
That's why Narada confused Kansa to kill all sons of Devaki, rather then only 8th.
Note: some facts are from this sources and this yahoo answer and few from my memories.