Another Myth :
Shiva and Parvati were married and the demi-Gods eagerly awaited their son. However Shiva and Parvati were so engrossed in their amorous activities that they had no time for the travails of the world. Ultimately the demi-Gods had to disturb Shiva and remind him of his duties.
Shiva gave his seed to Agni and vayu. They went near a river where the wives of the seven sages were bathing. Six of them went to the fire to warm themselves and were impregnated. On realising their predicament they left the seed in the form of an embryo in the Himalayas. The Himalayas, unable to bear the heat of the embryo, placed it in the river GangaGanga
According to Bhagavata Purana, Vishnu in one of....
. There Kartikeya was born.
Shiva gave his seeds, which were handled by vayu, the process of centrifugation, so that the healthy sperms can be taken out. Next they were handles by agni, the process of incubation. The next process was the entry into the river, which means the Petri dish containing the eggs. Now, starts the process of fusion into an embryo. The six headed infant here means the six cell stage of the embryo ready for implantation in the womb. The same was implanted in the Parvatis’ womb (in one story as she embraced the same) and in the Krttikas (the native girls) in another part of the story. Karttikeya is described as five or six-headed GOD representing that a cell of five or six cell stage can be implanted.