Question raised was good but here is the answer.
1stly if kansa was tht smart he would not ever try to kill babies or think of killing 8th child after hearing
the voice of god which they said used to happen often in those days, cause whatever god says everyone have to accept.
2ndly Like Hindu belives whatever is written to happen will happen anyways.. which is true no one can stop. So even if he would have kept vasudev and devki in seperate cell she would still concieve, now say it a miracle or destiny.
MY favt. and only idol krishna and his words.. my best line "what goes around comes around " in SHORT KARMA!
And I think whole world believes in it.
Kids are smarter than Kansa of Mahabharata
In some remote village of India, one masterji is teaching the Mahabharat katha to class 6 students. He is at the krishna janma' (Krishna's Birth) part of it.
Masterji: "Kansa heard the akashwani that his sister's 8th child is going to kill him. He was furious. He ordered to put vasudev n devki behind the bars. First son is born, and kansa kills him by poisoning...
Second one is born n kansa throws him off the mountain peak. Third one is born."
Now Ramu, who is smartest of the lot, puts up his hand and asks "Masterji, I have a doubt" (sounding nervous n confused)
Masterji: "Ramu bete, whole India does not have doubt in Mahabharata then how come you have one?"
Ramu: Masterji, if Kansa knew that Devaki's 8th child was going to kill him, " WHY THE HELL DID HE PUT VASUDEV AND DEVAKI IN THE SAME CELL?"