Some good friends of mine, he had a vascectomy after DD3 (unplanned) was born with severe CP and they knew coping with another DC would be a struggle.
DC4 arrived somewhile later, yes it was a little tense at the beginning, but these things (the snip) aren't 100% reliable.
If you have not cheated, unless you believe in the immaculate conception, there is no other explaination for it.