All three of my children were a surprise in their own way.
My first son was a surprise- but not the kind of surprise you probably have in mind. I was going about my business, living my life, and not particularly planning to adopt a baby; when a beautiful little baby boy came into my life and needed a mother. I just adored him, so that was it.
My younger son was a surprise in that he was born six weeks early and in about an an hour and a half.
My daughter was a surprise when she was a completely different looking baby (than her brother (the one I gave birth to six weeks early), but also when she turned out to be a girl. I guess I was expecting "another one just like the one before her" or else "another adorable little son, like the "little set of boys" that I already had.
All three were a surprise in what an absolute joy it was (and still is) to have each of them, but also to have all three of them as my children.
All three (now grown) continue to be a surprise as I continue to discover the different aspects of being the parent of grown sons and a daughter.
One way or another, children have their way of being surprises. :)