Issei's parents had great trouble conceiving a child, first having to get an abortion due to the health risks, then suffering a still-birth two years later. After that, with the difficulties of getting pregnant, and the low odds of success if fertilization is achieved, they gave up, until 8 years later Issei was conceived by accident. Naturally, they did everything possible for both mother and child to be as safe and healthy as possible.
As part of that, Issei's father walked barefoot through the snow at night a hundred times to pray at a Shinto temples, plus numerous prayers at a Buddhist temple. That prayer?
"I offer you my life. You can halve the remainder of my life. How about it? Just protect my child. I beg you. Please."
And then Issei was born, healthy, with his mother surviving as well.
People who investigate might think that Amaterasu might have heard, and granted the request, with Issei being blessed as a result.