I'm really just wondering if cubs are rolled at time of conception or time of birth, if anybody happens to know?

I'm really wondering, because, say, I use a skin changer and change my lion's skin after he has impregnated a lioness, but before the cubs are born. If they were rolled when he did the do, they'd receive the option for the old skin, but if they're rolled when they're born they have a chance for the new skin.

Mainly just curiosity!