Crucial to the character of Jenny Sparks is the fact that she is the "Spirit of the 20th Century". In
The Authority #20(?) she saves the world from "god", then dies at the turn of the century. As she is a "
spirit", she lives on, reincarnated in a child's body.
The child happens to be a girl, but we have yet to determine if she is consciously the same character as Jenny Sparks. The baby does demonstrate accelerated development and post-human abilities.
A curious footnote: other characters in Warren Ellis' world were born on January 1st, 1900. These have been at the forefront on the Planetary series of comic books.