Penelope Gate's first son

When they met during her first time-travel adventure, Penelope Gate told Chris Cwej that she and her husband had had a son until three years prior, when the baby had died, less than a year old. (PROSE: The Room With No Doors)

Behind the scenes
The Infinity Doctors and The Gallifrey Chronicles would later reveal Penelope Gate as the Doctor's mother, meaning that this first, human child would have technically been a half-sibling of the Doctor's.