Ian Andrew Troi II was an alien entity who wished to experience Human existence. In 2365 in the form of a glowing white light, he entered into Deanna Troi and began his Human experience as a fetus. Dr. Katherine Pulaski confirmed that the fetus would reach full term within 36 hours. He was born in early 2365 and named after Deanna's father, Ian Andrew Troi.
He aged rapidly and grew from an infant to an eight-year old in one day. At the same time, a plasma plague put everyone on the ship in danger. A contaminated specimen of the hazardous strain had begun to grow on the cargo deck due to radiation from an unknown source.
Ian turned out to be the source of the radiation. When the young boy realized this, he decided to sacrifice his life to save the crew. As Troi looked on, Ian died, but not before he told her that he was the alien and that he wanted to experience the Human life cycle including death. His body reverted to its original state and left the ship, but not before he had experienced Humanity, and Troi had experienced the joy and sadness of parenthood. (TNG episode: "The Child")
In 2380, Deanna had a conversation with Qui'chiri, a Pa'haquel matriarch, about children. Deanna admitted that technically, she did have one child, Ian, but that the alien being could not survive long in human form. Qui'chiri sympathized with her as she had also had several offspring not survive to term. (TTN novel: Orion's Hounds)
When William T. Riker and Troi had trouble conceiving a child, it was revealed that an unknown side-effect of Troi's pregnancy with Ian had resulted in the radiation he generated causing subtle but significant damage to her reproductive system, until the Caeliar were able to treat the damage, allowing her to conceive a daughter.