Meredith found out she was pregnant three weeks into her pregnancy, a few months after Mark died. She told Derek by buying a shirt for Zola that said "World's Best Big Sister". At first, she was convinced that she would miscarry, which is why she only told him, and not even Cristina. ("I Was Made for Lovin' You")
Meredith went into labor in the middle of a super storm. The labor was going smoothly until the OB determined that the baby had face presentation. He was then born via emergency c-section in the dark during the storm. In the operating room, the doctors had to hold flashlights so that Connie, the OB, could see to deliver him. After he was born, his breathing was shallow and his sats were low, so he was taken to the NICU to be monitored and receive oxygen. After he was born, Shane noticed that Meredith had suffered from a massive splenic hemorrhage caused by her fall down the stairs. Miranda Bailey performed surgery to stop the hemorrhage and saved Meredith's life, thus Meredith named her son after her. ("Perfect Storm")
A few weeks after Meredith was attacked by a patient, Arizona and Alex brought Zola, Bailey, and Ellis in to see her. However, the three of them were too scared by her appearance and the visit was brief. After she had recovered, the family was happily reunited at home. ("The Sound of Silence")
Before falling unconscious, Meredith said that she knew his name but was put under anaestethic before she could tell anyone. It is unknown if she had the name Bailey in mind at the time, as Bailey hadn't saved her yet.
Before his birth, Meredith and Cristina occasionally referred to him as "fetus."
Before his sex was determined, Meredith referred to him as "she" while Derek refered to him as "he".
When discussing names, Derek said he liked William and Meredith liked Sebastian.
He is the only one of his parents' three kids to be born in the hospital where they worked. ("Perfect Storm")