The singer also known as Ginger Spice, 33, told Hello! magazine she'd been inspired by seeing the spring flowers during her pregnancy.
"But what really clinched it for me was my mother telling me that the bluebell is increasingly rare — so it's precious flower, which seems just right for my daughter," Halliwell was quoted as saying.
Halliwell said she had chosen the middle name in honor of one of her heroines.
"As she came out of my tummy, Bluebell had both arms flung wide in the air as if announcing to the world, 'Hi! I'm here!'" the magazine quoted Halliwell as saying.
"She was screaming her head off, as though she was shouting, 'Hello, Wembley!' No one else has that name, apart from the Virgin Madonna and the singer, whom I love."
The baby was born May 14 at London's Portland Hospital weighing 5 lbs 12 ounces.
In the Hello! interview, Halliwell made no mention of the baby's father. He is reportedly Hollywood scriptwriter Sacha Gervasi, 40.