"I guess I will be handing over the 'Baby' tag now to a deserving little new owner," the 31-year-old known as Baby Spice said in a statement.
Bunton and boyfriend Jade Jones, 27, a former member of the British boy band Damage, have been together for eight years. They moved in together to a five-bedroom home in north London in 2005.
"We are so happy, it's unbelievable news," the couple said in a statement. "What a great start to the New Year." The baby's due date or sex was not revealed.
In 2000, Bunton had to have pre-cancerous cells removed from her cervix. Last year, she revealed she'd been diagnosed with endometriosis, a painful condition in which the lining of the uterus begins growing in other areas of the body, making it difficult to conceive.
In December, she told the U.K.'s The Mirror that she hoped to have a baby of her own, but feared she might not be able. "Obviously I'd like to try for my own first," she said. "But I would be open to adoption."
Bunton's new baby will join the rest of the growing Spice Girl brood: Geri Halliwell gave birth to her first child, Bluebell Madonna, eight months ago and Victoria Beckham has three children: Brooklyn, 7, Romeo, 4, and Cruz, who turns 2 on Feb. 20.