"Christian and I are delighted to announce our baby boy was born this morning weighing 7lbs 8oz," Halliwell, who is also mom to 10-year-old daughter, Bluebell, from a previous relationship, tweeted. Her husband also fathered a daughter, 3-year-old Olivia, with his former partner, Beverley Allen.
Halliwell later took to Instagram, sharing a precious black-and-white snap of her newborn's toes. His full name? Montague George Hector Horner!
"Montague George Hector Horner arrived this morning, a beautiful little brother for Bluebell and Olivia," she captioned it. "#amazing-day #grateful #monty."
Upon hearing the news, a few of Halliwell's Spice Girls bandmates congratulated her via Twitter. Ironically, Montague was born on the same day as Emma Bunton, aka Baby Spice, who turned 41 on Saturday.
"Amazing news, so happy! #birthdaytwins #birthdaymade love you all. Xxx"