The 45-year-old, who gave birth to her son Monty in January 2017, said she and her husband tried for a baby for more than a year after their wedding in 2015.
Horner also has 11-year-old daughter Bluebell from a former relationship and her husband is dad to four-year-old Olivia.
The former Spice Girl told Red Magazine: “I wanted another baby and I’d tried an assisted route and it didn’t work.”
“I feel for any woman who has that natural biological longing, it’s tough. I’d got to thinking I have a child, my husband has a daughter, and accepting this is our lot. Then Monty came naturally.”
But despite the difficulties Horner hasn’t been put off wanting to continue to expand her family.
Speaking to HELLO in August, Horner called Monty a “gift” and said she wouldn’t say no to having another baby.
“There’s room for another one, for sure, and if that happened that would be great,” she said.
“But either way is okay. We’ve got our hands pretty full and we’re very grateful for what we have.”
Horner recently shared an adorable photo of herself cuddling her 10-month-old son on Instagram, and wrote: “Sometimes, I’ve just got to pause – take a minute to say thank you #thankyou #blessed.”