The Saturdays singer, 26, and 34-year-old Wayne are already parents to son Parker, one, and discovered their happy news over Christmas.
But Frankie has been diagnosed with hyperemesis gravidarum, which is the same crippling morning sickness Kate Middleton had when pregnant with Prince George.
Speaking to OK! Magazine, Frankie said: “I discovered I was pregnant over Christmas. I was obviously over the moon, but the sickness is terrible.”
Frankie has been so sick that she’s been forced to pull out of the Strictly Come Dancing tour, and it was confirmed last week that Rachel Stevens would replace her.
She continued: “I haven’t been able to leave my bed for days and my doctor’s instructed me to rest and not exercise, so dancing was completely out of the question.”
But Frankie isn’t letting the sickness dampen her excitement at welcoming a baby brother or sister for Parker.
“I absolutely love being a mum and having Parker was the best thing that ever happened to me,” she added. “The thought of an addition to our little family is the most exciting thing ever.”