Rosanna Davison is expecting twin boys, due shortly after daughter Sophie celebrates her first birthday at the end of November.
She told the Herald she is "absolutely thrilled" at her surprise lockdown pregnancy with identical twin boys.
Rosanna said they were conceived naturally after she became a mum last year through gestational surrogacy.
It has been a long and arduous road for the former Miss World and her husband Wes Quirke, who suffered the devastation of 14 miscarriages.
Rosanna (36), who is at the midway point in her pregnancy, told of her hopes that couples struggling to start a family will take comfort from her story.
The daughter of musician Chris de Burgh revealed the news on Instagram yesterday in a post that quickly went viral, and said she was "blown away" by the messages she received afterwards.
She added that while she was "shocked" to discover she was pregnant with twins, she has relished every moment.
"It feels great to share the news that we're at the halfway point of the pregnancy and we've had such an incredible outpouring of love from everybody, so it's been an emotional afternoon," she said.
"The first trimester was tough with nausea and fatigue, but I was just so shocked and grateful to be pregnant that I tried to embrace the hormonal roller-coaster.
"I've been feeling really good since about 13 weeks, with plenty of energy. I'm mainly just extra hungry and need more sleep."
It will be a busy 2021 for the model, given she will welcome the twins just after Sophia turns one, meaning she will have three babies under 13 months.
"The twins are due just after Sophie's birthday. It just goes to show that miracles really do happen and I think for me, being relaxed during lockdown really helped," she said.
Rosanna had been told she would never be able to carry her own baby full-term due to a suspected immune system dysfunction that medical intervention had failed to resolve.
While she had no issue becoming pregnant, she had been unable to sustain the pregnancies, resulting in a string of heartbreaking miscarriages and a "challenging fertility journey".
"For this to happen naturally and to have twins too, by complete chance, is an absolute dream come true for us," she said.
Rosanna said the news has confounded medics, but she believes that slowing down and relaxing completely during lockdown helped her conceive.
The nutritionist and author said "fertility miracles" may take time, but they can happen in the "most unexpected and magical ways".
"As always, sending so much love to those of you still on your baby journey. Never give up hope," she said.
Rosanna has been praised for speaking about her struggle to become a mum and her gestational surrogacy journey.
She and Wes welcomed baby Sophia in a hospital in Ukraine last November after they used a surrogate to make their baby dream come true.
Rosanna kept her followers updated on her progress and said she would love to write a book and make a documentary on what it is like for couples experiencing fertility problems.
She recently said that having baby Sophia to look after during the pandemic was a huge blessing, as it meant she could focus on being a mum after the world went into lockdown.