The well-loved presenter announced the news in a broadcast earlier this week.
The couple, who live in York, already have 16-year-old twin boys Harvey and Harrison.
Following the announcement Mr Gration tweeted: "I do appreciate your kind comments.
"In truth it is my wife and twin boys who are the ones to admire in all this
"They give me the strength to challenge those who say I am mad! And I can change a nappy at 68! Regards."
Helen is now 20 weeks pregnant and due to give birth in October.
Speaking to the Telegraph she said she realised she wanted to try for another child with their teenage boys ready to leave for university soon.
After being advised by a fertility consultant that Helen would need an egg donor due to hers being "too old and viable", the couple started looking for an egg donor in January 2018.
Regulations established by the Human Fertilisation and Embryology Authority do not permit treatment in women aged over 51 because of menopause.
The couple travelled to Cyprus to have the IVF done and were delighted when they found out it had been successful.
A 12 week scan revealed a healthy baby much to the relief of Helen who says she is aware of health risks surrounding pregnancies in older women.
The couple do not yet know the sex of their baby, but say the child's name will start with the letter 'H'.
Their announcement was celebrated by Harry's journalism colleagues.
Christine Talbot said: "Congratulations Harry and Helen! They say they keep us young!"
Hi co-presenter Amy Garcia tweeted: "Well news is finally out! My co-host is going to be joining the sleep deprived, nappy changing gang again!
"Welcome back to the best club in town Helen Gration and Harry - huge congratulations xxx"