The Look North presenter revealed the happy news in a broadcast earlier this week.
The couple, who live in York, already have twin boys Harvey and Harrison, 16.
Harry took to Twitter to thank people for their kind words following his announcement.
He tweeted: "I do appreciate your kind comments. In truth it is my wife and twin boys who are the ones to admire in all of this.They give me the strength to challenge those who say I am mad! And I can change a nappy at 68!"
Helen, who is now 20 weeks pregnant, is due to give birth in October and said she realised she wanted to try for another child with their teenage boys ready to leave for university soon.
A fertility consultant told Helen she could have another baby, despite her age, but said she would need an egg donor due to hers being "too old and viable".
The couple started looking for an egg donor back in January 2018 and were advised to look into a clinic in Cyprus due to the lengthy process of finding a donor in the UK.
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!"