The couple - played by Ryan Moloney and Eve Morey in the soap - have been trying to conceive a child for several months to no avail.
However, after the pair decide to put their family plans on hold, Sonya discovers she is finally pregnant.
"Toadie and Sonya have been through so much together that when they decided they wanted to have a baby they thought it would all go to plan," a Neighbours source told the Daily Star.
"Sadly, the pressure of trying to conceive led to heartache and stress so they decided to give up on the idea - unaware Sonya was already pregnant.
"Sonya is keen to keep the news to themselves as she is worried that it is too early to tell friends and family.
"But, like a lot of things in Ramsay Street, it is not going to be a secret for very long."
The scenes are due to be broadcast in the UK later this summer, while the episodes will air earlier in the year in Australia.
Ryan Moloney praised the pairing of his character with Morey's Sonya Mitchell earlier this year.