The 39-year-old actress, who stars as Trixie Franklin in the period drama, appeared on The Zoe Ball Breakfast Show on BBC Radio Two on Wednesday to promote her upcoming appearance in the West End production of The King And I, as well as the new season of Call the Midwife.
When asked what Helen could tell viewers about the new episodes, she said: "We're filming at the moment. So I'm sort of discovering the stories as we go along. It's another exciting year, lots of babies.
"I can't tell you much more than that," she said, adding: "There's a few shocks."
Helen's update comes amid reports that her co-star Olly Rix, who plays Trixie's husband Matthew Aylward, has left the show.
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Olly has reportedly been written out scripts, however, the reasons why have not been explained.
Neither Olly nor show bosses have spoken out publicly about the reports. HELLO! has reached out to the actor's representatives and the BBC for comment.
Meanwhile, filming for the new series is currently underway, having officially begun back in May.
The show's social media team shared an update on production this week, revealing that there's only three episodes left to shoot.
Taking to Instagram, the drama shared a behind-the-scenes snap showing Linda Bassett and Georgie Glen, who play Nurse Crane and Miss Higgins, filming a scene on a beach in Sussex alongside an expectant mum.
"What do we have here?? Have Nurse Crane and Miss Higgins abandoned Poplar for the delights of the English Riviera? And who is that young mother-to-be that they're attending to....?" read the caption.
"Last week our team took a special excursion away from the smoke-filled streets of the Sixties East End to film in the fresh coastal air of Sussex."
The caption also provided an update on where the cast and crew are at with filming. "This sequence was for episode 5 of season 13.... which means that we're racing on with our filming for the next series! Just three more eps to film after this one," it read.
"The Autumn winds were blowing on the beach, and the light was closing in much earlier - which told us that it wouldn't be too long to wait now until our brand new Christmas episode!!"
Call the Midwife returns with a Christmas special in December later this year. Series 13 will premiere on BBC One in the New Year, although an official release date has yet to be announced.
If previous seasons are anything to go by, fans should expect to see the new episodes sometime in January.