She said: “It’s about keeping women working if they want to during their pregnancy – because a lot of women are hidden away in the back room when they are pregnant. “But if you want to keep on working then you should be able to and there shouldn’t be anyone who limits that. “I am in a really strong position to be able to do that in my chosen job and this drama and I think it doesn’t happen very often.
“Lots of actresses get sacked for being pregnant.”
When Helen, 37, was expecting her first child, she was diagnosed with a potentially serious liver disorder called ICP (Intrahepatic Cholestasis of Pregnancy).
Her daughter Wren was born earlier than expected in September 2017 by caesarean section.
But Helen, who has portrayed Trixie Franklin in the series since 2012, is taking extra care in this pregnancy.
She says the support of her co-star and partner Jack Ashton – Wren’s father, who she has been in a relationship with since 2016 – as well as co-star Jenny Agutter, has made life on set much easier.
“It’s supposed to be a Christmas baby but my last one was early, so we’ll see,” she revealed.
“It’s OK at the minute and we are just keeping our eye on [the ICP] and I’ve got good support around me so I know where to go to if it persists.
“I am lucky in that Call The Midwife is a lovely place to work and be pregnant.”
Asked about Wren’s birth, she explained: “It coincided with the fact that I had to deliver her early. But even without that, I would have gone for an elective caesarean because of what I’d learned on Call The Midwife.
“Working on the show means that lots of people tell you their horror stories about birth.”
Defending the decision, she added: “I’m not against natural birth, I’m pro whatever you feel is right for you.
“Some people may not understand why I elected to have a C-section, but it was right for me at the time.
“It’s not because I’m ‘too posh to push’, it’s about what I think my body is capable of.
“I’m not good with pain. I faint when I stub my toe.
“Not that a C-section is the easy way out. It’s a major operation.
“I have a large scar on my stomach. You can’t exercise for a long time and you need help to pick up the baby.”
• Helen was speaking at the Florence Nightingale Garden at the RHS Chelsea flower show