Bosses have been keeping the true and painful storyline under wraps, as they worked with mental health charities including Mind and Bipolar UK about the disease, which affects new mothers and can cause mood swings and hallucinations.
Sufferers of the condition have also met members of the show's staff, including actress Turner, to discuss and consult over the script.
Dominic Treadwell-Collins, the show's executive producer, said: "Stacey's mental health story will be one of our biggest storylines over Christmas and the beginning of 2016 as we explore Stacey's bipolar disorder and its effects on her and those closest to her. We have always had a big commitment to Stacey's story and this time we want to explore the effects of postpartum psychosis on those with bipolar, something which, although a rare illness, is unfortunately more common when mothers with bipolar give birth - and look at what we are doing as a country to help mothers experiencing this."
Clare Dolman, Bipolar UK's vice chair, said: "As the national charity supporting people with bipolar, we've been glad to work closely with the BBC on Stacey's storyline. There is a very high risk that women with bipolar will become ill when they have a child and 20%-25% of them will have a postpartum psychosis so it's fantastic that EastEnders are raising awareness of this devastating condition. When it happens it's a psychiatric emergency, and yet it's eminently treatable and the vast majority of women recover to be great mothers."
Alison Kerry, from Mind, said: "We are pleased EastEnders is tackling another important, often misunderstood, mental health problem. It is so important that we are raising awareness about this subject through the medium of soap as it brings the issue into the living rooms of a massive audience helping us to challenge stigma and improve understanding. We have been working with the EastEnders team to develop this storyline over several months and have been impressed by the dedication they have shown in portraying postpartum psychosis sensitively."
Stacey was diagnosed with bipolar disorder in 2009, and is expecting a baby with Kush Kazemi, without her boyfriend Martin Fowler's knowledge and is oblivious to the fact he is not the baby's father.