Motherhood isn’t all it’s cracked up to be. Or at least it isn’t black and white, more of a stretched, washed-out, flesh-coloured maternity-bra shade inbetween, according to screenwriter Diablo Cody’s new film Tully. A decade on from her Academy Award-winning teen pregnancy rom-com Juno, Cody has created another lead who is expecting, this time turning Charlize Theron into baggy-eyed mum Marlo, who is one frozen pizza dinner away from the edge. She shouts. She accidentally drops an iPhone on her baby’s head. She slumps in front of her children, post-birth belly hanging out, much to their horror. Deflated, running on empty, she resembles a whoopee cushion being sat on in slow motion. Parenting has never looked so relatable.

“The expectations [on] women are out of control,” says Cody, who takes our call in her garage, the only place in her LA home where there’s any quiet. She is herself juggling the Alanis Morissette musical she has scripted and which is about to open in Boston, a pilot for ABC that she is currently cutting, and being a mother of three. “I can’t believe the disdain towards women who have ‘let themselves go’. Why can’t I? I’m taking care of three small children; why am I also supposed to be skinny and hot?”

Cody, 39, says she is a different Diablo from the one who sashayed down the 2008 Oscars red carpet in head-to-toe leopard print to collect her award for Juno’s screenplay. She wrote the rom-com in a month while sitting in Starbucks, and it took her from part-time stripper-blogger to star overnight. Her projects since Juno haven’t quite matched that film’s pop culture-puncturing potential: Megan Fox horror dud Jennifer’s Body; a Spielberg-produced TV series, United States of Tara, which lasted three seasons; underrated rock comedy Ricki and the Flash; brilliant but cancelled Amazon Prime series One Mississippi, which she co-created with standup Tig Notaro. But one aspect that has remained constant is her focus on creating female characters that defy the ordinary. Meryl Streep’s ageing rocker in 2015’s Ricki and the Flash, based on Cody’s mother-in-law, was particularly important – “a woman who had chosen to pursue her passion at the expense of her children, and she was viewed as a monster because of that,” she says. “We’re surrounded by men who have made that choice and are not demonised in the same way.”

Tully is the third film in her trilogy with Jason Reitman, and the second with Theron after Young Adult, and again offers a view on the female experience that Cody rarely sees onscreen. And so there are money issues, a sexless marriage with a fairly useless husband, the exasperation at not knowing how to care for a young son with undiagnosed special needs. Sensing her desperation, Marlo’s brother buys the baby shower gift that every strained mother dreams of: help. Tully is a night nanny who breezily glides in, all toned abs and tanned limbs, to do the twilight shift so Marlo can rest. She’s a Manic Pixie Dream Girl version of Maria von Trapp, but also a device to help Marlo question what parts of herself have been lost under the tummy-huggers.

Cody can relate. After having kids, she says, the question of “How do you access the person that you once were?” is something she grapples with. “I feel like a completely different human being on a cellular level since having children.” Not just through a loss of identity – in the film, Marlo is also suffering from postnatal depression, which Cody says there’s still a stigma around. She doesn’t say whether she had post-natal depression herself but does confirm she felt “that emptiness” for a while. “We’re told that we should feel completely blissed out after we have a baby, and that’s not always how women feel. It comes down to this idea that mothers are supposed to be completely selfless and if you’re indulging feelings of sadness or depression, that could be perceived by other people as selfish.”

Tully isn’t just about parenting, however: “It’s about the great and intense transformation of middle age. It’s waking up and going: ‘Oh my God, I’m 40 – what happened?’” Is she feeling apprehensive about that coming milestone? “Yeah, and it’s strange. For years I’ve been regarded as ‘a young screenwriter’, or someone who has achieved a lot for her age. But no one’s going to give you a cookie for achieving something at 40. At this point, the pressure’s on me to perform and to sustain that. And also, we all know how older women are regarded, particularly in Hollywood. So it’s a little scary to think: ‘Oh, I will be discarded – that’s going to happen.’”

Talking to Cody about these pressures, you wonder why she hasn’t been much involved in the wider public conversation about women in Hollywood that has occurred in the past year. Reading old interviews with her, Cody was outspoken and outraged, but recently her voice has been eerily absent; she quit Twitter in 2015 and hasn’t considered rejoining. “I’ve been expressing my viewpoint in my art for more than a decade now, so everybody knows that I’m a feminist,” she says. “I don’t think my voice would’ve been valuable in that conversation because, honestly, I don’t think Twitter is desperate for another quirky white woman spouting her opinion. Let’s hear from voices we don’t hear from.”

Still, she is encouraged by the Time’s Up movement, even if she initially doubted its staying power. “I’m such a cynic that, when it first started happening, I was on a group text with a lot of my girlfriends, and they were all so excited and I was like: ‘This won’t last. Men in power are going to shut this down so fast.’ And then it didn’t, it just grew and grew. So I’m glad that I was proven wrong.”

Cody says she is “heartened” too by the progress that Hollywood is making with “the types of roles that are being written for women. When I started out, a TV show or a film about a difficult woman was a hard sell, and now it’s something that people are soliciting from me.” But she finds it frustrating that women aren’t allowed to fail in the same way as men. “It drives me crazy that if a woman directs a movie and it doesn’t perform, people say: ‘Oh, they shouldn’t have let a woman direct it.’” . Cody’s own directorial debut, the 2013 comedy drama Paradise, was made while she was pregnant, and was subsequently panned by critics. “If a man directs a movie and it flops, there’s literally no discussion of the director’s gender,” she adds. “As a woman in a field where women are underrepresented, you often feel like you are representing women, and that’s ridiculous.”

With Tully, Cody does seem to have come full circle since Juno. Knowing what she knows now about childbirth, would she have done anything about her first film differently? “In terms of pregnancy, no,” she says. “But I don’t feel I was clear enough in terms of why Juno chose to not have an abortion. It was simply because she did not want to.”

The film was criticised by some for portraying abortion in a “negative” light. “It was not about any type of feeling that abortion was wrong – I’m pro-choice,” Cody says. “So for it to be interpreted as an anti-choice movie, that’s upsetting to me.”

The Juno backlash is one reason that she has become “more private”. She refuses to self-censor – “it hasn’t affected my writing” – but says: “It’s certainly affected the way that I talk to people about the projects. I’ve become very boring because I want to protect myself and my children. I would like to just keep a low profile and continue to work, and I’ve had to really stay under the radar in order to accomplish that.”

It’s depressing that a forthright person such as Cody has been forced to retreat from the public eye. But, like Marlo, she sees a “joy” in embracing a life more ordinary. “I take great pride in my boring life, because it’s not easy for me to be sedate. I’m a naturally curious, impulsive, self-destructive person. And I’ve had to put all that aside as a mum. I’m proud of that because it’s required a lot of discipline.”

• Tully is released on 4 May in the US and the UK, and 10 May in Australia