Test audiences thought the shocking ending of Sydney Sweeney's horror movie had to be seen to be believed. The director says they're wrong.

• The bloody ending of "Immaculate" is purposely ambiguous.
• Sydney Sweeney nailed the final scene, in which she gives birth, on her first try.

When Sydney Sweeney brought director Michael Mohan the script for "Immaculate," she told him she just wanted to be "covered in blood and go crazy."

Mohan was more than happy to deliver.

The director, who'd previously worked with Sweeney on the erotic thriller "The Voyeurs" and the Netflix series "Everything Sucks!", had never made a horror movie before. So the bloody, emotionally raw ending of "Immaculate" became his thesis statement on the kind of horror movie he wanted to make.

"She brought me on for a reason, and that was the reason," Mohan told BI. "I was like, 'Let's not push the envelope, let's rip it to shreds.'"

Below, we break down the movie's polarizing ending and explain how it was made — including the key role sound played in keeping it ambiguous.

Warning: Spoilers ahead for the ending of "Immaculate."

What happens at the end of 'Immaculate'?

Sister Cecilia (Sweeney), a young American nun who's mysteriously impregnated after moving to an eerie yet picturesque Italian convent, finds out that, despite everyone treating the pregnancy as a miracle of immaculate conception, she's actually not the next Virgin Mary — just an unwitting participant in a sinister science experiment.

Father Sal Tedeschi (Álvaro Morte) explains to Cecilia, who is now near the end of her pregnancy, that he was a biologist before becoming a priest. He used that skill set to try to make a new savior by pulling some DNA off of a nail that was supposedly used to crucify Jesus and creating an embryo with it. His past attempts — the monstrous-looking jarred fetuses in various stages of development that Cecilia finds earlier in the film — were all failures until he impregnated Cecilia, whom he'd specifically hand-picked and invited to the convent just to be the vessel for his Jesus 2.0.

Understandably horrified, Cecilia goes on one of the most excellent final-girl rampages in recent memory, bludgeoning the complicit mother superior with a crucifix, strangling the cardinal with rosary beads, and finally setting Tedeschi's lab ablaze with ethanol.

In the midst of all this, her water breaks and labor begins. She finally manages to kill Tedeschi, stabbing him through the throat with the crucifixion nail as he's trying to perform a barbaric C-section on her, and escapes from the catacombs she'd been fleeing through — only to give birth alone, covered in blood, out in the countryside.

The catch, and what makes this one of the most memorable horror movie endings in years, is what doesn't happen next.

Cecilia reacts in horror seeing whatever she's just birthed and ultimately decides to crush the baby with a big rock. But at no point does the audience actually see the baby — so we don't know what it is, or why Cecilia decides to kill it.

The 'Immaculate' ending wasn't in the original script

Sweeney had auditioned a decade ago for a version of the film based on Andrew Lobel's original script, which followed a student in a boarding school rather than a nun in a convent. The movie was ultimately shelved, but Sweeney couldn't get the story out of her head, she told BI on the red carpet ahead of the movie's premiere at SXSW Film & TV Festival. With Lobel's permission, she ultimately resurrected it herself under the banner of her production company Fifty-Fifty Films.

Mohan and Sweeney decided to make Cecilia a nun, and Lobel came back in to rework the script to accommodate that change.

Mohan described Lobel's first script as a studio movie meant for a wider audience that had a "more traditional ending" to match. He envisioned something a bit grittier, and settled on a minutes-long extreme close-up of a blood-covered Sweeney screaming her lungs out while in labor.

"I wanted there to be this moment of real catharsis," Mohan told BI. "This is a character who's been tortured and held captive, and I felt like audiences just needed that sense of breathlessness and then that sense of relief that comes at the end of it."

Sweeney nailed the scene on the very first take

The intense final scene is almost exclusively a close-up of Sweeney's face as Cecilia goes through the pain of unmedicated childbirth, rips the umbilical cord with her teeth, reacts in horror seeing her child for the first time, and ultimately decides to pick up a rock and crush the baby with it.

It's a powerhouse performance from Sweeney, who did the whole thing in one unbroken take. In fact, as Sweeney and Mohan revealed at the Q&A after the film's SXSW Film Festival premiere, the scene we see in the final cut of the film was Sweeney's very first take.

"On the day, shooting it, it was actually really easy. We figured out the camera blocking, because it's really minimalist, it's all just about her performance," Mohan told BI. "We set up the blocking, we figured out where the camera was going to go, we walked it multiple times so that we knew it would always be in focus, and then we just let it rip."

According to Mohan, that level of unhinged is Sweeney's sweet spot.

"This is easy for her. She doesn't have to sit in her trailer and get worked up and then come out," he told BI. "It's not like I have to whisper something into her ear like a secret that suddenly unlocks this performance. She's able to go there at the drop of a hat."

Almost as noteworthy as Sweeney's raw performance is the storytelling choice to not show the baby at all.

The creature Cecilia just birthed — and promptly smashed with a rock — is merely a blurry, unfocused blob in the background as the camera stays focused solely on Sweeney. All the audience hears are unnerving gurgling sounds, leaving it unclear whether the child is actually a demonic antichrist or just a sickly, underdeveloped newborn.

That ambiguity was on purpose, and it was no simple feat.

Sound supervisor Bryan Parker was tasked with creating the creature's perfectly creepy cries

Mohan and the team knew that sound would be an important part of the final moments of the film, so much so that the ending was the first thing sound supervisor Bryan Parker was tasked with working on.

The sound needed to sound both "monstrous" and "sickly," Parker said — a tall order that required a bit of finesse. The sound had to be clear enough for audiences to mostly interpret it the same way, while still leaving room for ambiguity.

That ambiguity "enables us to hint very heavily at something that leaves the door open for each person to fill in the picture with what they find most scary," Parker said, "in a way that if you show too much of the monster on screen, people don't have as much room to interpret it with their own personal fears."

At one point, Parker's team tried to use the sound of a baby goat, but it didn't play the right way at all. "In our first test screening, everybody laughed, and so we realized, OK, that's not it," Mohan recalled.

Ultimately, Parker and his team settled on two sound layers — one for monstrous, and one for sickly — and wove them together to sound like they were coming from one creature. They used a cat wheezing as part of the sickly layer, and a recording of a "really phlegmy, mucusy, full-grown adult man" for the monstrous one.

There was even a little bit of Parker's own voice in the final product — "not very much of it, although that's always fun," he said.

People really wanted to see the monster baby in test screenings

It took multiple rounds of notes over three to four weeks to get the baby sound just right.

"In front of the test audiences, it made our friends and family feel the certain kind of uncomfortable we wanted them to feel and get them demanding the kinds of questions that we wanted people to be asking each other on the way out of the theater," Parker said of the finished product.

But sometimes, an audience member would give the note that they didn't want ambiguity. They just wanted to see the baby.

"Along the way, there were people who watched the film and were like, 'Well, I have a note, and you have to show me this,'" Parker said.

"And Michael was like, 'Oh, that's a very interesting note. Anyway, moving on.'"

"Immaculate" is in theaters now.